unit 4 a2 chemistry edexcel
DESCRIPTION
Unit 4 A2 Chemistry EdexcelTRANSCRIPT
+
ChemistryUnit 4
+ Reaction RatesIt’s the change in the amount of reactants or products per unit time
1st Order:• Rate = k[X]1
• Constant Half Lives2nd Order:• Rate = k[Y]2
• Increasing Half Lives0 Order:• Rate = k[Z]0
Continuous Rate Method
Initial Rate Method
1o Halogenoalkane + Hydroxide = SN2
3o Halogenoalkane + Hydroxide = SN1
If it’s in the Rate Equation then it’s the Rate Determining Step
+ Nucleophilic SubstitutionSN2
SN1
Iodine + Propanone
Slow Fast
Fast
Fast Fast
Slow
+ Arrhenius Equation
ln(k) = X + Constant-Ea 1 R T
R = Gas Constant (8.314)T = Temperature (Kelvin)
1/temp (k)
ln( 1/time)x
y
Gradient = XY
+ Heterogeneous Catalysts• In a different state to the reactants
• Large surface area as they’re usually powder or a mesh
• Easily separated from products & excess reactants
• Can be poisoned:• Adsorbs too strongly to surface of catalyst and doesn’t
allow other reactants to adsorb to the catalyst
• e.g. – Nickel in Hydrogenation of Vegetable Oil – Platinum in catalytic converters in cars
How do they work?
- Reactant adsorbed onto surface of catalysts at the active site- Interaction between reactant & catalyst - Reaction occurs from the interaction- Products are desorbed – breaks off catalyst
Adsorb – forms a temporary bond when something sticks to a surface
Homogeneous Catalyst is when catalyst is in the same state to reactants
+ EntropyEntropy change of a reaction is measure of order or disorder
The order within is a substance is how the quanta of energy are arranged
Reaction will occur if overall entropy is increasing, from order to disorder
If entropy is +ve then reaction will tend to occur
Ordered
Disordered
But doesn’t exist
More disorder = more +ve SΘ
Solid OrderedLiquid Disordered Gas Very Disordered
More Complex/Moles More Disordered
+
+ Stable or Inert
Stable if there is no tendency for the reaction to ‘go’
If system is +ve but surrounding are –ve we can ‘force’ the reaction by increasing the temp
C (Diamond) C (Graphite)
+ve entropy change
But Ea is too high – Kinetically Inert
= - ve
+ Solubility
Endo
Endo
Exo
Standard enthalpy change of Solution:• Enthalpy change when one mole of a compound
dissolves to form solution containing 1moldm-3 under standard conditions.
Standard enthalpy change of Hydration:• Enthalpy change when one mole of gaseous ions
is hydrated under standard conditions to form a solution in which the concentration of ions is 1moldm-3.
Factors affecting solubility:• Ionic Charge > Ionic Radii
Lattice Energy & Hydration Energy increase as Charge increases
Group 2 Compounds less soluble as solution energy is more endoGroup 1 Compounds more soluble as solution energy is less endo
+ Equilibria (Kc)
Dynamic Equilibria:- The forwards reaction and back reaction are at the same
rate so there’s no overall change in yield of products or reactants in a closed system
+ Kp
+ Effects on EquilibriaAdding a catalyst:• Equilibrium constants not affected• Position of equilibria not affected• Speeds up forward & backward reaction at same rate
Change in concentration:• Equilibrium constants not affected• Adding reactant shifts equilibria right• Adding product shifts equilibria left
Change in pressure:• Equilibria shifts to side with fewest molecules• Equilibrium constants not affected
Increase in temperature:• Endothermic = +ve shifts right, more product produced• Exothermic = -ve shifts left, more reactant produced• Kc & Kp:
• Increases if endothermic• Decreases if exothermic
• Total entropy change = RlnK (R = Gas Constant)
KTotal
EntropyProgression of Reaction
>10-10 >-191 Doesn’t Go
>10-5 >-96 Reversible pushed to Left
1 0 Equilibrium
<105 <+96 Reversible pushed to Right
<1010 <+191 Goes to Completion
+ Uses in Industry
They alter conditions to produce maximum yield
Requiring least amount of energy
Often looking for new more environmentally friendly catalysts
e.g. Ethene + H2O Ethanol Sped up by using catalyst (Silica soaked in H3PO4)
Remove product as it’s being formed
+
Arrhenius:
- Acids are H+ producers- Bases are OH- producers in H2O- Only used in aqueous solutions
Bronsted-Lowry:
- Acids are proton donors- Bases are proton acceptors
Acid Theories
A base has a lone pair of electrons which can form a dative covalent bond with a H+
Lewis:
- Acid is an electron pair acceptor- Bases is an electron pair donor
HA + H2O H3O+ + A-
Conjugate Pairs
H2O & HA are Acids & Bases as they give & accept H+
H+ = H3O+
HA is the acid & A- is it’s conjugate baseH2O is the base & H3O+ is it’s conjugate acid
Amphoteric Substances:• It acts a base or an acid H3O+ H2O OH-
AcidBase
+ pH Strong Acids or Strong BasespH = - log 10 [ H+ ] Kw @298k = 1x10
-14
[ H+ ] = 10-pH
+ pH Strong Acid & Strong Base
Excess [ H+ ]
Excess [ OH- ]
+ pH Weak AcidspKa = -log10( Ka )
Larger pKa = Weaker Acid
+ pH BuffersSubstances that resist change to pH when small amounts of acid/alkali are added
Assumptions:• [CH3COO-] = [H+]• [CH3COO-] = [CH3COONa]• [CH3COOH] same at
equilibrium & at start
[CH3COO-] from Salt
Acidic Buffers = Weak Acid + it’s Salt
+ Isomerism/Optical IsomersOnly occurs when chiral carbon present:
- Carbon with 4 different groups attached
Mirror
Enantiomers/Optical Isomers
Racemic Mixture:Contains equal amounts of each enantiomer
+ Carbonyls
Ethanal PropanoneCarbonyl Group
+ Reactions of Carbonyls
Reaction with Dichromate:- Aldehydes can be oxidised
- Orange Green- Ketones can’t be oxidised
3RCHO + Cr2O7- + 8H+ 3RCOOH + 2Cr3+ + 4H2O
Reaction with Tollens:- Aldehydes +ve Silver Mirror Forms
Reaction with Benedicts:- Aldehydes + ve Blue (Cu2+) Red Precipitate (Cu+)
Reaction with Brady’s:(2,4 DNP or 2,4 DiNitroPhenylhydrazine)- Carbonyls +ve Orange Precipitate
Reaction with Iodine:- Methyl group adjacent to C=O +ve Pale Yellow Precipitate, Antiseptic smell
Melting point used to identify Carbonyl compound
Dissolved in Methanol & conc H2SO4
AgNO3 dissolved in NH3(aq)
Dissolved in NaOH
dil H2SO4
LiAlH4 (in Dry Ether) to go from Carb Acid to 1o or Aldehyde
Presence of Alkali
Triiodomethane
+ HCN Reactions
Propanone + HCN
Ethanal + HCN
2 methyl 2 hydroxypropanenitrile
2 hydroxypropanenitrile
Nucleophilic Addition
In a lab HCN made by reacting KCN(s) + H2SO4
+ Carboxylic Acids
H-Bonding in Pure Ethanoic Acid (Dimer Shape)
Sodium Ethanoate
Identifying Carboxylic Acids:Add Sodium Carbonate – effervescence if +ve
Weak acid – Partially Dissociate
Very soluble in H2OLonger chain, less soluble
Formed from:- Oxidising a 1°- Hydrolysis of Nitrile
(Reflux with dil HCl and distil off)
Reaction with PCl5:CH3COOH + PCl5 POCl3 + HCl + CH3COCl
Ethanoyl Chloride
Add NH3 white smoke = +ve
Acid + Alcohol Ester + H2O
+ Esters
Acid Hydrolysis:- Reflux with dil HCl or H2SO4
Transesterification
Base Hydrolysis:- Reflux with dil Alkali (e.g. NaOH)
Dicarboxylic Acid + Diol Alcohol Polyester
+ Acyl ChloridesEthanoyl Chloride
Reaction with H2O:
CH3COCl(l) + H2O(l) CH3COOH(l) + HCl(g)
Reaction with Alcohol:
CH3COCl(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + HCl(g) Ethyl Ethanoate
Reaction with Ammonia:
CH3COCl(l) + NH3(aq) CH3CONH2(aq) + HCl(g)
Ethanamide Reaction with Ethylamine:
CH3COCl(l) + C2H5NH2(aq) CH3CONHCH3CH2(aq) + HCl(g)
N-Ethyl-Ethanamide
N-“substituted”-Amide
-amide
+ Soap & Triglycerides
Fats solid at RTP
Triglycerides have lower melting point due to less regular shape
Hydrogenation:- Nickel catalyst @ 150°C- Unsaturated Saturated FA- Solidifies fats
+ UV & Microwave RadiationUV can initiate reactions:• In the form of electromagnetic radiation• Wavelength between Visible and X-ray (400nm-10nm)• e.g. Cl-Cl = Clo + Clo homolytic fission
Free-radical Substitution:
• Initiation = Breaks homolytically (sufficient energy in sunlight)• Propagation = Cl* + CH4 = HCl + CH3* = CH3* + Cl2 = ClCH3 + Cl*• Termination = Cl* + Cl* = Cl2 = CH3* + Cl* = CH3Cl* = CH3* + CH3* = C2H6
Microwaves are used to heat things• Longer than IR also used for communications (1mm-1m)• Most molecules in food are polar (water, fats, sugars)• Microwaves are passed through, they create an electric field• Any polar molecules align with the electric field• Whilst rotating they collide with other molecules generating heat energy
Exo
+ Mass Spectroscopy
Vapourisation:Sample put into vacuum – analysed as a gas
Ionisation:High energy e-s knock off other e-s (+vely charged sample)Electron gun used
Acceleration:-vely charged plate pulls sample up the tube
Deflection:Magnetic field introduced – lighter atoms = deflect moreAll same charge, so only mass varies
Detection:Atoms hit charged plate – small charge is created
Uses: Drugs Testing & Carbon dating
Particle Charge Mass
Proton +1 1
Neutron 0 1
Electron -1 1/1840
Parent Ion Peak = Mr
Only Ions show up
+ NMR – Nuclear Magnetic Resonance• Any molecules with odd number of nucleons (protons & neutrons) has
nuclear spin• This causes a weak magnetic field• NMR looks at how this weak field reacts when you put it in a much
larger external field• When field is applied the protons align themselves with or against the
field• When aligned protons are at a lower energy level than opposing
protons, and they can absorb radio waves of the right frequency, they flip to a higher energy level
• The opposing protons can absorb the radio waves and flip to a lower energy level
• There tends to be more aligned protons, so there’s an absorption of energy overall. NMR measures this absorption
• Proton environments can affect the amount of absorption
Chemical Shift:
• Every energy peak is relative to the peak of tetramethylsilane at ‘0’• Height of peak is no of protons
2 Proton Environment
s
2 Proton Environment
s
Chemical Shift
+ NMRHigh Resolution NMR:
• Each peak is broken down into smaller peaks, this is due to neighbouring magnetic field interacting with each other (spin-spin coupling)
2 Peaks so 2 Proton
EnvironmentsPeak at
9.5ppm due to R-CHO
(aldehyde)
Peak at 2.5ppm due to
R-COCH3 (carbonyl)
DoubletQuartet
Tetramethylsilane
Uses:• To ensure pharmaceutical products are pure• Studies internal structures of the body
+ IR – Infrared Spectroscopy• A beam of IR radiation goes through the sample• Bonds absorb the IR energy, increasing the vibrational energy• Different bonds absorb different IR wavelengths• Bonds in different places of a molecule also absorb different
wavelengths
Uses:• Able to detect when one functional group has
been changed to another in a reaction• The degree of polymerisation that has occurred• Detects weaknesses in polymer is reacted with
O2
+ Chromatography• Mobile Phase – where the molecules can move• Stationary Phase – where the molecules can’t move
Gas Chromatography:• Stationary phase is a viscous liquid (e.g. oil) which coats a coiled tube• Mobile phase is N2(g) as it’s unreactive• Sample is injected into tube as gas• Each compound adsorbs to the stationary phase differently• So each compound takes a different amount of time to be recorded
(retention time)• Recorder produces a graph – area/height shows amount of each
compound
HPLC – High Performance Liquid Chromatography:• Stationary phase is small particles of a solid in a tube• Liquid mobile phase is usually a polar mixture (e.g. methanol & H2O)• Liquid is forced through tube under high-pressure• Sample added to liquid phase and forced through tube as a solution• Mass spectrometer is used to analyse each compound as it’s collected• Mixture separates as it adsorbs to the solid differently• UV shone through liquid stream at end• UV absorbed by mixture as it comes through. Graph is produced• Can be used when sample is heat sensitive or high boiling point
Uses:• Checks chemical
equipment for impurities
Uses:• Routinely check
purity of products in a continuous process