Rates of Reaction

Rates can be measured by the following methods:

For reactions involving acids and bases

• pH change by titrations
• pH change via pH meter

For reactions involving gases

• Change in volume or pressure
• loss of mass of reactants

For reactions that produce visual changes

• The formation of a precipitate
• A colour change

For reactant A

• rate ∝ [A]^x
• where x is an order with respect to A

Orders are always defined by the reactant.

Zero Order

If X=0, rate is unaffected by change in [A]

First Order

If X=1, rate ∝ [A]^1           (if [A] doubles, as will rate.)

Second Order

If x-2, rate ∝ [A]^2            (if [A] doubles, rate will quadruple.) 

Overall Order

• Sum of individual orders.
• eg, rate=k[B]^1[C]^2
• overall rate = 1+2 = 3

Units of rate constants

K = rate/[A]

rate units = moldm^-3 S^-1

[A] units = moldm^-3

A larger value for K means a faster reaction

Temperature

• Raising the temperature speeds up the rate of most reactions by increasing K
• For many reactions, the rate doubles for each 10 Degrees Celcius increase in temperature.

Half Lives

The time taken for [A] to reduce by half.

First Order - [A] decreases at a constant rate.  Half life decreases with time.

Second Order - [A] halves in equal time.  Half life is constant.

Third Order - [A] decreases rapidly but then rate of decrease slows down.  Half life increases with time.

Multistep reactions often have one step that is slower than others.

• Reactions can only become products as fast as they can get through this slow step.
• Overall rate can be no faster than the slowest step.

The rate of reaction is dominated by this slow step, called the rate-determining step.

RDS of a reaction between X and Y is

2X -> Z

Predict the rate equation for this reaction.

rate = K[X]^2

Why?

Order with respect to the reaction tells you how many particles of the reactant are involved in RDS therefore 2X gives X 2nd Order.