In this chapter of A2/A-Level Chemistry, we will learn how to find the rates of reaction with the Rate Equations.

Rate Equations

Rate equation = tell you how the rate is affected by the concentrations of reactants.

E.G. **Rate = k [A] ^{m }[B]^{n}**

Where:

m = order of A

n = order of B

n+m = overall order

k = rate constant (always the same for a reaction at specific temp and pressure, increase temp = increase k = bigger value of k = faster reaction)

__EXAMPLE__

Propanone + Iodine —> Iodopropanone + H^{+} + I^{-} (reaction occurs in acid)

Info: First order with respect to propanone and H^{+} and zero order with respect to iodine

Rate equation = k[propanone]^{1}[H^{+}]^{1}[iodine]^{0}

Simplify to;

Rate equation = k[propanone][H^{+}] (because anything to the power of 0 is 1)

__In A2/A-Level Chemistry, How to calculate rate constant from the orders and rate?__

Rearrange to make k the subject and calculate.

Units of k can be found as you know concentration is moldm^{-3} and rate is moldm^{-3}s^{-1} using a normal “cancelling” method.

__Using data to deduce the order__

1. The experiment: titrate sample solutions against sodium thiosulfate and starch to work out the concentration of the iodine. Repeat experiment, changing only the concentration for ONE REACTANT at a time.

Here, first we changed the concentration of propanone for experiments 1, 2 and 3.

Then, we changed the concentration of iodine in experiments 4 and 5.

Lastly, we changed concentration of H^{+} in experiments 6 and 7.

2. From this table we can plot 7 Concentration-Time graphs. Finding the gradient at time zero for each of these plots will give us the INITIAL rate of each.

3.Compare the results e.g.

*Reaction rates won’t be exactly double or treble due to experimental errors etc.

4. Now we can work out the rate equation:

- Rate is proportional to [propanone] so the reaction is of order 1 with respect to propanone.
- Rate does not change/is independent of [iodine] so the reaction is of order 0 with respect to iodine.
- Rate is proportional to [H
^{+}] so the reaction is of order 1 with respect to [H^{+}].

In A2/A-Level Chemistry, Rate determining step = slowest step in a multi-step reaction

(if a reactant appears in the rate equation it MUST be a rate determining step including catalysts which may appear in a rate equation)

That's how the rate equations and the calculation works. Good job!