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AP Chemistry 7.3 Reaction Quotient and Equilibrium Constant Study Notes

AP Chemistry 7.3 Reaction Quotient and Equilibrium Constant Study Notes - New Syllabus Effective fall 2024

AP Chemistry 7.3 Reaction Quotient and Equilibrium Constant Study Notes- New syllabus

AP Chemistry 7.3 Reaction Quotient and Equilibrium Constant Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Represent the reaction quotient Qc or Qp, for a reversible reaction, and the corresponding equilibrium expressions Kc  =Qc or Kp  =Qp.

Key Concepts: 

  • The Reaction Quotient
  • Calculating the Equilibrium Constant
  • Magnitude of the Equilibrium Constant
  • Manipulating the Equilibrium Constant

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry Exam Questions MCQs and FRQs

7.3.A.1 Reaction Quotient (Qc, Qp) and Equilibrium Constants (Kc, Kp):

1. Reaction Quotient (Qc and Qp):

– Qc (Concentration): Reactant to product concentration ratio at any point during a reaction.

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

– Qp (Partial Pressure): Ratio of reactant to product partial pressures for reactions involving gases.

Qp=(PC)c(PD)d(PA)a(PB)b

Relation to Equilibrium:
– At equilibrium,
(Q = K) (equilibrium constant).
– If Q < K: Shifts towards products.
– If Q > K: Shifts towards reactants.
– If Q = K: System is at equilibrium.

2. Equilibrium Constant (Kc and Kp):

– Kc: Concentration-based equilibrium constant.
– Kp: Partial pressure-based equilibrium constant for gases.

Qc and Qp Approach Kc and Kp:
– Qc and Qp are the reaction ratios at any point in time, not necessarily at equilibrium.
– At equilibrium:
– Qc = Kc or Qp = Kp.
– If Qc < Kc or Qp < Kp: The reaction shifts toward products.
– If Qc > Kc or Qp > Kp: The reaction shifts **toward reactants.

Qc and Qp approach Kc and Kp as the system reaches equilibrium.

3. Law of Mass Action:

The Law of Mass Action tells us that when a chemical reaction reaches equilibrium, the ratio of the concentrations (or partial pressures) of the products to the reactants remains constant. This constant is referred to as the equilibrium constant (K).

Equilibrium Expressions:

i. For Concentrations (Kc):

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

ii. For Partial Pressures (Kp):

Kp=(PC)c(PD)d(PA)a(PB)b

Both Kc and Kp stay constant when the system is at equilibrium, and the exponents in these equations correspond to the coefficients from the balanced chemical equation.

7.3.A.2 Exclusion of Solids and Pure Liquids in the Reaction Quotient:

1. Reaction Quotient (Q) Definition:

Reaction quotient (Q) is a ratio comparing reactant and product concentrations (or partial pressures) at any time in a reaction, not necessarily at equilibrium. It calculates the direction in which the reaction would move to reach equilibrium.

For a generalized reaction:
aA+bB⇌cC+dD

The reaction quotient ( Q ) is given by:

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

Where:
– [C], [D], [A], [B] are the concentrations (or partial pressures) of the reactants and products.

Exclusion of Solids and Pure Liquids:

– Solids and Pure Liquids are excluded from the reaction quotient since their concentrations (or densities) remain unchanged throughout the reaction.
– For solids, concentration does not vary because they exist in their pure form and do not contribute to altering the equilibrium.
– In pure liquids, their concentration likewise remains unaffected as their volume does not significantly alter during the reaction.

They would not provide useful information regarding the advancement of the reaction and are therefore not included in ( Q ).

2. Law of Mass Action: 

The Law of Mass Action informs us that at equilibrium, the ratio of product concentrations (or partial pressures) to reactant concentrations is always the same, and that constant is the equilibrium constant (K).

Exclusion of Solids and Pure Liquids:

– Solids and Pure Liquids are excluded from equilibrium equations (both concentration and partial pressures) because their concentrations (or activities) remain the same throughout the reaction.
– Solids: Their concentration remains unchanged as their number is fixed in a phase (has constant density). Thus, they do not alter the equilibrium position.

– Pure Liquids: Similarly, the same applies to a pure liquid; its concentration also remains unchanged as it has a constant density and volume throughout the reaction.
These constant concentrations (or activities) are indeed used as 1 in the equilibrium expression, and thus they are omitted for simplicity and clearness.

Example:

For the reaction:
A(s)+B(l)⇌C(g)+D(g)

The equilibrium expression would be:

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

A (solid) and B (liquid) are omitted from the expression because their concentrations do not vary with the reaction.

OLD Content

Reaction Quotient and Equilibrium Constant

  • For   the Equilibrium Constant Expression relates the concentrations of reactants and products once the reaction has reached equilibrium.
    • Equilibrium Constant Expression: (on RFS)
      • In some tests K is written as Kc
  • If asked to find Kc expression only include gasses and (aq), NOT solids (s) or liquids (l)

Equilibrium Expressions with Pressure

  • For reactions that occur all in the gas phase, the equilibrium expression can be written in terms of partial pressures
    • Only include gases in the Kp expression

Reaction Quotient: Q

  • Calculated the same as the equilibrium constant, but for a system not at equilibrium (usually initial conditions)
  • Ex: →
    • The subscript zeros indicate initial concentrations
  • To determine in which direction a system will shift to reach equilibrium, we compare the values of Q and K. There are three possible cases
    1. Q = K. The system is at equilibrium; no shift will occur.
    2. Q > K → too many products, will shift to consume products and produce more reactants (shift reverse reaction/left)
    3. Q < K → not enough products; will shift to consume reactants and produce more products (shift forward reaction/right )
      • Do not say shift right/left on AP exam (won’t receive credit)
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