The rate of reaction refers to the speed at which the products are formed from the reactants in a chemical reaction
2Al(s) + 3 CuCl2(aq) → 2AlCl3(aq) + 3 Cu(s)
The equation for reaction rate in this chemical reaction (for CuCl₂) is the following one (The value 1/3 comes from the coefficient in the chemical equation for the reaction):
Here, the negative sign is used to indicate the decreasing concentration of the reactant.