AP Chemistry Chemical Equilibria Concepts and Exercises

AP Chemistry Chemical Equilibria Concepts and Exercises

Chemical equilibria are fundamental concepts in AP Chemistry, focusing on the dynamic balance between reactants and products in reversible reactions. This resource explores the equilibrium state, the significance of the equilibrium constant (K), and the factors affecting equilibrium positions. It includes exercises on writing equilibrium expressions, calculating equilibrium constants, and applying Le Chatelier's Principle. Ideal for AP Chemistry students preparing for exams, this guide provides essential insights into reaction dynamics and equilibrium calculations.

Key Points

  • Explains the nature of equilibrium and the dynamic state of reactions.
  • Covers the Law of Mass Action and how to write equilibrium expressions.
  • Includes exercises for calculating equilibrium constants from given concentrations.
  • Discusses Le Chatelier's Principle and its application to predict shifts in equilibrium.
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  • MR ALi
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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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End of Document
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Faqs of AP Chemistry Chemical Equilibria Concepts and Exercises
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 a reaction and the direction in which it will shift under various conditions. It is crucial for students to grasp how K changes with temperature and how it relates to the stoichiometry of the balanced equation.
How does Le Chatelier's Principle apply to chemical reactions?
Le Chatelier's Principle states that if a system at equilibrium experiences a change in concentration, pressure, or temperature, the equilibrium will shift to counteract that change. For example, adding more reactants will shift the equilibrium toward the products, while removing products will shift it back toward the reactants. This principle is essential for predicting how systems respond to external stresses, making it a key concept in understanding chemical equilibria.
What are the steps to calculate equilibrium concentrations using a RICE table?
To calculate equilibrium concentrations, students should set up a RICE table, which stands for Reaction, Initial concentrations, Change, and Equilibrium concentrations. First, write the balanced chemical equation. Then, fill in the initial concentrations of all species involved. Next, determine the changes in concentration in terms of a variable (x) and express the equilibrium concentrations based on these changes. Finally, use the equilibrium expression to solve for x and find the equilibrium concentrations.
What factors can affect the position of equilibrium in a reaction?
Several factors can affect the position of equilibrium, including changes in concentration, pressure, and temperature. For instance, increasing the concentration of reactants will shift the equilibrium toward the products, while increasing pressure will favor the side with fewer moles of gas. Additionally, temperature changes can shift equilibrium depending on whether the reaction is endothermic or exothermic. Understanding these factors is crucial for predicting how a reaction will respond to various conditions.