Rates of Reaction

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  • Created by: Arianne
  • Created on: 25-01-13 11:31

Methods of Measurement

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
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Orders 1

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.) 

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Orders 2

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

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Rate Constant, K and Half Lives

A larger value for K means a faster reaction


  • 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.

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Rate Determining Step

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


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

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