Rate of reaction:- It is defined as, the rate at which the concentration of a reactant changes with time. The rate at which a reaction proceeds can be followed by measuring the concentration at either a reactant change during a small interval of time dt, then the rate of reaction is represented by dx/dt. If on other hand  dc represents the concentration of the reactant left behind after a short interval of time dt. then the rate of reaction is represented by dc/dt. The negative sign implies that the concentration of the reactants decreases with time or rate of reaction decreases with time.

Unit of rate:-

∵ Rate of reaction = {Concentration}/{time}

= {mol/litre}/{second}

Unit of rate = mol lit^-1 sec^-1

Factors affecting the rate of reaction:- Following are the factors which affect the rate of reaction-

(i) Concentration of reactants

(ii) Temperature

(iii) Nature of reactants

(iv) Catalyst

(v) Surface area of reactants

All above factors affect the rate of reaction.