AP Chemistry Chemical Equilibria Concepts

AP Chemistry Chemical Equilibria Concepts

Chemical equilibria in AP Chemistry explores the dynamic balance between reactants and products in reversible reactions. Key concepts include the equilibrium constant (K), the Law of Mass Action, and how changes in concentration, pressure, and temperature affect equilibrium positions. This resource is essential for AP Chemistry students preparing for exams, covering critical topics such as writing equilibrium expressions and calculating K values. It also includes exercises and applications of Le Chatelier’s Principle, providing a comprehensive understanding of chemical equilibria.

Key Points

  • Explains the nature of equilibrium and dynamic reactions in chemistry.
  • Covers the Law of Mass Action and equilibrium constant expressions.
  • Includes exercises for writing equilibrium expressions and calculating K values.
  • Discusses Le Chatelier’s Principle and its applications in predicting shifts in equilibrium.
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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 AP Chemistry Chemical Equilibria Concepts

What is the significance of the equilibrium constant K?
The equilibrium constant K quantifies the ratio of product concentrations to reactant concentrations at equilibrium for a given reaction. A K value greater than 1 indicates that products are favored at equilibrium, while a K value less than 1 suggests that reactants are favored. Understanding K is crucial for predicting the direction of a reaction and for calculating concentrations at equilibrium. It is also temperature-dependent, meaning that changes in temperature can alter the value of K.
How do changes in concentration affect chemical equilibrium?
According to Le Chatelier’s Principle, if a system at equilibrium experiences a change in concentration of reactants or products, the equilibrium will shift to counteract that change. For example, adding more reactants will shift the equilibrium towards the products to restore balance. Conversely, removing products will also shift the equilibrium towards the products. This principle helps in predicting how a reaction will respond to various stresses, making it a vital concept in chemical equilibria.
What role does temperature play in affecting equilibrium?
Temperature changes can significantly impact the position of equilibrium in a chemical reaction. For endothermic reactions, increasing temperature shifts the equilibrium towards the products, as heat is absorbed. In contrast, for exothermic reactions, raising the temperature shifts the equilibrium towards the reactants, as heat is released. Understanding this relationship is essential for manipulating reaction conditions in both laboratory and industrial settings.
What is the Law of Mass Action?
The Law of Mass Action states that at equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, leading to a constant ratio of concentrations of products and reactants. This law provides the foundation for writing equilibrium expressions, which are used to calculate the equilibrium constant K. It emphasizes that the concentrations of reactants and products remain constant over time, highlighting the dynamic nature of chemical reactions.
How can equilibrium expressions be written for reactions?
Equilibrium expressions are derived from the balanced chemical equation of a reaction. For a general reaction of the form aA + bB ⇌ cC + dD, the equilibrium expression is K = [C]^c[D]^d / [A]^a[B]^b, where square brackets indicate molarity. Pure solids and liquids do not appear in the expression, as their concentrations remain constant. This method allows chemists to quantify the relationship between reactants and products at equilibrium.

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