AP Chem Chpt 14 - Chemical Kinetics

Reaction Rate

The speed at which a chemical reaction occurs.

Chemical Kinetics

The study of how fast chemical reactions occur.

Reaction mechanism

Step-by-step pathway from reactants to products.

direct

Relationship between Surface Area/Volume and Reaction Rates:

SArea/V+, RR+

(one factor impacting rates of reaction)

direct

Relationship between concentration ([ ]) and reaction rates:

M+, RR+

(factor impacting rates of reaction)

(concentration+, probability of collision+ = rate+)

direct

Relationship between temperature and reaction rates:

T+, RR+

(factor impacting rates of reaction)

(temp+, speed+, frequency & force of collisions+ = reaction rate+)

more collisions

Why does an increase in temperature cause the reaction rate to increase?

(think about kinetic energy!!)

catalyst

The presence/absence of a _______________ affects rates of reaction.

Catalyst

Agent that speeds up a reaction without being used up (ex. enzyme).

higher

The greater the frequency of collisions, the _______________ the reaction rate.

A, reactant

Being used up in a reaction (disappearing)

B, product

Being formed in a reaction (appearing)

positive

Rates of disappearance and appearance are always ______________!!!

Rate of disappearing

-∆[ ]/∆t

Rate of appearance

∆[ ]/∆t

M/s

Units for rates of disappearing/appearing.

final - initial

∆ (change) =

smaller

As a reaction progresses, there is a ________________ reaction rate due to decreasing concentration of reactants.

directly

When volume is constant, moles and molarity are ______________ proportional.

decreases

The average rate ______________ with time.

Instantaneous rate

The rate at any instant in time.

tangent, triangle

Instantaneous rate can be found by drawing a line __________ to the graph/points and then drawing a _______________.

A → B

-∆[A]/∆t = ∆[B]/∆t

2A → B

-1/2 ∆[A]/∆t = ∆[B]/∆t

3A → 2B + C

-1/3 ∆[A]/∆t = 1/2 ∆[B]/∆t = ∆[C]/∆t

increase

Rates ____________ when reactant concentration is increased.

decrease

Rates ______________ as the concentration of reactants in reduced.

Rate 1/Rate 2 = [A₁]^m [A₂]ⁿ/[B₁]^m [B₂]ⁿ

Equation to determine Rate Law:

1) Pick 2 experiments where [A] remains constant and [B] changes to find n.

2) Pick 2 experiments where [B] remains constant and [A] changes to find m.

Rate Law

The overall concentration dependence of reaction rate.

k

Rate constant

Reaction orders

Exponents m and n in the Rate Law equation:

Rate = k[reactant 1]^m [reactant 2]ⁿ

Overall reaction order

Sum of the reaction orders

Yes

Are fractional or negative reaction order values possible?

fast

A large value of k (10⁹ or greater) means the reaction is…

slow

A small value of k (10 or lower) means the reaction is…

no

If a reaction is zero order, will changing the initial concentration of the reactant have an effect on the rate?

double

If a reaction is in First Order, doubling the concentration will cause the rate to…

(2²)-fold

If a reaction is in Second Order, doubling the concentration will result in a ________ increase in rate.

(3²)-fold

If a reaction is in Second Order, tripling the concentration will result in a _________ increase in rate.

(2ⁿ)-fold

A reaction is nth order if doubling the concentration causes a _________ increase in rate.

yes

Does the rate depend on concentration?

no

Does the rate constant depend on concentration?

yes

Is the rate constant affected by temperature and the presence of a catalyst?

k

Zero Order Rate Law

M/s

Zero Order units

k[A]

First Order Rate Law

s⁻¹ or 1/s

First Order units

k[A][B], k[A]², k[B]²

Second Order Rate Law (3)

M⁻¹s⁻¹ or 1/Ms

Second Order units

linear

Of given graphs, the one with the _____________ line/slope is the reaction.

ln[A]t = -kt + ln[A]₀

First Order Integrated Rate Law

-k

Slope of First Order graph

ln[A]₀

y-intercept of First Order graph

1/[A]t = kt + 1/[A]₀

Second Order Integrated Rate Law

k

Slope of Second Order graph

1/[A]₀

y-intercept of Second Order graph

Rate = k[A][B]

Reaction that is second order overall, but has first order dependence on A and B:

zero-order

A _______________ reaction is one whose rate is independent of the reactant concentration.

[A]t = -kt + [A]₀

Zero Order Integrated Rate Law

-k

Slope of Zero Order graph

[A]₀

y-intercept of Zero Order graph

half life

(t 1/2) The time required for the concentration of a reactant to decrease to half its original value.

indirect

Relationship between [A]₀ and t 1/2:

[A]₀ - , t ½ +

(The reaction rate slows down as [A] goes down, making the half-life longer)

t 1/2 = 0.693/k

First Order Reaction Half Life Equation

t 1/2 = 1/k[A]₀

Second Order Reaction Half Life Equation

k = 0.02 d⁻¹

(TEST QUESTION) The decomposition of a certain pesticide is a first order reaction. The half-life of the pesticide in 32.5 days. Solve for the rate constant.

first

The half-life or a ________ order reaction is independent of the initial concentration of the reactant.

second

The half-life or a ________ order reaction is dependent on the initial concentration of the reactant.

concentration, temperature

Rates of reaction are affected by _______________ and ______________.

Collision Model

Helps to explain how rates of reactions are affected. Based on the Kinetic Molecular Theory!

orientation, energy, frequency

3 things impacting Collision Model:

orientation, energy

In order for a reaction to occur, the reactant molecules must collide in the correct _____________ and with enough ___________ to form products.

orientation

HAVE to have proper ________________ to end up with a reaction!

broken

In order to form products, bonds must be ____________ in the reactants.

requires

Breaking bonds __________ energy.

Activation energy

(Ea) The minimum energy required to initiate a chemical reaction (will vary with the reaction).

[The energy barrier between the starting molecule and the highest energy state found along the reaction pathway.]

Endothermic

Reactions taking in (absorbing) energy.

Exothermic

Reactions releasing energy.

products

For endothermic reactions, the potential energy is higher in the _____________.

(Energy profile: reactants start low, products end high)

reactants

For exothermic reactions, the potential energy is higher in the _____________.

(Energy profile: reactants start high, products end low)

lower

For a reverse endothermic reaction, is the Ea higher or lower than in an endothermic forward reaction?

higher

For a reverse exothermic reaction, is the Ea higher or lower than in an exothermic forward reaction?

indirect

Relationship between activation energy and reaction rate:

Ea-, RR+

(can get to products more quickly!)

endothermic

If a forward reaction is exothermic, the reverse reaction is…

transition state

The species at the top of the barrier between staring molecule and highest energy state.

∆E

The difference in energy between the reactants and products in an energy profile.

no

Does ∆E impact reaction rate?

Ea

_________ is the difference in energy between reactants and transition state (forward) or products and transition state (reverse).

equal, greater than

Molecules that have an energy __________ or ________________ Ea have sufficient energy to react.

temperature, increases

As we increase the ________________, the fraction of the population that has an energy equal to or greater than Ea ______________.

(more molecules can react)

frequency

What does the Arrhenius Equation explain in terms of the Collision Model?

3

Reaction rates obey an equation based on _______ factors.

direct

Relationship between temperature and frequency (Arrhenius Equation):

T+, A+

molecules move faster → increased number of collisions

↑ temp, ↑ KE, ↑ force/frequency of collisions, ↑RR

Reaction Mechanism

The process by which the reaction occurs.

broken, formed

Mechanisms provide a picture of which bonds are ____________ and ___________ during the course of a reaction.

Elementary reactions

Any processes that occur in a single step.

Molecularity

The number of molecules present in an elementary step.

unimolecular

Reactions involving only ONE molecule.

bimolecular

Elementary reactions involving the collision of TWO molecules.

termolecular

Elementary reactions involving the simultaneous collision of three molecules (statistically impossible in the real world).