MCAT Study Guide Chemistry Ch. 9 – Chemical Reaction 2017-08-15T06:45:06+00:00

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

MCAT Study Guide Chemistry - Kim Matsumoto


More MCAT Study Guide Chemistry

1.

Ch. 3 Chemistry Basics

2.

Ch. 4 Atomic Structure

3.

Ch. 5 Chemical Bonds

4.

Ch. 6 Enthalpy + Entropy

5.

Ch. 7 Calorimetry + Phase Diagrams

6.

Ch. 8 Ideal Gas Law

7.

Ch. 9 Rate Laws

8.

Ch. 10 Equilibrium

9.

Ch. 11 Acids and Bases

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