Name: __________ _________________________
CHEM 1212 Module 7 Reading Guide
Equilibrium
Reversible reactions may proceed in both forward (left to right) and reverse (right to left) directions. Virtually all chemical reactions are __________ to some extent. Chemical __________ is the dynamic situation in which the forward and reverse occur at the same __________ , resulting in no __________ over time.
Under a given set of conditions, the composition of the equilibrium mixture is determined by the magnitudes of the __________ constants for the forward and the reverse reactions. This is described by the equilibrium constant, K, expressed as the concentrations of the __________ over the concentrations of the __________ , each raised to the power of its __________ in the balanced chemical equation. By convention, equilibrium constants are expressed without __________ because each term is referenced against a standard state (1 M or 1 bar).
For the general reaction ![]()
the equilibrium constant is expressed as ![]()
The law of __________ says that the ratio of products to reactants in the equilibrium expression is __________ for a system that is at equilibrium.
The value of K depends only on the specific chemical equation and is __________ dependent. It does not depend on any of the other factors that can affect the rate of a reaction, such as the __________ of the species in the reaction, the particle size of a solid in a reaction, or the presence of a __________ . The magnitude of K does not indicate how rapidly or slowly equilibrium will be reached. In a __________ equilibrium, the species are not all in the same phase. __________ and pure __________ are not included in equilibrium constant expressions.
A reaction exhibiting a __________ K will reach equilibrium when most of the reactant has been converted to product, whereas a small K indicates the reaction achieves equilibrium after __________ reactant has been converted. Generally, we say that if K > 1, the reaction is product-favored, and if K < 1, the reaction is __________ , but it is when the value of K is greater than 1000 that the reactants demonstrate a strong tendency to form __________ . Similarly, when the value of K is less than 0.001, the reactants do not form products readily and there is little __________ present at equilibrium. Between these values, there are significant amounts of both reactants and products at equilibrium. Practically speaking, this reaction would be considered incomplete and changes to the reaction conditions can be made to produce a better outcome.
In order to differentiate from rate constant, the equilibrium constant is written with a capital K. There are several different labels that appear as subscripts on K to give you a little more information. eq = equilibrium C= concentration P = __________
If amounts of gases are expressed as partial pressures, the number of moles of gas in the reaction and the ideal gas law are used to convert between Kc and KP. ![]()
Practice Problems
4. Determine whether the following statements are true or false. Correct the false statements.
a. The concentrations of the products are equal to the concentrations of the reactants at equilibrium.
b. The rates of the forward and reverse reactions are the same at equilibrium.
5. Consider the reversible reaction A + B ⇌ C + D.
c. If A and B are placed together in a flask and allowed to react, which species will be present at equilibrium?
d. If C and D are placed together in a flask and allowed to reaction, which species will be present at equilibrium?
6. For the following reactions, determine whether the forward or reverse reaction is favored.
a. 2 SO3(g) ⇌ 2 SO2(g) + O2(g) K = 2.5 × 1025
b. N2(g) + O2(g) ⇌ 2 NO(g) K = 4.1 × 10−31
c. H2(g) + Cl2(g) ⇌ 2 HCl(g) K = 5.1 × 108
9. Write the equilibrium expression, Kc, for each of the following reactions.
a. Ag2CrO4(s) ⇌ 2 Ag+(aq) + CrO42−(aq)
b. 2 NO(g) + O2(g) ⇌ 2 NO2(g)
c. HCN(aq) + OH−(aq) ⇌ CN−(aq) + H2O(l)
Calculating K and Q
The __________ for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these concentrations are known, the calculation is performed by substituting the values into the K expression. Alternatively, the equilibrium concentration of one species can be calculated if K and the equilibrium __________ of all the other species are known.
When a chemical equation is manipulated, its equilibrium constant also changes to represent the new reaction.
· When a reaction is reversed, its K value is __________ .
· When the coefficients of a reaction are __________ by a factor, the K value is raised to the power of that factor.
· When reactions are summed, their K values are __________ .
Q is the reaction __________ and describes the ratio of products to reactants at any __________ during the reaction. It is determined using the same process as K. The numerical value of Q __________ as a reaction proceeds towards equilibrium. The reaction quotient is used to predict how the concentrations or partial pressures of products and reactants must __________ to reach equilibrium.
If Q = K, the reaction is at equilibrium. If Q > K, the reaction will shift toward the __________ (forming more reactants) to achieve equilibrium. If Q < K, the reaction will shift toward the right (forming more __________ ) to achieve equilibrium.
Practice Problems
25. Consider the following reactions:
½ A + B ⇌ ½ C K1 = 115
3 D ⇌ 3 C K2 = 1.12 × 104
E ⇌ C K3 = 0.0278
Use the information above to calculate the equilibrium constant for the following reaction:
A + 2B ⇌ E K = ?
29. Balance the equation and write the reaction quotient expression, Qc, for the following synthesis of carbon disulfide, CS2:
CH4(g) + H2S(g) ⇌ CS2(g) + H2(g)
ICE
The most challenging type of equilibrium calculation occurs when equilibrium concentrations are derived from __________ concentrations and the equilibrium constant. For these calculations, you typically follow these steps:
1. Identify the __________ in which the reaction will proceed to reach equilibrium using Q.
2. Develop an ICE table.
3. Calculate the concentration changes and, subsequently, the equilibrium concentrations.
An ICE table is used to organize the information used to solve concentration problems for reversible reactions. The ICE table consists of a column for each species in the reaction and three rows. The first row represents the __________ concentrations (not the number of moles) of all species, the second row represents how the species will __________ based on the reaction stoichiometry, and the final row represents the concentrations of all species at __________ .
The mathematics to solve for the equilibrium changes can become complicated. In order to simplify problems, if the initial concentrations of reactants are __________ and the equilibrium constant is relatively __________ , we assume the concentration changes are negligible and remove the ‘-x’ from the equilibrium concentration expression of the reactants. In order for this assumption to be valid, the change represented by x must be less than _____ of the initial concentration.
Practice Problems
What are the concentrations of each species in the reaction when you reach equilibrium of a 0.15 M solution of HCN?
HCN(aq)⇌H+(aq)+CN−(aq) Kc = 4.9×10−10
Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@13.1.;
White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.;
Chemistry LibreTexts, Libretexts, [online] Available from: https://chem.libretexts.org/Courses/Heartland_Community_College/HCC:_Chem_161/5:_Thermochemistry/5.7:_Enthalpy_of_Formation.
Reading guide style Adapted from “Ionic equations Study Guide” by Montgomery College is licensed under CC BY 4.0
Document not to be reposted on the internet without express permission.
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