Which best describes a reaction in a state of equilibrium?

a. the rates of the forward and reverse reactions are zero and the concentrations of products

and reactants are equal.

b. the rate of the forward reaction equals the rate of the reverse reaction and the

concentrations of products and reactants are equal.

c. the rates of the forward and reverse reactions are zero and the concentrations of products

and reactants are constant.

d. the rate of the forward reaction equals the rate of the reverse reaction and the

concentrations of products and reactants are constant.

The best statement which describes a reaction in a state of equilibrium is letter D.  The rate of the forward reaction equals the rate of the reverse reaction and the concentrations of products and reactants are constant. 

>Chemical equilibrium is defined as the state in which both reactants and products are present in concentrations which have no further tendency to change with time. This state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are generally not zero, but equal. Thus, there are no net changes in the concentrations of the reactant(s) and product(s).

it's all four options.