Chemical Equilibrium

Many chemical reactions go to completion; i.e., all the reactants are converted to products. A good example is the reaction of calcium with cold water. . . Ca(s)+2 H2O(l) → Ca(OH)2(s)+ H2(g) . . . There is no evidence that the reverse reaction occurs. Such reactions are said to be irreversible. On the other hand, many reactions are reversible: the process can be made to go in the opposite direction. This means that both the reactants and products will be present at any given time. A reversible reaction is defined as one in which the products formed can react to give the original reactants. A double arrow is used to indicate that the reaction is reversible, as illustrated by the general equation:. . . aA+bB ⇌ cC +dD. . . At the start of the reaction, the reactants convert more quickly to products than products turn back to reactants because the reactants are present in much greater amount. Eventually the concentration of products is sufficient for the reverse reaction to become significant. The reaction is said to reach equilibrium when the net change in the products and reactants is zero, i.e., the rate of forward reaction equals the rate of reverse reaction. Chemical equilibria are dynamic equilibria because, although nothing appears to be happening, opposing reactions are occurring at the same rate. Figure 17-1 illustrates that for a reaction in chemical equilibrium, the rate of forward reaction equals the rate of reverse reaction. When a chemical reaction is at equilibrium, the concentrations of reactants and products are constant. The relationship between the concentrations of reactants and products is given by the equilibrium expression, also known as the law of mass action. For the general reaction:. . . aA+bB cC +dD. . . at a constant temperature, the equilibrium constant expression is written as: Kc = [C]c[D]d/[A]a[B]b where [A], [B], [C], and [D] are the molar concentrations or partial pressures of A,B,C, and D at equilibrium. The exponents a,b, c, and d in the equilibrium expression are the coefficients in the balanced equation; Kc is the equilibrium constant and is not given units. The subscript c shows that K is in terms of concentration. The numerical value for Kc is usually determined experimentally.