Define reaction rate, assuming constant temperature and a closed reaction vessel, why does the rate change with time?

The rate of a reaction is always depended on the factors like temperature, pressure of the vessel, surface area of the reactants, use of catalyst, etc. The catalysts are the substance that can be used to increase the rate of a reaction by decreasing the activation energy of the reactant substances.

The rate of the reaction or the reaction rate is the rate or speed at which the reactant substances in a closed vessel undergo reaction at a constant temperature. The reaction rate always changes with time. The change can be positive (increase) or negative (decrease). This change is due to the change in concentration of the reactant substances with time. As time goes, the reactant undergoes reaction to give the products.