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

**Example**

Reaction between calcium carbonate and hydrochloric acid.

CaCO

The rate of the reaction can be determined by_{3}+ 2HCl → CaCl_{2}+ CO_{2}+ H_{2}Oa. Measuring the reduces of the mass of the calcium carbonate (reactants) over time

$$\text{RateofReaction=}\frac{{\text{MassofCaCO}}_{\text{3}}\text{reacted}}{\text{Timetaken}}$$

b. Measuring the increases of the volume of the carbon dioxide gas produced over time.

$$\text{RateofReaction=}\frac{{\text{VolumeofCO}}_{\text{2}}\text{produced}}{\text{Timetaken}}$$

**Example**:

In a reaction, 5 g of calcium carbonate takes 250 seconds to completely react with 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]

**Answer**:

a.

$$\begin{array}{l}\text{RateofReaction}\\ \text{=}\frac{5g}{250s}\\ =0.02g{s}^{-1}\end{array}$$

b. $$\begin{array}{l}{\text{RelativeFormulaMassofcalciumcarbonate(CaCO}}_{3}\text{)}\\ =40+12+3(16)=100\\ \\ {\text{NumberofmoleofCaCO}}_{3}\\ =\frac{5g}{100gmo{l}^{-1}}\\ =0.05mol\\ \\ \text{RateofReaction}\\ \text{=}\frac{0.05mol}{250s}\\ =0.0002mol{s}^{-1}\end{array}$$