I. 9.1: REACTION MECHANISM: AN ANALOGY
A. Kinetics
The study of how reactions take place and how fast they occur (not spontaneity!)
B. Intermediate
A substance that is produced in one step then consumed in the next
C. EX:
1. dirty dish → soapy dish (5 dishes per minute)
2. soapy dish→ rinsed dish (8 dishes per minute)
3. rinsed dish → clean and dry dish (3 dishes per minute)
4. Rate-determining step is always the slowest step!
D. Rate = Δ
[A]/t
II. 9.2: REACTION RATE
A. Rate
How fast reactants are being consumed or products are being formed; depends on several factors:
1. How frequently the reactant molecules collide
2. The orientation of the colliding molecules
3. Their energy
B. ACTIVATION ENERGY
1. Activation energy (Ea) – the minimum energy required of reactant molecules during a molecular collision in order for the reaction to proceed to products
2. Transition state (activated complex) – high-energy, short lived state; different than intermediate (which is lower energy)
3. Reaction rate:
a) ↓ Ea = faster reaction rate
b) ↑ Reactantconc = faster reaction rate
c) ↑ T = faster reaction rate
4. Note that ΔG has no bearing on reaction rate!
III. 9.3: CATALYSTS
A. Catalysts increase reaction rate by lowering activation energy (lowering energy of highest-energy transition state)
B. Catalysts may undergo temporary change during a reaction, but must be converted back to its original state
C. Catalysts do not affect ⇋ or thermodynamics of a reaction
IV. 9.4: RATE LAWS
A. Rate law
Includes only those reactants that are involved in the rate-determining step; can be determined several ways, all experimentally (initial concentration method, graphical analysis, half life, etc)
1. EX: aA + bB → cC + dD
2. rate = k[A]x[B]y
a) x = order of rxn w/respect to A
(1) This means that the reaction rate is proportional to [A]x; x is the order
b) y = order of rxn w/respect to B
c) x + y = overall order of the reaction
d) k = rate constant
3. Can only be determined experimentally (except for the elementary step in rxn mechanism)
Experiment
[A]
[B]
[C]
Initial rxn rate [M/s]
1
0.2 M
0.1 M
0.05 M
1*10-3
2
0.4 M
0.1 M
0.05 M
2*10-3
3
0.2 M
0.2 M
0.05 M
4*10-3
4
0.2 M
0.1 M
0.1 M
1*10-3
4. Compare [A] in experiment 1 and 2: reaction rate doubles with doubling [A]
a) [A] is proportional to rate, ∴ [A]x → x = 1
5. Compare [B] in experiment 1 and 3: reaction rate quadruples with doubling [B]
a) [B]2 is proportional to rate, ∴ [B]y → y = 2
6. Compare [C] in experiment 1 and 4: reaction rate stays same with doubling [C]
a) [C] is not proportional to the rate at all, ∴ [C]z → z = 0
7. rate = k[A][B]2
B. THE RATE CONSTANT
1. Rearrange the rate equation:
a) k = rate/([A][B]2)
2. k is constant at any given temperature
3. Arrhenius equation:
a) k = Ae-(Ea/RT)
b) lnk = lnA – (Ea/RT)
(1) From this equation, you can see that lowering Ea (adding catalyst) OR ↑ T will ↑ k
(2) Note – reaction rate will ≈ double for every 10° C ↑ T
More MCAT Study Guide Chemistry
II. 9.2: REACTION RATE
A. Rate
How fast reactants are being consumed or products are being formed; depends on several factors:
1. How frequently the reactant molecules collide
2. The orientation of the colliding molecules
3. Their energy
B. ACTIVATION ENERGY
1. Activation energy (Ea) – the minimum energy required of reactant molecules during a molecular collision in order for the reaction to proceed to products
2. Transition state (activated complex) – high-energy, short lived state; different than intermediate (which is lower energy)
3. Reaction rate:
a) ↓ Ea = faster reaction rate
b) ↑ Reactantconc = faster reaction rate
c) ↑ T = faster reaction rate
4. Note that ΔG has no bearing on reaction rate!
III. 9.3: CATALYSTS
A. Catalysts increase reaction rate by lowering activation energy (lowering energy of highest-energy transition state)
B. Catalysts may undergo temporary change during a reaction, but must be converted back to its original state
C. Catalysts do not affect ⇋ or thermodynamics of a reaction
IV. 9.4: RATE LAWS
A. Rate law
Includes only those reactants that are involved in the rate-determining step; can be determined several ways, all experimentally (initial concentration method, graphical analysis, half life, etc)
1. EX: aA + bB → cC + dD
2. rate = k[A]x[B]y
a) x = order of rxn w/respect to A
(1) This means that the reaction rate is proportional to [A]x; x is the order
b) y = order of rxn w/respect to B
c) x + y = overall order of the reaction
d) k = rate constant
3. Can only be determined experimentally (except for the elementary step in rxn mechanism)
| Experiment | [A] | [B] | [C] | Initial rxn rate [M/s] |
| 1 | 0.2 M | 0.1 M | 0.05 M | 1*10-3 |
| 2 | 0.4 M | 0.1 M | 0.05 M | 2*10-3 |
| 3 | 0.2 M | 0.2 M | 0.05 M | 4*10-3 |
| 4 | 0.2 M | 0.1 M | 0.1 M | 1*10-3 |
4. Compare [A] in experiment 1 and 2: reaction rate doubles with doubling [A]
a) [A] is proportional to rate, ∴ [A]x → x = 1
5. Compare [B] in experiment 1 and 3: reaction rate quadruples with doubling [B]
a) [B]2 is proportional to rate, ∴ [B]y → y = 2
6. Compare [C] in experiment 1 and 4: reaction rate stays same with doubling [C]
a) [C] is not proportional to the rate at all, ∴ [C]z → z = 0
7. rate = k[A][B]2
B. THE RATE CONSTANT
1. Rearrange the rate equation:
a) k = rate/([A][B]2)
2. k is constant at any given temperature
3. Arrhenius equation:
a) k = Ae-(Ea/RT)
b) lnk = lnA – (Ea/RT)
(1) From this equation, you can see that lowering Ea (adding catalyst) OR ↑ T will ↑ k
(2) Note – reaction rate will ≈ double for every 10° C ↑ T
