chap 06a-chemical reactions 51 le… · formaldehyde formic acid oxidation (gain of oxygen) formic...
TRANSCRIPT
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Chapter 6A
Chemical
Reactions
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CHAPTER OUTLINE���
§ Chemical Reactions § Chemical Equation § Balancing Equations § Types of Chemical Reactions § Double Replacement Reactions § Oxidation-Reduction Reactions § Redox in Biological Systems § Activity Series of Metals
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CHEMICAL���REACTIONS���
q A chemical reaction is a rearrangement of atoms in which some of the original bonds are broken and new bonds are formed to give different chemical structures.
q In a chemical reaction, atoms are neither created, nor destroyed.
q A chemical reaction, as described above, is supported by Dalton’s postulates.
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CHEMICAL���REACTIONS���
6 oxygen atoms 6 oxygen atoms =In a chemical reaction, atoms are neither created, nor destroyed
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CHEMICAL���REACTIONS���
q A chemical reaction can be detected by one of the following evidences:
1. Change of color (formation of a solid)
2. Formation of a gas
3. Exchange of heat with surroundings
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CHEMICAL���EQUATIONS���
q A chemical equation is a shorthand expression for a chemical reaction.
Word equation:
Aluminum combines with ferric oxide to form iron and aluminum oxide.
Chemical equation:
Al + Fe2O3 → Fe + Al2O3
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CHEMICAL���EQUATIONS���
q Reactants are separated from products by an arrow.
Al + Fe2O3 → Fe + Al2O3
q Coefficients are placed in front of substances to balance the equation.
2 Al + Fe2O3 → 2 Fe + Al2O3
Subscripts
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CHEMICAL���EQUATIONS���
q Reaction conditions are placed over the arrow.
Al + Fe2O3 → Fe + Al2O3
q The physical state of the substances are indicated by the symbols (s), (l), (g), (aq).
2 Al (s) + Fe2O3 (s) → 2 Fe (l) + Al2O3 (s)
Δ
heat
solid liquid
Δ
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BALANCING���EQUATIONS���
q A balanced equation contains the same number of atoms on each side of the equation, and therefore obeys the law of conservation of mass.
q Many equations are balanced by trial and error; but it must be remembered that coefficients can be changed in order to balance an equation, but not subscripts of a correct formula.
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BALANCING���EQUATIONS���
q The general procedure for balancing equations is:
Write the unbalanced equation:
CH4 + O2 → CO2 + H2O
Make sure the formula for each
substance is correct
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BALANCING���EQUATIONS���
q The general procedure for balancing equations is:
Balance by inspection:
CH4 + O2 → CO2 + H2O
Count and compare each element on both sides of the
equation
1 C = 1 C
4 H 2 H
2 O 3 O
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BALANCING���EQUATIONS���
q Balance elements that appear only in one substance first. Balance H
CH4 + O2 → CO2 + H2O
1 CH4 + O2 → CO2 + 2 H2O
4 H present on each side
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BALANCING���EQUATIONS���
Balance O
1 CH4 + O2 → CO2 + 2 H2O
1 CH4 + 2 O2 → CO2 + 2 H2O
4 O present on each side
When finally done, check for the smallest coefficients possible
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Examples:���
AgNO3 + H2S → Ag2S + HNO3
Al(OH)3 + H2SO4 → Al2(SO4)3 + H2O
Fe3O4 + H2 → Fe + H2O
C4H10 + O2 → CO2 + H2O
2 AgNO3 + H2S → Ag2S + 2 HNO3
2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6 H2O
Fe3O4 + 4 H2 → 3 Fe + 4 H2O
2 C4H10 + 13 O2 → 8 CO2 + 10 H2O
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CONCEPT���CHECK���
q If red spheres represent oxygen atoms and blue spheres represent nitrogen atoms, write a balanced equation for the reaction shown below.
2 NO + O2 → 2 NO2
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TYPES OF���CHEMICAL REACTIONS���
q Chemical reactions are classified into five types:
1. Synthesis or combination
2. Decomposition
3. Single replacement
4. Double replacement
5. Combustion
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SYNTHESIS or���COMBINATION���
q In these reactions, 2 elements or compounds combine to form another compound.
A + B → AB
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DECOMPOSITION���
q In these reactions, a compound breaks up to form 2 elements or simpler compound.
AB → A +B
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SINGLE���REPLACEMENT���
q In these reactions, a more reactive element replaces a less reactive element in a compound.
A + BC → B + AC
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DOUBLE���REPLACEMENT���
q In these reactions, two compounds combine to form two new compounds.
AB + CD → AD + CB
q The cation from one compound replaces the cation in another compound.
+ +
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COMBUSTION���
q A reaction that involves oxygen as a reactant and produces large amounts of heat is classified as a combustion reaction.
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)
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Examples:���
Classify each of the reactions below: 1. Mg + CuCl2 → MgCl2 + Cu 2. CaCO3 → CaO + CO2 3. 2 HCl + Ca(OH)2 → CaCl2 + 2 H2O 4. 4 Fe + 3 O2 → 2 Fe2O3
Single replacement
Mg is more reactive than Cu
Decomposition
Double replacement
Synthesis
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DOUBLE REPLACEMENT���REACTIONS���
q Double replacement reactions can be subdivided into one the following subgroups:
1. Precipitation: formation of a solid
2. Neutralization: formation of water
3. Unstable product: formation of a gas
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PRECIPITATION���REACTIONS���
q In these reactions one of the products formed is an insoluble solid called a precipitate.
q For example, when solutions of potassium chromate, K2CrO4 , and barium nitrate, Ba(NO3)2 , are combined an insoluble salt barium chromate, BaCrO4 , is formed.
K2CrO4 (aq) + Ba(NO3)2 (aq) BaCrO4 (s) + 2 KNO3 (aq) precipitate
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NEUTRALIZATION���REACTIONS���
q The most important reaction of acids and bases is called neutralization.
q In these reactions an acid combines with a base to form a salt and water.
2HCl (aq) + NaOH (aq) NaCl (aq) + H O (l)æ æÆ
Acid Base Salt
q Salts are ionic substances with the cation donated from the base and the anion donated from the acid.
q In the laboratory, neutralization reactions are observed by an increase in temperature (exothermic reaction).
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UNSTABLE PRODUCT���REACTIONS���
q Some chemical reactions produce gas because one of the products formed in the reaction is unstable.
q Two such products are:
H2CO3 (aq) → CO2 (g) + H2O (l) Carbonic acid:
Sulfurous acid: H2SO3 (aq) → SO2 (g) + H2O (l)
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UNSTABLE PRODUCT���REACTIONS���
q When either of these products appears in a chemical reaction, they should be replaced with their decomposition products.
2 HCl + Na2CO3 → 2 NaCl + H2CO3
2 HCl + Na2CO3 → 2 NaCl + CO2 (g) + H2O (l)
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Complete and balance each neutralization reaction below:
Examples:���
HNO3 + Ba(OH)2 → Ba(NO3)2 + H2O 2 2
H2SO4 + NaOH → Na2SO4 + H2O 2 2
HC2H3O2 + KOH → KC2H3O2 + H2O
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OXIDATION-REDUCTION���REACTIONS���
q Reactions known as oxidation and reduction (redox) have many important applications in our everyday lives.
q Rusting of a nail or the reaction within your car batteries are two examples of redox reactions.
q In an oxidation-reduction reaction, electrons are transferred from one substance to another.
q If one substance loses electrons, another substance must gain electrons.
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OXIDATION-REDUCTION���REACTIONS���
q Oxidation is defined as loss of electrons, and reduction is defined as gain of electrons.
q One way to remember these definitions is to use the following mnemonic:
q Combination, decomposition, single replacement and combustion reactions are all examples of redox reactions.
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
OIL
RIG
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OXIDATION-REDUCTION���REACTIONS���
q In general, atoms of metals lose electrons to form cations, and are therefore oxidized, while atoms of non-metals gain electrons to form anions, and are therefore reduced.
Ca + S CaS
Ca Ca2+ + 2 e- S + 2 e- S2-
Oxidation
Reduction
q For example, in the formation of calcium sulfide from calcium and sulfur
q Therefore, the formation of calcium sulfide involves two half-reactions that occur simultaneously, one an oxidation and the other a reduction.
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OXIDATION-REDUCTION���REACTIONS���
Mg + 2 HCl MgCl2 + H2
Mg Mg2+ + 2 e- 2 H+ + 2 e- H2
Oxidation
Reduction
q Similarly, in the reaction of magnesium metal with hydrochloric acid
In every oxidation-reduction reaction, the number of electrons lost must be equal to the number of electrons gained.
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OXIDATION-REDUCTION IN���BIOLOGICAL SYSTEMS���
q Many important biological reactions involve oxidation and reduction.
q In these reactions, oxidation involves addition of oxygen or loss of hydrogen, and reduction involves loss of oxygen or gain of hydrogen.
q For example, poisonous methyl alcohol is metabolized by the body by the following reaction:
CH3OH H2CO + 2H•
methyl alcohol formaldehyde
Oxidation (loss of
hydrogen)
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OXIDATION-REDUCTION IN���BIOLOGICAL SYSTEMS���
q The formaldehyde is further oxidized to formic acid and finally carbon dioxide and water by the following reactions:
2 H2CO + O2 2H2CO2 formic acid formaldehyde
Oxidation (gain of oxygen)
formic acid
2 H2CO2 + O2 CO2 + H2O
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q In many biochemical oxidation-reduction reactions, the transfer of hydrogen atoms produces energy in the cells.
q For example, cellular respiration is an oxidation-reduction process that transfers energy from the bonds in glucose to form ATP.
OXIDATION-REDUCTION IN���BIOLOGICAL SYSTEMS���
C6H12O6 + 6 O2 6 CO2 + 6 H2O + ATP + Heat
Glucose + Heat
C6H12O6 O2 CO2 H2O ATP 6 6 6
Loss of hydrogen atoms (becomes oxidized)
Gain of hydrogen atoms (becomes reduced)
OXIDATION-REDUCTION IN���BIOLOGICAL SYSTEMS���
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OXIDATION-REDUCTION IN���BIOLOGICAL SYSTEMS���
q The oxidation of a typical biochemical molecule can involve the transfer of hydrogen atoms to a proton acceptor such as coenzyme FAD to produce its reduced form FADH2.
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REDOX IN���BIOLOGICAL SYSTEMS���
q In summary, the particular definition of oxidation-reduction depends on the process that occurs in the reaction.
q A summary of these definitions appears below:
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Example 1:���Linoleic acid, an unsaturated fatty acid, can be converted to a saturated fatty acid by the reaction shown below. Is linoleic acid oxidized or reduced in this reaction?
C18H32 O2 + 2 H2 C18H36 O2
Gain of hydrogen Reduction
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Example 2:���The reaction of succinic acid (C4H6O4) provides energy for the ATP synthesis and is shown below: a) Is succinic acid oxidized or reduced? b) Is FAD oxidized or reduced?
C4H6 O4 C4H4 O4 + 2 H•
Loss of hydrogen Gain of
hydrogen
FAD + 2 H• FADH2
oxidation reduction
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ENZYMES IN���BIOLOGICAL SYSTEMS���
q In biochemical reactions, enzymes are necessary to oxidize glucose and other foods.
q For example, oxidation of glucose involves the transfer of hydrogen atoms and electrons to an enzyme, such as NAD+ to produce its reduced form NADH.
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ENZYMES IN���BIOLOGICAL SYSTEMS���
q Similarly, oxidation of methanol involves transfer of 2 hydrogen atoms and 2 electrons to NAD+ to form the reduced form NADH.
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ENZYMES IN���BIOLOGICAL SYSTEMS���
q Molecules such as NAD+ are called “electron carriers” since they carry electrons in their reduced form.
q The electron carriers collectively are called electron transport chain.
q As electrons are transported down the chain, ATP is generated.
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ENZYMES IN���BIOLOGICAL SYSTEMS���
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ACTIVITY SERIES���OF METALS���
q Activity series is a listing of metallic elements in descending order of reactivity.
q Hydrogen is also included in the series since it behaves similar to metals.
q Activity series tables are available in textbooks and other sources.
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ACTIVITY SERIES���OF METALS���
q Elements listed higher will displace any elements listed below them.
q For example Na will displace any elements listed below it from one of its compounds.
2 Na (s) + MgCl2 (aq) → 2 NaCl (aq) + Mg (s)
Na (s) + AgCl (aq) → NaCl (aq) + Ag (s)
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ACTIVITY SERIES���OF METALS���
q Elements listed lower will not displace any elements listed above them.
q For example Ag cannot displace any elements listed above it from one of its compounds.
Ag (s) + CuCl2 (aq) → No Reaction
Ag (s) + HCl (aq) → No Reaction
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Example 1:���Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”.
Pb (s) + HCl (aq) → Metals Fe Ni Sn Pb H
Cu Ag
Pb is more reactive than H
PbCl2 (aq) + H2 (g) 2
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Example 2:���Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”.
Zn (s) + MgCl2 (aq) → Metals Na Mg Al Zn Fe Ni
Zn is less reactive than Mg
No Reaction
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Example 3:���Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”.
Ni (s) + CuCl2 (aq) → Metals Fe Ni Sn Pb H
Cu Ag
Ni is more reactive than Cu
NiCl2 (aq) + Cu (s)
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Example 4:���Use activity series to complete each reaction below. If no reaction occurs, write “No Reaction”.
Mg (s) + AlCl3 (aq) → Metals Na Mg Al Zn Fe Ni
Mg is more reactive than Al
MgCl2 (aq) + Al (s) 2 3 3 2
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THE END