AQA A Level Chemistry

Revision Notes

5.2.1 Rate Equations

Rate Equations

  • The rate of reaction refers to the change in the amount or concentration of a reactant OR product per unit time
  • It can be found by:
    • Measuring the decrease in the concentration of a reactant over time
    • Measuring the increase in the concentration of a product over time
    • The units for rate of reaction are mol dm-3 s-1

Rate equation

  • The following general reaction will be used as an example to study the rate of reaction

D (aq) → E (aq) + F (g) 

  • The rate of reaction at different concentrations of D is measured and tabulated

Rate of reactions table

Table to show rates [D], downloadable AS & A Level Chemistry revision notes

  • A directly proportional relationship between the rate of the reaction and concentration of D is observed when a graph is plotted

Rates [D] graph, downloadable AS & A Level Chemistry revision notes

Rate of reaction over various concentrations of D

  • Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations

Rate of reaction = k [A]m [B]n

[A] and [B] = concentrations of reactants

m and n = orders of the reaction

  • All of the reactant concentrations will have an order of 0, 1 or 2, depending on the effect that they have on the rate of the reaction
  • The products are never involved in the rate equation, as they have no effect on the rate of the reaction
  • For the above reaction, the rate equation would be

    Rate = k [D]

  • Let’s take a real life example:

2NO (g) + 2H2 (g) → N2 (g) + 2H2O (g)

  • The rate equation for the formation of nitrogen gas (N2) from nitrogen oxide (NO) and hydrogen (H2) is:

rate = k [NO]2 [H2]

  • Notice that the [H2] does not have an order of 2
    • This is because the order must be determined experimentally, not from the equation
  • The orders of the reaction will be calculated from a table of experimental data, or from a graph
  • The rate equation for the reaction above shows that:
    • When changing the concentration of NO to determine how it affects the rate, while keeping [H2] constant
    • The change in rate is proportional to the square of [NO]

Rate = k1 [NO]2

  • And, when changing the [H2] to determine how it affects the rate while keeping [NO] constant
  • The change in rate is proportional to [H2]

Rate = k2 [H2]

  • Combining the two equations gives the overall rate equation (where k = k1 + k2)

Rate = k [NO]2 [H2]

  • For a catalyst to appear in the rate equation:
    • It must have a measurable and quantifiable effect on the rate of reaction
    • The catalyst must be homogeneous
    • If a chemical appears in a rate equation but is not one of the reactants, then it is a catalyst

Order of reaction

  • The order of a reactant shows how the concentration of a reactant affects the rate of reaction
    • It is the power to which the concentration of that reactant is raised in the rate equation
    • The order can be 0, 1 or 2
    • When the order of reaction of a reactant is 0, this means that it has no effect on the rate of the reaction and therefore is not included in the rate equation at all
    • When the order of reaction of a reactant is 1, the rate is directly proportional to the concentration of that reactant
    • When the order of reaction of a reactant is 2, the rate is directly proportional to the square of the concentration of that reactant
  • The overall order of reaction is the sum of the powers of the reactants in a rate equation
  • For example, in the following rate equation, the reaction is:

Rate = k [NO]2 [H2]

    • Second-order with respect to NO
    • First-order with respect to H2
    • Third-order overall (2 + 1)

Half-life

  • The half-life (t1/2) is the time taken for the concentration of a limiting reactant to become half of its initial value
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top