5 Steps to a 5 AP Chemistry

4 .Physical state of reactants—When reactants are mixed in the same

physical state, the reaction rates should be higher than if they are in

different states, because there is a greater chance of collision. Also, gases

and liquids tend to react faster than solids because of the increase in

surface area. The more chance for collision, the faster the reaction rate.

5 .Catalysts—Acatalystis a substance that speeds up the reaction rate and

is (at least theoretically) recoverable at the end of the reaction in an

unchanged form. Catalysts accomplish this by reducing the activation

energy of the reaction. Activation energyis that minimum amount of

energy that must be supplied to the reactants in order to initiate or start

the reaction. Many times the activation energy is supplied by the kinetic

energy of the reactants.

Keywords and Equations

ln[A]t−ln[A] 0 =–kt(first order)

(second order)

t= time (seconds)

Ea= activation energy

k= rate constant

A= frequency factor

Gas constant, R= 8.314/J mol−^1 K−^1

Rates of Reaction

The rate (or speed) of reaction is related to the change in concentration of either a reactant or

product with time. Consider the general reaction: 2A +B → C+3D. As the reaction proceeds,

the concentrations of reactants A and B will decrease and the concentrations of products

C and D will increase. Thus, the rate can be expressed in the following ways:

The first two expressions involving the reactants are negative, because their concentra-

tions will decrease with time. The square brackets represent moles per liter concentration

(molarity).

The rate of reaction decreases during the course of the reaction. The rate that is calcu-

lated above can be expressed as the average rate of reaction over a given time frame or, more

commonly, as the initial reaction rate—the rate of reaction at the instant the reactants are

mixed.

Rate

A] B] C] D]

=−

Δ

Δ

=−

Δ

Δ

=

Δ

Δ

=

Δ

Δ

1

2

1

3

[[[[

ttt t

Ink a In

E

RT

= A

− ⎛

⎝

⎜

⎞

⎠

⎟+

1

t

1/2 kk

In2

==

0 693.

11

[] [AA

kt

t0]

−=

198  Step 4. Review the Knowledge You Need to Score High

KEY IDEA