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AP Chemistry 7.7 Calculating Equilibrium Concentrations Study Notes

AP Chemistry 7.7 Calculating Equilibrium Concentrations Study Notes - New Syllabus Effective fall 2024

AP Chemistry 7.7 Calculating Equilibrium Concentrations Study Notes- New syllabus

AP Chemistry 7.7 Calculating Equilibrium Concentrations Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Identify the concentrations or partial pressures of chemical species at equilibrium based on the initial conditions and the equilibrium constant.

Key Concepts: 

  • Calculating Equilibrium Concentrations
  • Representations of Equilibrium

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry Exam Questions MCQs and FRQs

7.7.A.1 Predicting Equilibrium Concentrations Using K:

1. Balanced Chemical Equations:

A balanced chemical equation ensures the number of atoms of each element is the same on both sides, following the law of conservation of mass.

Example:
Unbalanced:

Balanced:

2\text{C}_2\text{H}_6 + 7\text{O}_2 \rightarrow 4\text{CO}_2 + 6\text{H}_2\text{O}

 

Stoichiometry and Equilibrium

Stoichiometry calculates the ratios of reactants and products from a balanced equation. It is used to calculate the amount of one substance needed or produced from another.

In chemical equilibrium, stoichiometry is used to:
i. Determine concentrations of reactants and products at equilibrium.
ii. Relate concentrations to the equilibrium constant (K).
iii. Predict the impact of changes in conditions (concentration, pressure, temperature) on the system using Le Chatelier’s Principle.

2. Equilibrium Constant (K):

The equilibrium constant (K) is a quantitative measure of product concentration to reactant concentration ratio at equilibrium in a reversible reaction.

For the reaction:

K=[C]c[D]d[A]a[B]bK = \frac{[\text{C}]^c[\text{D}]^d}{[\text{A}]^a[\text{B}]^b}

Relation to Equilibrium Position
– K > 1: In favor of products (excess of products).
– K < 1: In favor of reactants (excess of reactants).
– K = 1: Products and reactants are in equal proportions.

3. ICE Tables (Initial, Change, Equilibrium):

An ICE table is employed to track concentrations in a reaction.

i. Initial: Record initial concentrations.
ii. Change: Determine the changes in terms of ( x ) in concentrations as the reaction occurs.
iii. Equilibrium: Sum initial concentrations and changes to find equilibrium concentrations.

Example:
For the reaction:

 ABCD
Initial1.0 M1.0 M0 M0 M
Change−x-x

−x-x

+x+x

+x+x

Equilibrium1−x1 – x

1−x1 – x

xx

xx

x

4. Equilibrium Expression:

For a reaction:

The equilibrium expression is:

K=[C]c[D]d[A]a[B]b

Key Points:
– Products over Reactants: Products in the numerator, reactants in the denominator.
– Exponents: Use coefficients from the balanced equation.
– Exclude solids/liquids: Don’t include pure solids or liquids.
Example::

K=[NH3]2[N2][H2]3

5. Solving for Equilibrium Concentrations:

i. Construct an ICE table: Establish initial, change, and equilibrium concentrations.
ii. Make the equilibrium expression according to the balanced equation.
iii. Substitute known values into the equilibrium expression.
iv. Calculate ( x ): Employ the equilibrium expression to solve for ( x ) (change in concentration).
v. Calculate equilibrium concentrations by substituting ( x ) into the ICE table.

Example:

For the reaction :

 ABCD
Initial1.0 M1.0 M0 M0 M
Change−x-x

−x-x

+x+x

+x+x

Equilibrium1−x1 – x

1−x1 – x

xx

xx

Solve for ( x ) using the equilibrium equation and then substitute ( x ) to determine equilibrium concentrations.

7.7.A.2 Relationship Between Q and K in Chemical Reactions:

1. Reaction Quotient (Q) vs. Equilibrium Constant (K):

– Q: Product to reactant ratio of concentrations at any time during a reaction.
– K: Product to reactant ratio of concentrations at equilibrium.

Predicting Reaction Direction:
– If ( Q < K ): Reaction shifts right (to products).
– If ( Q > K ): Reaction shifts left (to reactants).
– If ( Q = K ): System is in equilibrium.

2. Le Châtelier’s Principle:

If Q not equal to K , the system shifts to re-establish equilibrium:

– If ( Q < K ): Shifts right (toward products).
– If ( Q > K ): Shifts left (toward reactants).

Adjustments:
– Increase reactants: Shifts right (more products).
– Increase products: Shifts left (more reactants).
– Increase pressure: Shifts to the side with fewer gas moles.
– Increase temperature:
– Exothermic: Shifts left.
– Endothermic: Moves right.

3. Dynamic Equilibrium:

When ( Q = K ), the system is in dynamic equilibrium:
– Forward and reverse reaction rates are equal.
– No net change in the concentrations, although the reactions are occurring.
– The system is in equilibrium with constant concentrations of reactants and products.

OLD Content

Calculating Equilibrium Concentrations

Type 1: Given initial concentrations and one equilibrium concentration

  • Write balanced, dissolution equation →Set up ICE table below
    • I: initial concentrations
    • C: Change in concentration
    • E: Equilibrium concentrations
  • Solve for x
  • If question doesn’t state molarity of substance, assume that it is 0

Type 2: Given initial concentrations and K

  • Find Q using inital concentrations and compare to K
    • Q < K → reactants will have –nx and products will have +nx (products/forward reaction favored)
    • Q > K → reactants will have +nx and products will have –nx (reactants/reverse reaction favored)
  • If k is very small can assume that x is insignificant
  • Shortcut using calculator
    • Y1 = value of K
    • Y2 = plug in entire equilibrium equation
    • Graph → 2nd Trace → #5 → don’t move cursor → enter x3
    • X-value will be the answer for x
    • Plug in value for x
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