AP Chemistry Chemical Equilibria General Concepts

AP Chemistry Chemical Equilibria General Concepts

Chemical equilibria is a fundamental concept in AP Chemistry, focusing on the dynamic balance between reactants and products in reversible reactions. This guide explains the nature of equilibrium, the equilibrium position, and the significance of the equilibrium constant (K). It covers essential topics such as writing equilibrium expressions, the impact of changing stoichiometric coefficients, and the role of Le Chatelier’s Principle in predicting shifts in equilibrium. Ideal for AP Chemistry students preparing for exams, this resource provides clear explanations and exercises to reinforce understanding of chemical equilibria.

Key Points

  • Explains the dynamic nature of chemical equilibrium and its significance in reactions.
  • Covers the equilibrium constant expression and its calculation for various reactions.
  • Includes exercises for writing equilibrium expressions and calculating K values.
  • Discusses Le Chatelier’s Principle and its application in predicting equilibrium shifts.
, ,
  • newtopiccyclegrowin
332
/ 11
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
/ 11
End of Document
332
You May Also Like

FAQs of AP Chemistry Chemical Equilibria General Concepts

What is the significance of the equilibrium constant K?
The equilibrium constant K is a crucial value that 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 chemists predict the extent of a reaction and how changes in conditions can affect the equilibrium position. It is essential for students to grasp how to calculate K from equilibrium concentrations and how it relates to reaction dynamics.
How do changes in concentration affect chemical equilibrium?
According to Le Chatelier’s Principle, if a stress is applied to a system at equilibrium, the equilibrium will shift in a direction that reduces that stress. For example, adding more reactants will shift the equilibrium toward the products to re-establish balance. Conversely, removing products will also cause the equilibrium to shift toward the right. This principle is vital for predicting how changes in concentration, pressure, or temperature can influence the outcome of chemical reactions.
What are the steps for calculating equilibrium concentrations?
Calculating equilibrium concentrations involves setting up a RICE table, which stands for Reaction, Initial concentrations, Change, and Equilibrium concentrations. First, write the balanced chemical equation and input initial concentrations. Next, determine the changes in concentrations using a variable (often x) to express how much the concentrations will change as the system reaches equilibrium. Finally, substitute these values into the equilibrium expression to solve for x and find the equilibrium concentrations of all species.
What role do pure solids and liquids play in equilibrium expressions?
In equilibrium expressions, pure solids and liquids do not appear because their concentrations remain constant and do not affect the equilibrium position. For example, in a reaction involving a solid catalyst or a liquid solvent like water, these components are omitted from the equilibrium constant expression. This simplification is crucial for accurately calculating the equilibrium constant and understanding the dynamics of the reaction.
How does temperature affect the equilibrium constant?
Temperature changes can significantly impact the value of the equilibrium constant K. For endothermic reactions, increasing temperature generally raises the value of K, favoring product formation. Conversely, for exothermic reactions, increasing temperature typically decreases K, favoring reactants. Understanding this relationship is essential for predicting how temperature variations influence chemical equilibria and reaction yields.

Related of AP Chemistry Chemical Equilibria General Concepts