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AP Chemistry 7.12 Common-Ion Effect Study Notes

AP Chemistry 7.12 Common-Ion Effect Study Notes - New Syllabus Effective fall 2024

AP Chemistry 7.12 Common-Ion Effect Study Notes- New syllabus

AP Chemistry 7.12 Common-Ion Effect Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Identify the solubility of a salt, and/or the value of  Ksp for the salt, based on the concentration of a common ion already present in solution.

Key Concepts: 

  • Common-Ion Effect

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry Exam Questions MCQs and FRQs

7.12.A.1 Common-Ion Effect on Solubility

1. Solubility and Ksp

  • Solubility: The amount of a salt that dissolves in a solvent to form a saturated solution.
  • Ksp (Solubility Product Constant): The equilibrium constant for the dissolution of a salt. It is the product of ion concentrations:
    • AB(s) ⇌ A⁺(aq) + B⁻(aq)
    • Ksp = [A⁺][B⁻]
    • Higher Ksp → More soluble
    • Lower Ksp → Less soluble

2. Le Châtelier’s Principle

When a system at equilibrium is disturbed (e.g., change in concentration, temperature, or pressure), it shifts to minimize the disturbance and restore equilibrium.

Example: Adding more ions pushes the reaction to form more solid, counteracting the disturbance.

3. Common-Ion Effect

  • Definition: Adding a common ion to a solution reduces the solubility of a salt.
  • Why? The equilibrium shifts to the left (towards undissolved salt), decreasing dissolved salt.

4. Calculating Solubility with Common Ion

  1. Dissociation Equation:
    AB(s) ⇌ A⁺(aq) + B⁻(aq)
  2. Write the Ksp Expression:
    Ksp = [A⁺][B⁻]
  3. Adjust for Common Ion:
    If [A⁺] from another source = x, and s = solubility of AB:
    [A⁺] = x + s, [B⁻] = s
  4. Solve for Solubility (s):
    Plug into the Ksp formula and solve. The new solubility (s) will be lower due to the common ion.

5. Applications

  • Precipitation Control:
    Use common ions to lower solubility and force precipitation of specific salts.
  • Buffer Solutions:
    Common ions help buffers resist pH changes by shifting equilibrium reactions.

Summary: The common-ion effect is key to understanding solubility, precipitation, and equilibrium in chemistry.

OLD Content
Common-Ion Effect

  • Common-Ion Effect: when you try to dissolve the solid in a solution with either the cation or anion already present less solid will dissolve/more solid will be produced → the solubility of that salt is going to be less that it was in pure water
    • Q > Ksp → reaction will shift to consuming more ions and producing more reactant (shift left) (more solid forming)

 Calculations

  • Asked to calculate the solubility of solid (s) in a solution of another compound
    • Write balanced equation [dissolution/ions on product side] and ICE table
      • Nothing in the Solid column, difference is that will have initial value for the ion that is present in solution
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