Orders and rate equations

Rate equation

Show how the rate of reaction is affected by the concentration of reactants

For the general equation A + B → C + D, what is the rate equation

Rate = K[A]^m[B]ⁿ

In the rate equation Rate = K[A]^m[B]ⁿ, what is m

The order of reaction with respect to reactant A and shows how the concentration of A affects the rate of reaction

In the rate equation Rate = K[A]^m[B]ⁿ, what is n

The order of reaction with respect to reactant B and shows how the concentration of B affects the rate of reaction

What is K in Rate = K[A]^m[B]ⁿ

• The rate constant

• A bigger rate constant, means the reaction is faster

• The rate constant is always the same for a reaction at a particular temperature

• The units of K vary and can be worked out

If [A] changes and the rate stays the same, what is the order

The order of reaction with respect to A is 0

If the rate is proportional to the [A] (rate ∝ [A]), what is the order

The order of reaction with respect to A is 1

If the rate is proportional to the [A]² (rate ∝ [A]²), what is the order

The order of reaction with respect to A is 2

What is the overall order of a reaction

The sum of the orders of all the different reactants

Methods of finding orders

• Use initial rates to construct a rate-concentration graph and examine its shape

• Directly compare the initial rates for different concentrations of reactant

• Use the continuous monitoring method to construct a concentration time graph and compare successive half lives

How to ensure the concentrations of reactants not being investigated don’t affect the rate

Present in excess so their concentration don’t change significantly during the reaction

Which shapes of concentration-time graphs correspond to which order

• A straight line indicates 0 order

• A curved indicates first or second order

Which shapes of rate-concentration graphs correspond to which order

• A straight horizontal graph indicates 0 order

• A straight line going through the origin indicates first order

• A curved line indicates second order

How does temperature affect K

K increases as temperature increases