Chapter U6.120, Problem 1E

Interpretation Introduction

Interpretation: The reason for the systems at equilibrium to not contain the same amounts of the reactants and the products needs to be explained.

Concept Introduction: Consider an equilibrium reaction

  $aA+bB\rightleftharpoons cC+dD$

Where A and B are the reactants and C and D are the products. a, b, c and d are the stoichiometric coefficients of the reactants and the products in the equilibrium reaction.

The equilibrium constant for the above reaction is written as

  $K=\frac{{\left[C\right]}^c{\left[D\right]}^d}{{\left[A\right]}^a{\left[B\right]}^b}$

Where [..] denote the molar concentrations at equilibrium.

Answer to Problem 1E

The systems at equilibrium do not contain the same amounts of the reactants and the products because the equilibrium is defined with specified amounts of the reactants and the products and these amounts are not equal. At equilibrium, the rates of the two reactions are equal; but the amounts of the reactants and the products are different.

Explanation of Solution

A chemical reaction starts with the conversion of the reactants to the products. As time elapses, the concentrations of the reactants decrease while the concentrations of the products build up. When the concentrations of the products have reached an optimum value, then, depending on the reaction conditions, the products (may) re-combine to give the reactants. The process continues until the rate of the recombination of the products become equal to the rate of the conversion of the reactants. The system attains chemical equilibrium when these two rates are equal.

For a chemical system to be at equilibrium, the concentrations of the reactants and the products must have specified values. These values are definitely not equal; however, the rates of the forward and the reverse reactions are equal at equilibrium.

Conclusion

The concentrations of the reactants and the products are not equal at equilibrium; however, the rates of the forward and the reverse reactions are equal at equilibrium.