The lessons in this group introduce the concept of equilibrium at an elementary level, and go on to describe the LeChâtelier principle, the equilibrium quotient Q, and the equilibrium constants K_{c} and K_{p}. Most of this will be suitable for all levels of General Chemistry. The section on equilibrium calculations is a bit more advanced, and might well be skipped by students in “prep” courses. The problem exercises in this section illustrate the use of iterative approximation and graphic calculations to solve quadratic equations.

Please note that certain special types of equilibria such as acid-base, oxidation-reduction and solubility are not covered in this lesson set.

Number of lessons |
9 |

Chem1 Reference Text |
pdf and Web versions containing all major illustrations appearing in the lessons |

Note: parenthesized numbers in lesson titles indicate number of interactions in lesson.

Definition of equilibrium, reversible reactions; HI synthesis as example of an incomplete reaction; identifying equilibrium state on conc’n-vs-time plot; reversibility; dynamic nature of equilibrium with forward and reverse rates identical; Law of Mass Action. Criteria for equilibrium; examples judged against these criteria. Finding extent of reaction in N_{2}O_{4} 2NO_{2} example. Comparing reaction rates from plots of conc’ns vs. time; catalysts.

Shifts in composition resulting from non-identity of forward and reverse rates; how adding more reactant can bring this about; effects on conc’n-time plot for HI synthesis. LeChâtelier principle; using it to predict effects of changes in various reaction systems.

Extent of reaction; the equilibrium quotient Q, writing equilibrium expressions. The equilibrium constant K as a special case of Q. How ratio Q/K affects “shift” of reaction to right or left.

Dealing with unchanging concentrations. Reactions in aqueous solution involving water; molar concentration of pure water. Definition of a heterogeneous reaction; example of thermal decomposition of CaCO_{3}. Hydrates; K_{p} for decomposition of CuSO_{4}·5H_{2}O from its vapor pressure. Vapor pressure of water and its equilibrium constant; [non]-influence of liquid volume. Finding Kp from vapor pressure table; value of K_{p} at boiling point. Condition at which vapor pressures of water and ice are identical. Relative humidity produced by Na_{2}SO_{4}·10H_{2}O in a closed container.

This section is presented as a series of guided problem exercises that are intended to illustrate various aspects of equilibrium calculations.

Solid ammonium hydrosulfide dissociates according to NH_{4}HS(s) NH_{3}(g) + H_{2}S(g).

If the total gas pressure in a sealed container of ammonium hydrosulfide is 280 torr, find the value of K_{p} at this temperature.

Nitrosyl chloride is an orange gas that dissociates at high temperatures into chlorine and nitric oxide:

2NOCl(g) 2NO(g) + Cl_{2}(g)

In a certain experiment, 2.0 moles of NO, 3.0 moles of Cl_{2}, and 1.4 moles of NOCl were introduced into a 10 liter container.

a) What is the value of the equilibrium quotient Q_{c} under these conditions?

b) After equilibrium was reached, there were 3.2 moles of NOCl in the container. Use this information to evaluate Q_{c} and the equilbrium constant K_{c}.

The next two lessons refer to the reaction

C_{2}H_{5}OH(l) + CH_{3}COOH(l) CH_{3}COOC_{2}H_{5}(l) + H_{2}O(l)

When 1 mole of ethanol reacts with 1 mole of acetic acid, the equilibrium mixture consists of 3/7 mole each of water and the ester, ethyl acetate. Use this information to evaluate K_{c}.

Calculate the equilibrium composition after 2 mol of alcohol and 5 mol of acid have been combined if K_{c}=4.

If K_{p} = 1.78 at 250°C, calculate the percentage of PCl_{5} that will be dissociated when 0.07 mol of PCl_{5} is placed in a closed vessel at 250°C and the total pressure at equilibrium is 2 atm.