Chemistry, Module Two

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What determines molecular shape?

• Electron pairs around the central atom.• Lone pairs of electrons are electrons that are

not shared.• Bonding pairs of electrons are electrons that

are shared with another atom in a covalent bond.

• Electron pairs repel each other.• The molecular shape is when the electron pair

are as far apart from each other as possible.

What is the electron-pair repulsion theory?

• Electron pairs repel each other because all electrons are negatively charged.

• Lone pairs repel more than bonding pairs.• Each lone pair reduces the bond angles by 2.5

degrees.

What are linear molecules?

• 2 electron pair on the central atom.• Bond angle of 180 degrees.• E.g. BeCl2 and CO2

What are trigonal planar molecules?

• 3 electron pairs on the central atom.• No lone pairs.• Bond angle of 120 degrees.• E.g. BCl3, CO3

2- and NO3-

What are ‘bent’/non-linear molecules?

• Molecules with two bonding pairs of electrons and one or more lone pairs of electrons.

• Bond angle = bond angle for number of electron pairs on the central atom – 2.5 degrees for every lone pair.

• E.g. SO2 and H2O

What are tetrahedral molecules?

• 4 electron pairs on the central atom.• No lone pairs.• Bond angles of 109.5 degrees.• E.g. NH4

+

What are trigonal pyramidal molecules?

• 4 electron pairs on the central atom.• One lone pair.• Bond angles of 107 degrees.• E.g. NH3 and SO3

2-

What are trigonal bipyramidal molecules?

• 5 electron pairs on the central atom.• No lone pairs.• 3 bond angles of 120 degrees and 2 bond

angles of 90 degrees.• E.g. PCl5

What are octahedral molecules?

• 6 electron pairs on the central atom.• No lone pairs.• Bond angles of 90 degrees.• E.g. SF6

What are allotropes?

• Different forms of the same element in the same state.

• There are 3 allotropes of carbon; diamond, graphite and fullerenes.

• Each allotrope has a different giant molecular structure.

What is diamond?• Allotrope of carbon.• Each carbon atom covalently bonded with sigma bonds to four

other carbon atoms.• Tetrahedral shape.• Sparkle because regular structure refracts light.• Strong covalent bonds:

– Very high melting point.– Sublimes (turns from solid to gas).– Hard; used in drill tips and saws.– Good thermal conductor as vibrations travel easily through the stiff

lattice.– Doesn’t conduct electricity because electrons are held in localised bonds.– Insoluble in all solvents.

What is graphite?

• Allotrope of carbon.• Sheets of flat hexagons of carbon atoms.• Each carbon atom bonded with sigma bonds to 3 other carbons.

– Very high melting point; sublimes.– Insoluble in all solvents.

• 4th outer electron of each carbon forms a delocalised system.– Conducts electricity because delocalised electrons are free to move

along the sheets.• Sheets joined by weak London forces.

– Sheets slide over each other; used as dry lubricant and in pencils.– Less dense than diamond; used in strong, lightweight sports

equipment.

What are fullerenes?• Allotropes of carbon.• Hollow balls and tubes.

– Used to cage other molecules; delivery of drugs to specific cells in the body.• Each carbon atoms forms 3 sigma bonds with neighbouring carbon atoms.• 4th outer electron of each carbon forms a delocalised system.• Buckimisterfullerene, C60, was the first fullerene discovered.

– Hollow ball.– Occurs naturally in soot.

• Many fullerenes are soluble in organic solvents to form brightly coloured solutions.

• Nanotubes are a single layer of graphite rolled into a tiny hollow cylinder.– Sigma bonds so strong; used to reinforce graphite in tennis rackets and to make strong,

lightweight building materials.– Delocalised electrons so conducts electricity; used as wires in circuits for computer chips.– Can be ‘capped’ to form a cage structure.

What is nanotechnology?

• Making materials and devices from nanoparticles.• Nanoparticles in sunscreen (zinc oxide and

titanium dioxide) reflect UV radiation but are so small they appear transparent.– People more likely to wear transparent sunscreen so

can reduce risk of skin cancer.– Some nanoparticles can pass through the skin and into

cells and so could damage cells.– Researchers believe nanoparticles in sunscreen are not

capable, but cannot be 100% sure.

What are the limitations of the models of bonding?

• Dot-and-cross diagrams of covalent bonds only show the electrons that are shared, not the lengths of bonds, or shape of the molecule.

• Most bonds are neither pure ionic or pure covalent but an intermediate due to bond polarisation.

Why is there a gradual transition form ionic to covalent bonding?

• Few compounds are purely ionic.• Only bonds between atoms of the same

element can be purely covalent.• Compounds between the extremes have a mix

of ionic and covalent properties.• Due to bond polarisation.

What is electronegativity?

• Ability to attract bonding electrons in a covalent bond.

• Measured using Pauling scale.• Fluorine most electronegative element with a

value of 4.0 on Pauling scale.• Higher number on the Pauling scale = more

electronegative.

How do differences in electronegativity cause bond polarisation?

• Bonding electrons pulled towards most electronegative atom.

• Bond is polar.• Causes a dipole; difference in charge between

the atoms.• Greater the difference in electronegativity, the

more polar the bond, giving more ionic character.

What are polar molecules?• Molecules have permanent dipoles.• Permanent dipole depends on the shape of the molecule and the polarity of

bonds.• In simple molecules (with one bond), if the bond is polar, the molecule is also

polar.• E.g. HCl• If all the polar bonds point in roughly the same direction, the molecule is polar.

– E.g. HCCl3

• In symmetrical molecules, if all the bonds are polar, the molecule is non-polar.– Polar bonds are arranged in opposite directions.– Dipoles cancel.– E.g. CO2

• Lone pairs are slightly negative, so may help cancel out dipoles, making the molecule non-polar.– E.g. NCl3

How does bond length affect strength of bonds?

• The two nuclei in a bond are both positively charged so repel each other.

• Electrons are negatively charged so repel each other.• Nuclei and shared electrons attract each other.• Bond length is the distance between the two nuclei where

the attractive and repulsive forces are balanced.• Stronger attraction means shorter bond length because

the force of attraction is stronger than the force of repulsion.

• C=C is shorter than C-C because there is a greater electron density between the two carbons in C=C than in C-C.

What are intermolecular forces?

• Forces between molecules.• Weaker than covalent, ionic and metallic

bonds.• 3 types; London forces, permanent dipole-

dipole interactions and hydrogen bonding.

What are London forces?• Instantaneous dipole-induced dipole effect.• Cause all atoms and molecules to be attracted to each other.• Electrons in charge clouds are always moving quickly.• At any particular moment, the electrons in an atom are likely to

be more to one side of the atom than the other; the atom has a temporary dipole.

• This dipole causes a temporary dipole on a neighbouring atom in the opposite direction.

• The two dipoles attract each other.• The second dipole causes a dipole on a third atom etc. etc.• Electrons moving so dipoles are created and destroyed all the

time; overall effect of atoms being attracted to each other.

What effects the strength of London forces?

• The following results in stronger London forces:– Larger molecules because larger electron cloud.– Larger surface area because there is a bigger

exposed electron cloud.• Stronger London forces results in higher

melting and boiling points because the intermolecular forces must be overcome to change the state.

What affects the melting and boiling points of alkanes?

• Chain length:– Smallest alkanes are gases at room temp and pressure.– Larger alkanes are liquids at room temp and pressure.– Longer chain results in more electrons and a larger

molecular surface area, so the London forces are stronger.• Branching:

– Straight chain alkanes have higher melting and boiling points than branched alkanes.

– Branching causes a smaller molecular surface area and molecules can’t pack as closely together, so the London forces are weaker.

What are permanent dipole-dipole forces?

• Dipoles on polar molecules cause weak electrostatic forces of attraction between molecules.

• Polar liquids contain permanent dipoles, which attract to an electrostatically charged rod, causing the liquid to bend towards the rod.

What is hydrogen bonding?

• Only occurs when hydrogen is covalently bonded to fluorine, nitrogen or oxygen.– Fluorine, nitrogen and oxygen are very electronegative so draw

electrons away form the hydrogen atom.– Hydrogen has a high charge density because it’s small.– Weak bond between hydrogen and lone pair on fluorine,

nitrogen or oxygen is formed.• Bond angle of hydrogen bond is 180 degrees.• Strongest intermolecular force:

– High melting and boiling points.– Ice is less dense than water because it forms more hydrogen

bonds than water, and the hydrogen bonds are relatively long.

How do intermolecular bonds affect solubility?

• For a substance to dissolve in another:– Bonds in the substance must break.– Bonds in the solvent must break.– New bonds form between the substance and the

solvent.• Strength of new bonds formed must be the

same or greater than the strength of the bonds broken.

What is the difference between polar and non-polar solvents?

• Polar solvents form permanent dipole-dipole forces.– E.g. water, which also forms hydrogen bonding.

• Non-polar solvents only form London forces.– E.g. alkanes such as hexane.

What type of solvents do ionic substances dissolve in?

• Polar solvents.• Ions attracted to the oppositely charged dipoles of

the polar solvent.• Ions are pulled away from the ionic lattice by the

polar solvent.– Hydration is when ions are pulled away from the ionic

lattice by water molecules.• Some substances won’t dissolve because the forces

between the ions are stronger than the new bonds that form.

What type of solvents do alcohols dissolve in?

• Alcohols have covalent bonds but can dissolve in water due to it’s polar O-H bond.

• Polar O-H bond of alcohols attracts the polar O-H bonds of water.

• Carbon chain is not soluble in water as it cannot form hydrogen bonds with water, so the longer the carbon chain, the less soluble the alcohol will be.

Why don’t halogenoalkanes dissolve in water?

• Dipoles of the polar bonds aren’t strong enough to form hydrogen bonds.

• Hydrogen bonding between water molecules is stronger than the bonds that would be formed between the halogenoalkane and water, so the halogenoalkane does not dissolve.

What type of solvent do non-polar substances dissolve in?

• Non-polar solvents.• Non-polar substances have London forces

between their molecules.• Non-polar substances form London forces with

non-polar solvents and so can dissolve in them.• Molecules of polar solvents are attracted to each

other more than they are attracted to molecules of non-polar substances, so non-polar substances do not dissolve easily in polar solvents.

What is a redox reaction?

• Transfer of electrons.• Oxidation is loss of electrons.• Reduction is gain of electrons.• Oxidation and reduction must always occur

simultaneously; redox reaction.• Oxidising agents accept electrons and so are reduced.• Reducing agents donate electrons and so are oxidised.• Ionic half-equations can be used to show oxidation or

reduction.– Combined to form the full redox equation.

What are the rules of oxidation numbers?

• Roman numerals in chemical names show the (positive) oxidation number.• All atoms treated as ions even if they form covalent bonds.• Uncombined elements have an oxidation number of 0.• Elements bonded to identical atoms have oxidation number of 0.• Monatomic ions have oxidation number the same as the charge on the ion.• In compound ions, the sum of oxidation numbers of the atoms equals the overall

charge of the ion.• In neutral compounds, the sum of oxidation numbers of the atoms is 0.• Combined oxygen is -2 except:

– In peroxides it is -1.– In OF2 it is +2.

– In O2F2 it is +1.

• Combined hydrogen is +1 except:– In NaH (and other metal hydrides) it is -1.

What do oxidation numbers show?

• Oxidation number of an atom increases by 1 for each electron lost; shows oxidation.

• Oxidation number of an atom decreases by 1 for each electron gained; shows reduction.

• Disproportionation is when a species is simultaneously oxidised and reduced:– Oxidation number of atoms increases for one

product and decreases for another.

Why do ionisation energies decrease down group 2?

• Each element down the group has an extra electron shell compared to the one above.

• Extra inner shells shield the outer electrons from attraction to the positive nucleus.

• Extra shells means outer electrons are further form the nucleus, reducing the force of attraction of electrons to the nucleus.

• Effect of extra shells is greater than the effect of increasing positive charge of the nucleus, so electrons become easier to remove causing lower ionisation energies.

How do group 2 elements react with water?

• Produce metal hydroxide and hydrogen.• Become more reactive down the group

because ionisation energies decrease.• M(s) + 2H2O(l) M(OH)2(aq) + H2(g)

How do group 2 elements react with oxygen?

• Produce solid white oxides.• Observe characteristic flame colours.• 2M(s) + O2(g) 2MO(s)

How do group 2 elements react with chlorine?

• Produce white solid chlorides.• M(s) + Cl2(g) MCl2(s)

Why are the oxides and hydroxides of group 2 elements basic?

• Oxides react with water to form metal hydroxides that are soluble.

• Hydroxide ions, OH-, make solutions strongly alkaline.• Magnesium oxide is an exception as it reacts slowly

with water and produces a hydroxide which is only slightly soluble.

• Oxides become more alkaline down group 2 because the hydroxides become more soluble.

• Oxides and hydroxides are bases so neutralise dilute acids, forming solutions of salts.

How do group 2 oxides and hydroxides react with water and acids?

Reaction with water

Oxide MO(s) + H2O(l) M(OH)2(aq)

Hydroxide M(OH)2(s) (+H2O(l)) M2+(aq) + 2OH-

(aq)

Reaction with acid

Oxide MO(s) + 2HCl(aq) MCl2(aq) + H2O(l)

Hydroxide M(OH)2(aq) + 2HCl(aq) MCl2(aq) + 2H2O(l)

What affects the solubility of group 2 compounds?

• Depend on the compound anion.• Group 2 compounds with singly charged anions

increase in solubility down the group.– E.g. Hydroxides (OH-)– Magnesium hydroxide is sparingly soluble in water.

• Group 2 compounds with doubly charged anions decrease in solubility down the group.– E.g. Sulfates (SO4

2-)– Barium sulfate is insoluble in water.

What are the trends of thermal stability of carbonates and nitrates of group 1 and group 2

elements?• Thermal decomposition is when a substance breaks down when it is

heated.• Thermal stability is a substances resistance to decomposition on heating.• Thermal stability increases down group the group:

– Carbonate and nitrate ions are large and made unstable by the presence of a cation.

– Cation polarises the anions (carbonate and nitrate ions).– Polarising power of group 1 and group 2 cations decreases down the group as

the ions become larger, so thermal stability increases.• Group 2 compounds are less thermally stable than group 1 compounds:

– Greater charge on the cation causes greater polarisation of the anion.– Group 2 have a 2+ charge compared to 1+ charge of group 1 cations, so group 2

compounds are less thermally stable.

How do carbonates and nitrates of group 1 and 2 elements decompose on heating?

Group 1 Group 2

Carbonates are thermally stable.Heating with a bunsen burner will not cause thermal decomposition.Except Li2CO3, which decomposes to Li2O and CO2.

Carbonates decompose to form the oxide and carbon dioxide.MCO3(s) MO(s) + CO2(g)

Nitrates decompose to form the nitrite and oxygen.2MNO3(s) 2MNO2(s) + O2(g)

Except LiNO3, which decomposes to form Li2O, NO2 and O2.

Nitrates decompose to form the oxide, nitrogen dioxide and oxygen.2M(NO3)2(s) 2MO(s) + 4NO2(g) + O2(g)

How can the thermal stability of nitrates and carbonates be tested?

• Decomposition of nitrates can be tested by measuring:– How long it takes until oxygen is produced (to relight a

glowing splint).– How long it takes for brown gas of NO2 to be

produced (done in fume cupboard as NO2 is toxic).• Decomposition of carbonates can be tested by

measuring:– How long it takes for carbon dioxide to be produced

(turns limewater cloudy).

What are the flame colours of group 1 and 2 compounds?

• Flame test is carried out by:– Mixing a small amount of compound with HCl.– Heating platinum or nichrome wire in a hot bunsen flame to clean it.– Dipping wire into compound/acid mixture and holding it in a hot bunsen

flame.• Different colours are observed:

– Li = red– Na = orange/yellow– K = lilac– Rb = red– Cs = blue– Ca = brick-red– Sr = crimson– Ba = green

Why do flame colours occur?

• Energy absorbed from the flame causes electrons to move to higher energy levels; electron transition.

• Energy is released in the form of light.• Difference in energy between higher and lower

energy levels determines the wavelength of light emitted.

• Different wavelengths of light appear different colours.

What are the halogens?

• Non-metals of group 7.• Exist as covalent diatomic molecules.• Electronegativity decreases down the group.

Halogen Formula, X2 Colour State Electronic structure

Fluorine F2 Pale yellow Gas 1s2 2s2 2p5

Chlorine Cl2 Green Gas 1s2 2s2 2p6 3s2 3p5

Bromine Br2 Red-brown Liquid 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5

Iodine I2 Grey Solid 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p5

What are the trends in solubility of the halogens?

• Low solubility in water because they are covalently bonded.

• Dissolve in organic, non-polar solvents.

Halogen Colour in water Colour in hexane

Chlorine Virtually colourless Virtually colourless

Bromine Yellow/orange Orange/red

Iodine Brown Pink/violet

What are the trends of reactivity of the halogens?

• React by gaining an electron in the outer p sub-shell.• Act as oxidising agents; halogens are reduced as they

oxidise other substances.• Less reactive as you go down the group because the

atoms become larger so the outer electrons are further from the nucleus and there is a greater effect of shielding.

• As the halogens become less reactive they can be described as less oxidising.

• Melting and boiling points increase down the group.

Why do halogens undergo disproportionation reactions?

• With hot or cold alkali solutions.• Halogens can undergo disproportionation

because they can exist in more than 2 different oxidation states.

• With cold alkali:– X2 + 2NaOH NaXO + NaX + H2O

• With hot alkali:– 3X2 + 6NaOH NaXO3 + 5NaX + 3H2O

How do halogens react with metals?

• Oxidise metals; redox reaction.• Fluorine and chlorine react with hot iron to form iron

(III) halides.– Iron taken to its highest oxidation state, +3, because

chlorine and fluorine are strong oxidising agents.– 2Fe(s) + 3Cl2(g) 2FeCl3(s)

• Bromine is a weaker oxidising agent so a mixture of iron (II) and iron (III) bromide is formed.

• Iodine is an even weaker oxidising agent so only iron (II) iodide forms.

How do halogens react with non-metals?

• Halogens act as an oxidising agent in a redox reaction.

• The non-metal is oxidised.• The halogen is reduced.• E.g. S8(s) + 4Cl2(g) 4S2Cl2(l)

How do halogens react with ions?

• All halogens except iodine oxidise iron (II) ions to iron (III) ions in solution.

• Solution changes colour from green to orange.• X2 + 2e- 2X-

• 2Fe2+ 2Fe3+ + 2e-

What are the trends of reducing power of halides?

• Halides act as reducing agents by losing an electron from it’s outer shell.

• Reducing power increases down the group as the attraction between the nucleus and the electrons gets weaker due to increase in size of the ion, so outer electrons are further from the nucleus, and the effect of shielding due to the inner electron shells.

How do the halides react with sulfuric acid?

• All halides react with conc sulfuric acid to produce a halogen halide.• KF or KCl with H2SO4:

– KX(s) + H2SO4(l) KHSO4(s) + HX(g)

– See misty fumes of HX gas as it comes into contact with the air.– HF and HCl aren’t strong enough to reduce sulfuric acid.– NOT A REDOX REACTION; the oxidation states remain the same.

• KBr with H2SO4:– KBr(s) + H2SO4(l) KHSO4(s) + HBr(g)

– See misty fumes of HBr gas.– HBr reduces H2SO4 in a redox reaction.

– 2HBr(aq) + H2SO4(l) Br2(g) + SO2(g) + 2H2O(l)

– See orange fumes of Br2 gas.

• KI with H2SO4:– KI(s) + H2SO4(l) KHSO4(s) + HI(g)

– HI reduces H2SO4 in a redox reaction.

– 2HI(g) + H2SO4(l) I2(s) + SO2(g) + 2H2O(l)

– HI reduces SO2 to H2S in a redox reaction.

– 6HI(g) + SO2(g) H2S(g) + 3I2(s) + 2H2O(l)

– H2S gas is toxic and smells of rotten eggs.

What are hydrogen halides?

• Colourless gases.• Very soluble.• Dissolve in water to give strong acids:– HX(g) H+

(aq) + Cl-(aq)

– Turns blue litmus paper red.– Hydrochloric, hydrobromic and hydroiodic acids.

• React with ammonia gas to give white fumes of ammonium hydride:– Acid-base reaction.– NH3(g) + HX(g) NH4X(s)

How do halide ions react in displacement reactions?

• Displaced from solution by more reactive halogens.• Shake mixture with an organic solvent to see reaction more clearly:

– Halogen dissolves in organic solvent.– Halide ions dissolve in water.– The organic solvent and water settle as two layers in a test tube.

• Chlorine displaces bromide and iodide.– Cl2(aq) + 2Br-

(aq) 2Cl-(aq) + Br2(aq)

• Orange solution of Br2

– Cl2(aq) + 2I-(aq) 2Cl-

(aq) + I2(aq)

• Brown solution of I2

• Bromine displaces only iodide.– Br2(aq) + 2I-

(aq) 2Br-(aq) + I2(aq)

• Brown solution of I2

• Iodine cannot displace halide ions.

How do halide ions react with silver nitrate solution?

• Add dilute nitric acid to remove interfering ions.• Add silver nitrate solution, AgNO3(aq).• Precipitate of the silver halide is formed.

– Ag+(aq) + X-

(aq) AgX(s)

– F- forms no precipitate.– Cl- forms a white precipitate.– Br- forms a cream precipitate.– I- forms a yellow precipitate.

• Add ammonia solution.– White precipitate of AgCl dissolves in dilute ammonia solution.– Cream precipitate of AgBr dissolves in conc ammonia solution.– Yellow precipitate of AgI does not dissolve in conc ammonia solution.

How do silver halides react with sunlight?

• Decompose when light shines on them to produce silver and the halogen.

• Used in film photography:– Film contains AgBr particles that turn opaque

silver when exposed to light.– 2AgBr 2Ag + Br2

How are acid-base titrations done accurately?

• Find out how much acid is needed to neutralise a quantity of alkali.• Alkali measured out with a pipette and put in a conical flask with

an indicator.• Burette filled with the acid.• Rough titration done to find roughly where the end point is by

adding acid from the burette into the conical flask and swirling until the indicator changes colour.

• Accurate titration done by adding acid into the conical flask until 2cm3 before the end point of the rough titration, then adding the acid dropwise until the end point is reached.

• Repeat until you get results close to each other, then find the average titre of these results.

What are indicators?

• Change colour quickly over a small pH range, so lets you know when neutralisation has occurred.

• Methyl-orange turns yellow to red when acid is added to alkali.

• Phenolthalein turns red to colourless when adding acid to alkali.

How are concentrations calculate from titrations?

• Write a balanced equation.• Calculate the number of mols of solution of known

concentration:– Mols = vol(dm3) x conc

• Use stoichiometry of the balanced equation to find the number of mols of the solution of unknown concentration.

• Find concentration of the solution:– Conc = mols/vol(dm3)– Units are mol dm-3

How do you calculate percentage error?

• Percentage error = (uncertainty/reading) x 100• Two readings are taken from the burette so the total

uncertainty is 2x uncertainty of the burette. • Uncertainty of burette = 2x 0.05cm3 = 0.1cm3

• Uncertainty of scales = 0.005g• Total percentage error = sum of percentage errors• Total error = total percentage error x conc found

What types of errors are there when doing titrations?

• Systematic errors:– Occur even if you repeat the experiment.– Caused by set-up or equipment.

• Random errors:– Reading from equipment.– Reduced by repeating the experiment, as they will

usually cancel each other out.

What is an iodine-sodium thiosulfate titration?

• Way of finding the conc of an oxidising agent.– E.g. potassium iodate (V)

• A specific volume of potassium iodate (V) added to excess acidic potassium iodide solution.

• Iodate (V) ions oxidise some iodide ions to iodine:– IO3

-(aq) + 5I-

(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)

• Solution titrated with sodium thiosulfate solution of known concentration.• Iodine reacts with thiosulfate ions:

– I2 + 2S2O32- 2I- + S4O6

2-

• When the solution in the conical flask turns pale yellow, starch solution is added.• Starch solution turns blue-black to colourless as the iodine reacts.• If starch is added too soon, the iodine sticks to the starch and won’t react as

expected with the thiosulfate.• Solutions react slowly with air so should be made up for the titration.

How can the accuracy of titrations be improved?

• Cleaning the burette with the solution that will go in it during the titration before the titration is completed to ensure it is not contaminated.

• Burette must be read from the bottom of the meniscus, with your eyes level with the liquid.

• Repeat experiment to reduce random errors.• Wash conical flask out with solution that will go

in it between experiments, or use a clean conical flask.

What is the collision theory?• Particles in liquids and gases are always moving and colliding with

each other.• A reaction will only take place with collision of two particles if:

– They collide in the right direction.– They collide with at least the minimum amount of kinetic energy to react.

• Activation energy is the minimum kinetic energy needed for two particles to react on collision.– Energy is required to break the bonds.– Reactions with low activation energies are more likely to occur than

those with higher activation energies because it is easier to break the bonds.

– Particles can be given more kinetic energy by heating the reactant mixture.

What is the Maxwell-Boltzmann distribution?

• Particles in liquids and gases don’t all have the same amount of kinetic energy.

• The Maxwell-Boltzmann distribution shows how the kinetic energy of particles is distributed.

• Graph with kinetic energy on the x-axis and number of molecules in the y-axis.

• Goes through the origin because no molecules have no kinetic energy.

• Peaks at moderate kinetic energy, then asymptotes to the x-axis.

What affect does temperature have on rate of reaction?

• Increasing temperature gives molecules more kinetic energy so they move faster.

• Greater proportion of molecules have the activation energy required to break the bonds to start a reaction.

• This causes the Maxwell-Boltzmann curve to shift right.

• Also particles move faster so collide more.• This means that increasing the temperature

increases the rate of reaction.

How can rate of reaction be increased?

• Increasing concentration or pressure of reactants means particles are closer together, so they collide more often, increasing the rate of reaction.

• Increasing surface area of reactants increases rate of reaction by making collisions between reactants more likely.

• Adding a catalyst can speed up rate of reaction by providing an alternate route of reaction with a lower activation energy.

What are catalysts?

• Used to increase the rate of reaction by providing an alternate route of reaction with a lower activation energy.

• The catalyst is chemically unchanged at the end of the reaction.

• Homogeneous catalysts are in the same phase as the reactants, and work by forming intermediates.– The activation energies needed to form intermediates from the

reactants, and products from the intermediates are both lower than the activation energy when there is no catalyst.

– Enthalpy profile diagram will show two activation energies.• Heterogeneous catalysts are in a different phase to the

reactants.

How is rate of reaction measured?

• If a gas is produced, a gas syringe can be used to measure the rate of gas production and hence the rate of reaction.

• If a precipitate is produced then rate of reaction can be measured by how long it takes for enough precipitate to be produced to cloud the solution enough so that a marker can no longer be seen.– E.g. Reaction between sodium thiosulfate and HCl produces

a yellow precipitate of sulfur.– A problem with this method is it is subjective, as people may

not exactly agree when the marker can no longer be seen.

What are reversible reactions?

• Reactions where that can go both ways.• As reactants are used, the rate of the forward

reaction decreases.• As products are formed, the rate of the backward

reaction increases.• Can reach a dynamic equilibrium in a closed system.– Forward reaction and the backward reaction occur at

the same rate.– Concentrations of the reactants and products stays

constant.

What factors affect the position of equilibrium of a reversible reaction?

• If the position of equilibrium shifts left, you get more reactants being made.• If the position of equilibrium shifts right, you get more products being made.• Le Chatelier’s principle says the equilibrium shifts to counter any changes

made.• Catalysts have no effect on the position of equilibrium:

– Can’t be used to increase yield.– Allow equilibrium to be reached faster.

• Increasing the concentration of reactants shifts the equilibrium right.• Increasing the concentration of the products shifts the equilibrium to the left.• Increasing pressure of a system of gases causes the equilibrium to shift to the

side with fewer gas molecules.• Increasing temperature shifts the equilibrium in the endothermic direction.

What are compromise conditions?• Businesses want to make money by make as many useful products as

cheaply as possible.• Conditions are controlled to be cheap to maintain, obtain a good

yield and allow the reaction to occur at a suitable rate.• There often has to be a compromise between high yield, fast rate and

low cost.– Methanol is made from hydrogen and carbon monoxide.– The reaction is exothermic so lower temperatures give a better yield, but

also reduce the rate of reaction so a compromise of 250 degrees is used.– There are fewer molecules of gas on the right hand side, so a high pressure

favours a high yield and also increases the rate of reaction, however high pressures are expensive to produce so a compromise of 50-100 atm is used.

– A catalyst is used to make the reaction reach equilibrium more quickly.

What are alcohols?

• Functional group O-H.• General formula CnH2n+1OH• Can be primary, secondary or tertiary:– Primary alcohols have 1 carbon attached to the

carbon of the O-H group.– Secondary alcohols have 2 carbons attached to the

carbon of the O-H group.– Tertiary alcohols have 3 carbons attached to the

carbon of the O-H group.

How are alcohols used to make halogenoalkanes?

• -OH group of alcohol swaps with –X of halogen to form a halogenoalkane.• Tertiary alcohols are more reactive than primary and secondary alcohols.

– Chloroalkanes can be made by shaking a tertiary alcohol with HCl, but the product is impure.

– Chloroalkanes can be made using phosphorus (V) chloride:• ROH(l) + PCl5(l) RCl(l) + HCl(g) + POCl3(l)

– Bromoalkanes and iodoalkanes can be made using tertiary alcohols by making HBr and HI in situ:• Phosphoric (V) acid and a metal halide are mixed to produce HBr and HI.• Sulfuric acid is not used because HBr and HI are oxidised by sulfuric acid to iodine and

bromine, reducing the yield of the halogenoalkane.

• Halogenoalkanes can be made using any alcohol by reacting the alcohol with the phosphorus (III) halide.– 3ROH + PX3 3RX + H3PO3

– PBr3 and PI3 are made in situ by refluxing the alcohol with ‘red phosphorus’ and the halogen.

What is the test for the hydroxyl group?

• The hydroxyl group is the –OH group.• Add phosphorus (V) chloride to the unknown

liquid.• If an –OH group is present you get steamy

fumes of HCl.– Turns blue litmus paper red because it is a strong

acid.– Dissolves in water to give chloride ions.• Test for chloride ions using the silver nitrate test.

How do alcohols react with sodium?

• Produce alkoxides.– When ethanol reacts with sodium metal, the O-H

bonds break to produce ionic sodium ethoxide and hydrogen.

– 2CH3CH2OH + 2Na 2CH3CH2O-Na+ + H2

• The longer the hydrocarbon chain, the less reactive the alcohol is with sodium.

What properties does hydrogen bonding give alcohols?

• Strongest intermolecular force so alcohols have high boiling points compared to non-polar compounds of similar sizes.– Low volatility; low tendency to evaporate into a gas.

• Miscible with water if the alcohol is small because hydrogen bonds can form between the –OH group of alcohols and the –OH group of water.– Larger alcohols are not miscible with water because the majority

of the molecule is non-polar hydrocarbon chain, so the energy required for breaking the hydrogen bonds of water is not compensated for by formation of new bonds.

• Small alcohols also miscible in non-polar solvents.

What happens when alcohols are burnt?

• Oxidised to carbon dioxide and water.• C-C and C-H bonds are broken.• Combustion reaction.• E.g. Ethanol burns with a pale blue flame.– C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g)

What determines the extent to which alcohols are oxidised?

• Oxidising agent of potassium dichromate (VI) used.• Orange dichromate (VI) ions reduced to green chromium (III)

ions.• Primary alcohols are oxidised to aldehydes, then to carboxylic

acids.– Carbonyl with one hydrogen and one alkyl group on the carbon

attached to the carbon of the carbonyl group.• Secondary alcohols are oxidised to ketones.

– Carbonyl with two alkyl groups attached to the carbon of the carbonyl group.

• Tertiary alcohols are not oxidised by potassium dichromate (VI).

How are aldehydes made from primary alcohols?

• Primary alcohol + [O] aldehyde + water• Potassium dichromate (VI) solution and

sulfuric acid gently heated in a test tube with the primary alcohol.

• The aldehyde must be distilled off as it is produced to prevent the aldehyde being oxidised to a carboxylic acid.

How are carboxylic acids made from primary alcohols?

• Primary alcohol + [O] aldehyde + water• Aldehyde + [O] (reflux) carboxylic acid• Alcohol vigorously oxidised.• Primary alcohol mixed with the oxidising agent

and heated under reflux to prevent volatile compounds being lost from the mixture.

How are ketones made from secondary alcohols?

• Alcohol + [O] (reflux) ketone + water• Acidified potassium dichromate (VI) used as

oxidising agent.• Ketones are not easily oxidised so no further

reactions occur.

What test can be used to determine if a ketone or aldehyde is produced?

• Fehling’s solution/Benedict’s solution test:– Deep blue Cu2+ complexes.– Reduced to brick-red Cu2O by aldehydes.– Stay blue when mixed with ketones as no reaction occurs.

• Tollen’s reagent, [Ag(NH3)2]+:– Reduced to silver when warmed with an aldehyde.– Silver coats the inside of the apparatus forming a silver

mirror.– This does not happen when warmed with a ketone

because no reaction occurs.

What are halogenoalkanes?

• Alkanes with at least one halogen in place of a hydrogen atom.

• Can be primary, secondary or tertiary.– Primary halogenoalkanes have 1 alkyl group.– Secondary halogenoalkanes have 2 alkyl groups.– Tertiary halogenoalkanes have 3 alkyl groups.

How do halogenoalkanes react with water?

• Produces an alcohol.• RX + H2O ROH + H+ + X-

• X- ions can be identified using the silver nitrate test.

• Tertiary halogenoalkanes are the most reactive and primary halogenoalkanes are the least reactive with water.

What are halogenoalkanes used for?

• Can be used to make polymers.– E.g. poly(chlorethene) and poly(tetrafluoroethene)

• Used as refrigerants because they’re easily compressed and don’t corrode pipework.

• Non-flammable halogenoalkanes used as fire retardants/flame retardants.

What are CFC’s?• Chlorofluorocarbons.• Halogenoalkane where all the hydrogen atoms have been replaced

with halogen atoms.• Used to be used as refrigerants, solvents etc because they’re non-

toxic and unreactive.• In the 1970’s scientists discovered CFC’s were damaging the ozone

layer, causing environmental problems.• Different halogenoalkanes are now used instead of CFC’s:

– A mixture of difluoromethane and pentafluoroethane used as a refrigerant.• Non-flammable, low toxicity.• Expensive, greenhouse gas.

– Hydrochlorofluorocarbons used in place of CFC’s as they are less stable than CFC’s, so decompose lower in the atmosphere.

How do halogenoalkanes react by nucleophilic substitution?

• Halogens are more electronegative than carbon so the C-X bond is polar.• Carbon is slightly positive so it is vulnerable to attack by nucleophiles (electron

pair donors).• The C-X bonds breaks heterolytically, with the halogen taking both electrons.• The nucleophile substitutes for the halogen.• The nucleophile can be:• OH- ions

– Alcohol heated under reflux with warm aqueous sodium or potassium hydroxide.– RX + OH- (reflux) ROH + X-

• H2O– Weak nucleophile.– RX + H2O ROH + H+ + X-

• NH3– Warm with excess ethanolic ammonia (ammonia dissolved in ethanol).– RX + 2NH3 (reflux, ethanol) RNH2 + NH4X

How do halogenoalkanes react by elimination?

• Add halogenoalkane to warm alkali dissolved in alcohol and heat under reflux.

• Produces an alkene.• RX + KOH (reflux, ethanol) alkene + water + KBr• OH- acts as a base and takes a proton from the carbon

not attached to the halogen.• This means the carbon now has an extra electron, so it

forms a double bond with the other carbon atom.• To form the double bond, the halogen must be lost

from the second carbon.

What types of reaction are there?• Addition = joining 2+ molecules to form a larger molecule.• Polymerisation = joining together lots of simple molecules to form

a giant molecule.• Elimination = when a small group of atoms breaks away from a

larger molecule.• Substitution = when one species is replaced by another.• Hydrolysis = splitting a molecule into two new molecules by

adding H+ and OH- derived from water.• Oxidation = any reaction where an atom loses electrons.• Reduction = any reaction where an atom gains electrons.• Redox = any reaction where electrons are transferred between

two species.

How are reagents classified?• Nucleophiles:

– Electron pair donors because they are electron rich.– React with positive ions.– Molecules with polar bonds are attacked by nucleophiles because they

have slightly positive areas.• Electrophiles:

– Electron pair acceptors because they are electron deficient.– React with negative ions.– React with the electron-rich area around a carbon double bond.

• Free radicals:– Have an unpaired electron so are very reactive.– React with anything, even stable, non-polar C-C and C-H bonds of

alkanes.

What types of bond fission are there?

• Bond fission is breaking a covalent bond.• Heterolytic fission:– Bond breaks unevenly.– Both electrons move to one atom.– Two different species are formed; an electrophile and a

nucleophile.• Homolytic fission:– Bond breaks evenly.– One electron moves to each atom.– Forms two free radicals.

What is the ozone layer?• In the layer of the atmosphere called the stratosphere.• Contains most of the atmospheres ozone molecules, O3.• Removes dangerous UV radiation that can damage DNA in cells and cause

skin cancer.• Formed when UV radiation from the sun hits oxygen molecules.

– UV absorbed by oxygen molecules.– Oxygen molecules split into separate free radicals by homolytic fission.– O2 + UV O. + O.

– Free radicals combine with oxygen molecules to form ozone.– O2 + O. O3

• Constantly destroyed and replaced:– O3 + UV O2 + O.

– O2 + O. O3

– An equilibrium is set up so the concentration of ozone remains constant.

How was the thinning of the ozone layer discovered?

• In the 1970’s the British Antarctic Survey team found the concentration of ozone over Antarctica was very low compared to previous measurements.

• A satellite mapping ozone levels also showed ozone depletion when low values that were ignored as errors were re-examined.

• Holes in the ozone layer allow harmful UV light to reach the Earth so are dangerous.

How did the ozone layer become thinner?

• Ozone was being destroyed by chlorine free radicals formed when CFC’s were broken down by UV light.– Chlorine free radicals act as catalysts for the reaction of 2O3(g) 3O2(g).

– CCl3F(g) (+UV) CCl2F.(g) + Cl.

(g)

– Cl.(g) + O3(g) O2(g) + ClO.

(g)

– ClO.(g) + O3(g) 2O2(g) + Cl.

(g)

• Montreal Protocol of 1989 was an international treaty to phase out the use of CFC’s by 2000, except in exceptional cases such as medical inhalers and fire extinguishers in submarines.

• Nitrogen oxides produced by car and aircraft engines also contribute to the depletion of the ozone layer by forming nitric oxide free radicals, NO., which react in the same way as chlorine free radicals.

How can chemical industries become more sustainable?

• Sustainable chemical industry is not using up all the Earth’s natural resources or producing too many chemicals that damage the environment.

• Use renewable raw materials.• Use renewable energy sources.

– Use microwave radiation to heat the vessel directly.– Use plant-based fuels.– Use solar power, wind power etc.

• Ensure all chemicals involved are as non-toxic as possible.– Lead used to be used in paint, petrol etc but is harmful to human health so

alternatives are now used.– Foams in fire extinguishers used to contain CFC’s which deplete the ozone layer but

alternatives are now used.– Dry cleaners used to use a solvent based on chlorinated hydrocarbon, but these are

carcinogenic so alternatives are now used.• Make sure products and waste are biodegradable or recyclable.

Why are catalysts and high atom economy important in the chemical industry?

• High atom economy reduces waste and makes better use of resources.• Catalysts reduce the activation energy of a reaction, so allows lower temperatures and

pressures to be used.• The efficiency of manufacturing ethanoic acid has increased over the years:

– First made by oxidation of butane or naptha (crude oil fractions).• Temperature of 150-200 degrees.• Pressure of 40-50 atm.• Cobalt catalyst.• Low atom economy.

– BASF developed a process of making ethanoic acid from methanol and carbon monoxide in 1963.• 100% atom economy.• High yield.• Temperature of 300 degrees.• Pressure of 700 atm.• Cobalt iodide catalyst.

– In 1970, Monsanto developed a rhodium iodide catalyst to be used in the BASF process.• Temperature of 150-200 degrees.• Pressure of 30-60 atm.• Very high yield.• Process still used today.

What is the greenhouse effect?• The Sun emits electromagnetic radiation; UV radiation, IR radiation and

visible light.• When radiation from the Sun reaches the Earth’s atmosphere, most UV

radiation and IR radiation is absorbed by atmospheric gases.• Some radiation is reflected back into space by clouds.• Some radiation is reflected back into space from the Earth’s surface by shiny

surfaces of ice and snow.• The remaining radiation that reaches the Earth’s surface is absorbed,

causing the Earth to heat up.• The Earth radiates energy back into space as IR radiation.• Gases in the troposphere (lowest layer of the atmosphere) absorb some of

the IR radiation and re-emit it in all directions, including back towards the Earth.

• Keeps the Earth warm enough to maintain life.

What are greenhouse gases?

• Molecules that absorb IR radiation to make bonds in the molecule vibrate more.

• Extra vibrational energy is passed onto other molecules in the air via collisions.

• Other molecules gain kinetic energy, so the overall temperature increases.

• The contribution of a greenhouse gas to the greenhouse effect depends on:– How much radiation one molecule of that gas absorbs.– How much of that gas there is in the atmosphere.

• Greenhouse gases include carbon dioxide, water vapour and methane.

How does an enhancement of the greenhouse effect cause global warming?• Concentrations of carbon dioxide and methane in the

atmosphere have been increasing.• Industrialisation has led to burning of fossil fuels and

deforestation, increasing levels of carbon dioxide in the atmosphere.

• Increased demand for food has caused increasing levels of methane in the atmosphere as it is produced by cows, paddy fields etc.

• Increase in greenhouse gases causes more IR radiation emitted from the Earth to be re-emitted back to the Earth by the gas molecules, so the Earth gains too much heat.

What are natural changes in the Earth’s climate?

• Changes in the Earth’s orbit around the Sun causes ice ages followed by interglacials (warm periods).

• Changes in the Sun’s activity such as sun spots can cause climate change.

• Volcanic eruptions and meteor impacts cause smoke and dust to enter the air, causing global cooling.

What is anthropogenic climate change?

• Increase rate of global warming due to human activities.• Unpolluted air samples from remote islands have shown

increases in average temperatures and carbon dioxide levels.

• Sea water is becoming more acidic as more carbon dioxide dissolves in the water, forming carbonic acid.

• Mass spectrometry of samples of air trapped in ice in polar regions have shown recent changes in the atmosphere are more dramatic than changes in the past.

• Correlation between average temperatures and carbon dioxide levels.

What are carbon footprints and carbon neutral?

• Carbon footprints are the amount of greenhouse gases something causes to be released.• Carbon neutral activities have no overall carbon emissions.• Trees remove carbon dioxide by photosynthesis so planting trees can be used to reduce

a carbon footprint.• Burning petrol has a large carbon footprint.• Bioethanol has a small carbon footprint because the carbon dioxide produced when the

fuel is burnt is removed form the atmosphere as the maize crops grow.– Still has carbon emissions due to making fertilizers and powering agricultural machinery.– Land that could be used for growing crops for food would have to be used to grow crops for

fuels.• Hydrogen gas has a small carbon footprint.

– Fuel cells convert hydrogen and oxygen to water, producing electricity.– Energy is needed to extract hydrogen from water.– Hydrogen is extremely flammable so people are afraid of using it as a fuel.– Hydrogen cars cost more than petrol cars.– Difficult to store large quantities of hydrogen gas at refuelling stations safely.

What is mass spectrometry?• Used to find relative isotopic masses,

abundance of isotopes and relative molecular mass (Mr) of compounds.

• Relative molecular mass found by the mass/charge ratio of the molecular ions.

• The molecular ion is the peak second from the right on the spectrum.

• Molecular ion is broken into smaller fragments, causing a fragmentation pattern on the spectrum.

• This can be used to work out the compounds structure and differentiate between similar molecules.

Fragment MrCH3 15C2H5 29C3H7 43OH 17

What effect does IR radiation have on bonds?

• Some molecules absorb IR radiation, making their covalent bonds vibrate more.

• Only molecules with different atoms can absorb IR radiation because their polarities change as the bonds vibrate.

• Gases that absorb IR radiation are called greenhouse gases.

• Gas molecule bonds have fixed energy levels called quantised levels so different molecules absorb different frequencies of IR radiation.

What is IR spectroscopy?• A beam of IR radiation is passed through the sample.• IR radiation is absorbed by covalent bonds in the molecule, increasing their

vibrational energy.• Bonds between different atoms absorb different frequencies of IR radiation.• Bonds in different places in a molecule absorb different frequencies of IR radiation.• IR spectroscopy can be used to identify the functional group of a molecule.

Functional group Where found? Frequency Type of absorption

C-H Organic molecules 2800-3100 Strong, sharp

O-H Alcohols 3200-3550 Strong, broad

O-H Carboxylic acids 2500-3300 Medium, broad

N-H Amines 3200-3500 Strong, sharp

C=O Aldehydes, ketones, carboxylic acids 1680-1750 Strong, sharp

C-X Halogenoalkanes 500-1000 Strong, sharp