# Average Rate of Reaction – Immeasurable Quantities

## Finding Average Rate of Reaction from Immeasurable Quantities

1. In some reaction, some changes are observable but difficult to be measured. For example
1. Change in colour
2. Precipitation
2. The time taken for the colour of a reactant to change or certain amount of precipitate to form can be used to measure the rate of reaction.
3. If the quantity change is immeasurable
$\begin{gathered} {\text{Rate or Reaction }} \hfill \\ {\text{ = }}\frac{1}{{{\text{Time Taken for the Reaction}}}} \hfill \\ \end{gathered}$
Example When the aqueous of ethanadioic is mixed with acided potassium manganate(VII) , the reaction happen slowly at room temperature. The purple colour of the solution is bleached after 20 seconds. Calculate the average rate of reaction. Answer Rate of reaction = 1/time = 1/25 =0.04 s-1

Example

2HCl(aq) + Na2S2O3(aq)
→2NaCl(aq) + S(s) + SO2(g) + H2O(l)

In a reaction between dilute hydrochloric acid and sodium thiosulphate, sulphur precipitate was produced after 2 minutes. Find the average rate of the reaction?