General Equilibrium Concepts in Chemistry

General Equilibrium Concepts in Chemistry

General Equilibrium explores the principles of chemical equilibria, focusing on the dynamic balance between reactants and products in reversible reactions. It covers key concepts such as the equilibrium constant, Le Chatelier's Principle, and the factors influencing equilibrium positions. This resource is essential for AP Chemistry students preparing for exams, providing detailed explanations and exercises related to equilibrium expressions and calculations. Topics include the Law of Mass Action, the significance of K values, and practical applications in various chemical reactions.

Key Points

  • Explains the nature of equilibrium in chemical reactions and its dynamic characteristics.
  • Covers the Law of Mass Action and how to write equilibrium expressions for various reactions.
  • Discusses the impact of temperature, pressure, and concentration changes on equilibrium positions.
  • Includes exercises for calculating equilibrium constants and predicting shifts using Le Chatelier's Principle.
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AP* Chemistry
CHEMICAL EQUILIBRIA:
GENERAL CONCEPTS
*AP is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this
product.© 2008 by René McCormick. All rights reserved.
THE NATURE OF THE EQUILIBRIUM STATE: Equilibrium is the state where the rate
of the forward reaction is equal to the rate of the reverse reaction. At these conditions,
concentrations of all reactants and products remain constant with time once equilibrium
has been established at constant temperature. (In stoichiometry, we dealt with equations that
went to completion; often equilibrium equations are going to fall short of this goal.)
Reactions are reversible. This is indicated by double arrows. R
• dynamic--R indicates that the reaction is proceeding in the forward and in the reverse
direction and once equilibrium is established, the rate of each direction is equal. This
keeps the concentration of reactants and products equal.
• the nature and properties of the equilibrium state are the same, no matter what the
direction of approach.
• Examples: Look at the following plot of the reaction between steam and carbon
monoxide in a closed vessel at a high temperature where the reaction takes place rapidly.
H
2
O(g) + CO(g)
R H
2
(g)
+ CO
2
(g)
THE EQUILIBRIUM POSITION: Whether the reaction lies far to the right or to the left
depends on three main factors.
• Initial concentrations (more collisions--faster reaction)
• Relative energies of reactants and products (nature goes to minimum energy)
• Degree of organization of reactants and products (nature goes to maximum disorder)
• The significance of K: K > 1 means that the reaction favors the products at equilibrium
K < 1 means that the reaction favors the reactants at equilibrium
THE EQUILIBRIUM EXPRESSION: A general description of the equilibrium condition
proposed by Gudberg and Waage in 1864 is known as the Law of Mass Action. Equilibrium is
temperature dependent, however, it does not change with concentration or pressure.
• equilibrium constant expression--for the general reaction
aA + bB R cC + dD
Equilibrium constant: K = [C]
c
[D]
d
* Note* K, K
c
, K
eq
may all be used here!
[A]
a
[B]
b
. Chemical Equilibria: General Concepts
2
The product concentrations appear in the numerator and the reactant concentrations in the
denominator. Each concentration is raised to the power of its stoichiometric coefficient in the
balanced equation.
- [ ] indicates concentration in Molarity (mol/L)
- K
c
--is for concentration (aqueous)
- K
p
--is for partial pressure (gases)
- “K” values are often written without units
• USING EQUILIBRIUM CONSTANT EXPRESSIONS
• Pure solids--do not appear in expression—you’ll see this in K
sp
problems soon!
• Pure liquids--do not appear in expression—H
2
O(l) is pure, so leave it out of the
calculation
• Water--as a pure liquid or reactant, does not appear in the expression. (55.5 M will not
change significantly)
o Weak acid and weak base equations are heterogeneous [multi-states of matter;
pure liquid and aqueous components] equilibria.
o Solubility of salts also fits into this category. The initial solid component has a
constant concentration and is therefore left out of the equilibrium expression.
Exercise 1 Writing Equilibrium Expressions
Write the equilibrium expression for the following reaction:
4 NH
3
(g) + 7 O
2
(g) R 4 NO
2
(g) + 6 H
2
O(g)
K = [NO
2
]
4
[H
2
O]
6
[NH
3
]
4
[O
2
]
7
Exercise 2 Equilibrium Expressions for Heterogeneous Equilibria
Write the expressions for K and K
p
for the following processes:
a. The decomposition of solid phosphorus pentachloride to liquid phosphorus trichloride and chlorine gas.
b. Deep blue solid copper(II) sulfate pentahydrate is heated to drive off water vapor to form white solid
copper(II) sulfate.
A: K = [Cl
2
]
K
p
= P
Cl2
B: K = [H
2
O]
5
K
p
= P
H2O
5
. Chemical Equilibria: General Concepts
3
• CHANGING STOICHIOMETRIC COEFFICIENTS
when the stoichiometric coefficients of a balanced equation are multiplied by some
factor, the K is raised to the power of the multiplication factor (K
n
). 2x is K squared;
3x is K cubed; etc.
• REVERSING EQUATIONS
take the reciprocal of K ( 1/K)
• ADDING EQUATIONS
multiply respective Ks (K
1
× K
2
× K
3
…)
Exercise 3 Calculating the Values of K
The following equilibrium concentrations were observed for the Haber process at 127°C:
[NH
3
] = 31 × 10
−2
mol/L
[N
2
] = 8.5 × 10
−1
mol/L
[H
2
] = 3.1 × 10
−3
mol/L
a. Calculate the value of K at 127°C for this reaction.
b. Calculate the value of the equilibrium constant at 127°C for the reaction:
2 NH
3
(g) R N
2
(g) + 3 H
2
(g)
c. Calculate the value of the equilibrium constant at 127°C for the reaction given by the equation:
1
2
N
2
(g) +
3
2
H
2
(g) R NH
3
(g)
A: K = 3.8 × 10
4
B: K’ = 2.6 × 10
-5
C: K” = 1.9 × 10
2
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FAQs of General Equilibrium Concepts in Chemistry

What is the significance of the equilibrium constant K?
The equilibrium constant K indicates the ratio of product concentrations to reactant concentrations at equilibrium. A K value greater than 1 suggests that products are favored, while a K value less than 1 indicates that reactants are favored. Understanding K helps predict the extent of reactions and is crucial for calculations in chemical equilibria.
How does Le Chatelier's Principle apply to chemical equilibria?
Le Chatelier's Principle states that if a stress is applied to a system at equilibrium, the system will shift in a direction that counteracts the stress. This principle helps predict how changes in concentration, pressure, or temperature will affect the position of equilibrium. For example, increasing the concentration of reactants will shift the equilibrium toward the products.
What factors influence the position of equilibrium in a reaction?
The position of equilibrium is influenced by several factors, including initial concentrations of reactants and products, temperature, and pressure. Changes in these factors can shift the equilibrium position to favor either the reactants or products. Understanding these influences is essential for manipulating reaction conditions in chemical processes.
What are the steps to calculate equilibrium concentrations?
To calculate equilibrium concentrations, one can use a RICE table, which stands for Reaction, Initial concentrations, Change, and Equilibrium concentrations. Start by writing the balanced equation, filling in initial concentrations, determining the change in concentrations in terms of a variable (x), and then expressing the equilibrium concentrations. Finally, set up the equilibrium expression and solve for x to find the equilibrium concentrations.
How do temperature changes affect equilibrium constants?
Temperature changes can significantly affect the value of the equilibrium constant K. For endothermic reactions, increasing temperature generally increases K, favoring product formation. Conversely, for exothermic reactions, increasing temperature typically decreases K, favoring reactants. This relationship is crucial for understanding how thermal conditions impact chemical equilibria.

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