3.1.9 Rate Equations

Define rate of reaction.

Change in concentration of any reactant or product per unit time.

What does a rate expression tell us?

How different species influence reaction rate; shows how rate depends on concentration.

What is the general form of a rate equation?

rate = k [A]^m [B]ⁿ

What is k in a rate equation?

The rate constant; specific to each reaction and varies with temperature.

What are the units of rate?

mol dm⁻³ s⁻¹

What is the order with respect to a reactant?

The exponent (power) of that reactant’s concentration in the rate equation.

How is overall order of a reaction determined?

Sum of all individual orders in the rate equation.

For rate = k [X][Y]², what is the order with respect to X?

1

For rate = k [X][Y]², what is the order with respect to Y?

2

For rate = k [X][Y]², what is the overall order?

3

What are the units of k for a zero-order reaction?

mol dm⁻³ s⁻¹

What are the units of k for a first-order reaction?

s⁻¹

What are the units of k for a second-order reaction?

dm³ mol⁻¹ s⁻¹

What are the units of k for a third-order reaction?

dm⁶ mol⁻² s⁻¹

What is the rate-determining step (RDS)?

The slowest step in a reaction mechanism.

Why is the RDS important?

It determines the overall rate; the rate expression includes only species up to and including the RDS.

If a reaction has multiple steps, which step appears in the rate expression?

Only the species involved in steps up to and including the RDS.

What is an intermediate?

A species formed in one step and consumed in a later step; not in overall equation.

If doubling [A] doubles the rate, what is the order with respect to A?

1 (first order)

If doubling [A] quadruples the rate, what is the order with respect to A?

2 (second order)

If doubling [A] does not change the rate, what is the order with respect to A?

0 (zero order)

What is the Arrhenius equation?

k = A e^(-Ea/RT)

What does the Arrhenius equation describe?

How rate constant k depends on temperature and activation energy.

How is activation energy related to reaction rate?

Higher Ea means slower rate (at a given temperature).

What does a rate constant’s units tell you?

The overall order of the reaction.

If rate = k [P]²[Q], what is the overall order?

3

What is the role of a catalyst?

Lowers activation energy, increases rate, not consumed in reaction, provides alternate pathway for reaction with lower Ea

Does a catalyst appear in the rate expression?

Sometimes, if involved in the RDS.

What is a reaction mechanism?

A step-by-step sequence of elementary reactions that sum to the overall reaction.

Can the rate equation be predicted from the overall equation?

No; must be determined experimentally.

What does a straight line with no gradient on a concentration vs. time graph indicate in terms of order of reactant?

Zero order.

What does a straight steep line on a concentration vs. time graph indicate in terms of order of reactant?

First order.

What does a non- straight line on a concentration vs. time graph indicate in terms of order of reactant?

Order greater than 1

What’s the logarithmic form of the Arrhenius equation?

lnK = -Ea/RT +lnA

When the logarithmic form of the Arrhenius equation is plotted on a graph, what are the y, m, x and c components.

Y= lnK

M= -Ea/ R

X= 1/T

C= lnA