Quick questions on Kinetics II (Topic 15) - Edexcel A-Level Chemistry

Compare experiments in which only one concentration changes: - rate unchanged when a concentration doubles, order $0$; - rate doubles, order $1$; - rate quadruples ($\times 2^2$), order $2$.

For a first-order reaction the half-life is constant (independent of concentration), which is the quickest way to spot first order. A first-order rate constant can also be found from $k = \ln 2 / t_{1/2}$. For a zero-order reaction the concentration falls linearly with time; for second order the half-life increases as the reaction proceeds.

If the rate equation is $\text{rate} = k[\text{A}][\text{B}]$ but the overall equation involves two moles of A, the mechanism must have only one A and one B reacting in (or before) the slow step. Proposed mechanisms must be consistent with the experimentally found rate equation.

The rate constant increases with temperature according to the Arrhenius equation, $k = Ae^{-E_a/RT}$, where $A$ is the pre-exponential (frequency) factor. Taking natural logs gives the linear form:

A reaction obeys $\text{rate} = k[A][B]^2$. State the overall order and the effect of doubling $[B]$. [2 marks]

Explain how a constant half-life on a concentration-time graph identifies the order. [1 mark]