0.20 Reaction equilibrium in the gas phase

Introduction

We performed “chemical algebra” or stoichiometric calculations in the early Concept Development Studies, using a balanced chemical equation to determine the masses or numbers of moles of product created from the masses or numbers of moles of the reactants. For example, if we burn methane gas, CH ₄ (g), in excess oxygen, the reaction

CH ₄ (g) + 2O ₂ (g) → CO ₂ (g) + 2 H ₂ O(g)

occurs, and we assumed, correctly in this case, that the number of moles of CO ₂ (g) produced is equal the number of moles of CH ₄ (g) we start with. This follows directly from the balanced equation but requires us to assume that all of the CH ₄ is converted into CO ₂ during the reaction.

From our study of phase transitions and solubility, we have learned the concept of equilibrium. We observed that, in the transition from one phase to another for a substance, both phases are found to coexist under certain conditions, and we refer to this as phase equilibrium. For example, when a liquid is in equilibrium with its vapor, not all of the liquid converts into vapor and not all of the vapor converts into the liquid. In this case, we would not be able to calculate the number of moles of vapor from the number of moles of liquid.

It should not surprise us that these same concepts of equilibrium used to describe the coexistence of phases can apply to chemical reactions as well. In Reaction (1) above, therefore, we should determine whether the reaction actually produces exactly one mole of CO ₂ for every mole of CH ₄ we start with or whether we wind up with an equilibrium mixture containing both CO ₂ and CH ₄ . In this case, the answer is that the reaction does “go to completion,” meaning that with very high accuracy we can assume that every CH ₄ reacts to produce a CO ₂ . However, in studying many reactions, we will find that different reactions provide us with varying outcomes. In many cases, virtually all reactants are consumed, producing the amount of product predicted by stoichiometric calculation. In many other cases, substantial amounts of reactant are still present along with product when the reaction achieves equilibrium. And in other cases, almost no product is present at equilibrium. Our goal will be to understand, describe, and predict the reaction equilibrium.

An important corollary to this goal is to attempt to control the equilibrium. We will find that varying the conditions under which the reaction occurs can vary the amounts of reactants and products present at equilibrium. We will develop a general principle for predicting how the reaction conditions affect the amount of product produced at equilibrium.

Foundation

In beginning our study of the reactions of gases, we will assume a knowledge of the physical properties of gases as described by the Ideal Gas Law and an understanding of these properties as given by the postulates and conclusions of the Kinetic Molecular Theory. We assume that we have developed a dynamic model of phase equilibrium in terms of competing rates. We will also assume an understanding of the bonding, structure, and properties of individual molecules.