# 11: Chemical Equilibrium

• • Contributed by Stephen Lower
• Professor Emeritus (Chemistry) at Simon Fraser University

The laws of chemical equilibrium define the direction in which a chemical reaction will proceed, as well as the quantities of reactants and products that will remain after the reaction comes to an end. An understanding of chemical equilibrium and how it can be manipulated is essential for anyone involved in Chemistry and its applications.

• 11.1: Introduction to Chemical Equilibrium
Chemical change is one of the two central concepts of chemical science, the other being structure. The very origins of Chemistry itself are rooted in the observations of transformations such as the combustion of wood, the freezing of water, and the winning of metals from their ores that have always been a part of human experience. It was the quest for some kind of constancy underlying change that led the Greek thinkers of around 200 BCE to the idea of elements and later to that of the atom.
• 11.2: Le Chatelier's Principle
The previous Module emphasized the dynamic character of equilibrium as expressed by the Law of Mass Action. This law serves as a model explaining how the composition of the equilibrium state is affected by the "active masses" (concentrations) of reactants and products. In this lesson, we develop the consequences of this law to answer the very practical question of how an existing equilibrium composition is affected by the addition or withdrawal of one of the components.
• 11.3: Reaction Quotient
Consider a simple reaction such as the gas-phase synthesis of hydrogen iodide from its elements:  $$H_2 + I_2 \rightarrow 2 HI$$ Suppose you combine arbitrary quantities of $$H_2$$, $$I_2$$ and $$HI$$. Will the reaction create more HI, or will some of the HI be consumed as the system moves toward its equilibrium state? The concept of the reaction quotient, which is the focus of this short lesson, makes it easy to predict what will happen.
• 11.4: Equilibrium Expressions
You know that an equilibrium constant expression looks something like K = [products] / [reactants]. But how do you translate this into a format that relates to the actual chemical system you are interested in? This lesson will show you how to write the equilibrium constant expressions that you will need to use when dealing with the equilibrium calculation problems in the chapter that follows this one.
• 11.5: Equilibrium Calculations
This page presents examples that cover most of the kinds of equilibrium problems you are likely to encounter in a first-year university course. Reading this page will not teach you how to work equilibrium problems! The only one who can teach you how to interpret, understand, and solve problems is yourself.
• 11.6: Phase Distribution Equilibria
If two immiscible liquid phases are in contact and one contains a solute, how will the solute tend to distribute itself between the two phases? One’s first thought might be that some of the solute will migrate from one phase into the other until it is distributed equally between the two phases. This, however, does not take into the account the differing solubilities the solute might have in the two solvents; the solute will preferentially migrate into the phase in which it is more soluble.