CB08 - Chemical Equilibrium

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 Kc and Kp. 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.

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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.

Lesson 1: Introduction to chemical equilibrium (16)

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 N2O4 2NO2 example. Comparing reaction rates from plots of conc’ns vs. time; catalysts.

Lesson 2: LeChâtelier Principle (8)

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.

Lesson 3: Equilibrium expressions: Q and K (8)

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.

Lesson 4: More on equilibrium expressions (17)

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 CaCO3. Hydrates; Kp for decomposition of CuSO4·5H2O 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 Kp at boiling point. Condition at which vapor pressures of water and ice are identical. Relative humidity produced by Na2SO4·10H2O in a closed container.

Equilibrium Calculations

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

Lesson 5: Kp for the dissociation of a solid (2).

Solid ammonium hydrosulfide dissociates according to NH4HS(s) NH3(g) + H2S(g).

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

Lesson 6: Dissociation of nitrosyl chloride (8).

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

2NOCl(g) 2NO(g) + Cl2(g)

In a certain experiment, 2.0 moles of NO, 3.0 moles of Cl2, and 1.4 moles of NOCl were introduced into a 10 liter container.

a) What is the value of the equilibrium quotient Qc under these conditions?

b) After equilibrium was reached, there were 3.2 moles of NOCl in the container. Use this information to evaluate Qc and the equilbrium constant Kc.

Kc for Formation of an ester.

The next two lessons refer to the reaction

C2H5OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

Lesson 7: K from equilibrium composition (3)

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 Kc.

Lesson 8: Equilibrium composition from Kc (5)

Calculate the equilibrium composition after 2 mol of alcohol and 5 mol of acid have been combined if Kc=4.

Lesson 9: Percent dissociation of PCl5 (5)

If Kp = 1.78 at 250°C, calculate the percentage of PCl5 that will be dissociated when 0.07 mol of PCl5 is placed in a closed vessel at 250°C and the total pressure at equilibrium is 2 atm.

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