AP Chemistry Chemical Equilibria General Concepts

AP Chemistry Chemical Equilibria General Concepts

Chemical equilibria are fundamental concepts in AP Chemistry, focusing on the dynamic state where the rate of the forward reaction equals the rate of the reverse reaction. This guide explores the equilibrium position, the significance of the equilibrium constant (K), and how changes in concentration, pressure, and temperature affect equilibrium states. It includes detailed explanations of the Law of Mass Action and equilibrium expressions, essential for AP Chemistry students preparing for exams. The document also features exercises to practice writing equilibrium expressions and calculating equilibrium constants.

Key Points

  • Explains the dynamic nature of chemical equilibrium and its significance in reactions.
  • Covers the Law of Mass Action and how to write equilibrium expressions for various reactions.
  • Includes exercises for calculating equilibrium constants and understanding their implications.
  • Discusses the effects of concentration, pressure, and temperature changes on equilibrium states.
, ,
  • newtopiccyclegrowin
293
/ 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
293
You May Also Like

FAQs of AP Chemistry Chemical Equilibria General Concepts

What is the Law of Mass Action in chemical equilibria?
The Law of Mass Action states that at equilibrium, the rate of a chemical reaction is constant and can be expressed as a ratio of the concentrations of products to reactants, each raised to the power of their coefficients in the balanced equation. This principle allows chemists to derive the equilibrium constant (K), which quantitatively describes the position of equilibrium for a given reaction at a specific temperature. Understanding this law is crucial for predicting how changes in conditions will affect the equilibrium state.
How does temperature affect the equilibrium constant?
Temperature has a significant impact on the equilibrium constant (K) of a reaction. For endothermic reactions, increasing the temperature shifts the equilibrium position to favor the formation of products, resulting in a higher K value. Conversely, for exothermic reactions, raising the temperature shifts the equilibrium towards the reactants, leading to a lower K value. This relationship is essential for predicting the behavior of chemical systems under varying thermal conditions.
What role do catalysts play in chemical equilibrium?
Catalysts speed up the rate at which equilibrium is reached but do not affect the position of the equilibrium itself or the value of the equilibrium constant (K). By lowering the activation energy for both the forward and reverse reactions equally, catalysts allow the system to achieve equilibrium faster without altering the concentrations of reactants and products at equilibrium. This characteristic makes catalysts vital in industrial processes where time efficiency is crucial.
What are the implications of Le Chatelier's Principle?
Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium. For example, if the concentration of a reactant is increased, the system will shift towards the products to reduce that concentration. This principle is fundamental in predicting how changes in concentration, pressure, or temperature will affect the equilibrium position of a reaction.
How do you calculate equilibrium constants from concentrations?
To calculate the equilibrium constant (K) from concentrations, one must first write the balanced chemical equation and the corresponding equilibrium expression. The concentrations of the products are placed in the numerator and the reactants in the denominator, each raised to the power of their stoichiometric coefficients. By substituting the equilibrium concentrations into this expression, K can be determined, providing insight into the favorability of the reaction at equilibrium.

Related of AP Chemistry Chemical Equilibria General Concepts