# Average Rate of Reaction – Measurable Quantities

## Finding Average Rate of Reaction from measurable quantities

1. During a chemical reaction, two things happen
1. The quantities of reactants reduce.
2. The quantities of products increase.
2. Therefore, the rate of the reaction can be determined by
1. measuring the decrease of the amount of the reactants over time.
2. measuring the increase in the amount of products over time.
3. However, the quantity (or change) of the reactants/product may be measurable or immeasurable.
4. The easily measured quantity changes include
1. Mass
2. Concentration (Conductivity)
3. Volume of gas

Example
The reaction between calcium carbonate and hydrochloric acid.

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

The rate of the reaction can be determined by

1. Measuring the reduces of the mass of the calcium carbonate (reactants) over time
$\begin{gathered} {\text{Rate or Reaction }} \hfill \\ {\text{ = }}\frac{{{\text{Mass of CaC}}{{\text{O}}_3}}}{{{\text{Time Taken for Reaction}}}} \hfill \\ \end{gathered}$
2. Measuring the increases of the volume of the carbon dioxide gas produced over time.
$\begin{gathered} {\text{Rate or Reaction }} \hfill \\ {\text{ = }}\frac{{{\text{Volume of C}}{{\text{O}}_2}{\text{ Produced}}}}{{{\text{Time Taken for Reaction}}}} \hfill \\ \end{gathered}$

Example:
In a reaction, 5 g of calcium carbonate takes 250 seconds to completely react with the solution of hydrochloric acid. Calculate the average rate for this reaction in units
(a) g s-1 and
(b) mol s-1
[ Relative atomic mass: C 12; O, 16; Ca, 40]

a.
Rate of Reaction
= 5g/250s
=0.02gs-1

b. Relative Formula Mass of calcium carbonate (CaCO 3 )
=40+12+3(16) = 100

Number of mole of CaCO3
= 5g/100gmol-1
=0.05 mol

Rate of Reaction
= 0.05mol/250s
=0.0002mols-1

We shall discuss the immeasurable quantity (or changes) in next section.