Fundamentals of Chemistry II
Reaction equations describe substances, called reactants, which when put together, react and produce other different substances, called products. It may appear as if only products remain after the reaction is finished. In reality, many reactions do not go to completion, even if the reactants are present in stoichiometric ratios or amounts. Rather, they reach a condition known as equilibrium, denoted by double, reversible arrows in the reaction equation. Equilibrium means that there is a balance between the reactant side and the product side, or simply between the reactants and the products, and that the reaction is reversible. The chemistry of many air pollutants falls under the heading of equilibrium reactions. The equilibrium condition is dynamic, not static, allowing microscopic changes in reactant and product concentrations to take place, such that no net change in reactant or product concentrations occurs, provided that no external stresses are applied. At any given time, all species in the reaction equation— reactants and products—are present at equilibrium in varying amounts. The relationship among these varying amounts can be described by a mathematical formula known as the equilibrium constant expression or simply the equilibrium expression. The equilibrium expression is set equal to an equilibrium constant symbolized by Kc.
An equilibrium reaction can be generally represented as:
The Kc expression can then be expressed as:
where a, b, g, and h represent the stoichiometric coefficients in the balanced reaction, and the brackets [ ] indicate molar concentrations. The simplest interpretation of Kc is that it is a measure of the extent to which a reaction goes toward completion, that is, a reaction where the product side is favored. The meaning of Kc is discussed in more detail later in this section.
Four points of caution regarding the use of this expression are noteworthy:
- • Only concentrations in units of molarity are permitted.
- • Only species that have concentrations can appear. Thus, species in the solution state (aq) or in the gaseous state (g) are included, but species that are pure solids (s) or liquids (l) are not.
- • Kc is a function of temperature (check the van’t Hoff equation, equation 2.5, later in this chapter).
- • All concentrations substituted into this expression must be taken at equilibrium.
A reaction in which all species are present in the same physical state (e.g., all gases or all solutions) is called a homogeneous reaction, while a reaction in which species are present in more than one physical state (i.e., mixed physical states) is called a heterogeneous reaction. If all species in a reaction are present in the gaseous state, an alternative form of the equilibrium constant expression can be written as Kp:
Here, P represents the partial pressure of each gas present at equilibrium. Any pressure unit is acceptable as long as it is consistent with the others and with the units of Kp, but atmospheres and torr are common.
A formula to convert Kc to Kp, and vice versa is as follows:
T = the absolute temperature in kelvins
R = the gas constant to be used in units consistent with partial pressure hn = the change in the number of moles of gas, that is, the total moles of product minus the total moles of reactant in the balanced reaction.
Write the equilibrium constant expression Kc for the following reaction at 25°C:
Note that this is a homogeneous, gas-phase reaction. All three substances can have concentrations in units of molarity and should appear in the Kc expression:
Given a value for Kc and asked to convert it to Kp, then, since all species are in the gaseous state, the equation would be:
Note that although the temperature is given, it is not used in the calculation. It may be useful in other calculations, however.
Sulfur dioxide, SO2 (g), is a common air pollutant. For the reaction
At 827°C, the value for Kc = 37.1. What is the value for the reverse reaction?