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AP®ChemistryCollege BoardStudy GuidesUnit 7: EquilibriumLe Châtelier’s PrincipleReaction Quotient & Le Châtelier’s Principle

Reaction Quotient & Le Châtelier’s Principle (College Board AP® Chemistry)

Study Guide

Philippa Platt

Written by: Philippa Platt

Reviewed by: Stewart Hird

Reaction Quotient & Le Châtelier’s Principle

What is the Reaction Quotient, Q?

  • The reaction quotient or Q is the value obtained by entering all of the required concentrations into the equilibrium expression and calculating the result

  • For a general equation:

aA + bB rightwards harpoon over leftwards harpoon cC + dD

Q = fraction numerator open square brackets straight C close square brackets to the power of straight c open square brackets straight D close square brackets to the power of straight d over denominator open square brackets straight A close square brackets to the power of straight a open square brackets straight B close square brackets to the power of straight b end fraction

  • The equilibrium constant is the numerical value of Kc when the reaction is at equilibrium 

  • If the reaction is not at equilibrium, the numerical value of the equilibrium expression is called the reaction quotient, Q

Summary of Reaction Quotient Principles

Statement

Result

If Q does not change with time, the reaction is in a state of equilibrium

Q = Kc

Reaction is in equilibrium

Q = Kc

Reaction move to the right hand side (forward) to reach equilibrium

Q < Kc

Reaction move to the left hand side (backwards) to reach equilibrium

Q > Kc

  • We can use these values to determine whether or not a reaction is in equilibrium

Worked Example

Determine whether the following reactions are in equilibrium.

If the reaction is not in equilibrium, predict whether it will proceed in the forward or backward direction:

  1. N2 (g) + 3H2 (g) rightwards harpoon over leftwards harpoon 2NH3 (g)    Kc = 9.40
    [N2] = 3.0 x 10-2 M
    [H2] = 3.7 x 10-2 M
    [NH3] = 1.6 x 10-2 M
     

  2. 2N2O (g) rightwards harpoon over leftwards harpoon 2N2 (g) + O2 (g)    Kc = 7.0 x 1034 

    [N2O] = 1.4 x 10-18 M
    [N2] = 0.0510 M
    [O2] = 0.0100 M

Answers:

Answer 1:

  • K subscript c space equals space fraction numerator open square brackets NH subscript 3 close square brackets squared over denominator open square brackets straight H subscript 2 close square brackets cubed open square brackets straight N subscript 2 close square brackets end fraction

  • Q space equals space fraction numerator blank over denominator stretchy left parenthesis 3.0 space cross times space 10 to the power of negative 2 end exponent stretchy right parenthesis stretchy left parenthesis 3.7 space cross times space 10 to the power of negative 2 end exponent stretchy right parenthesis cubed end fraction equals space 10529.15

  • The value of Q is greater than that of Kc (Q < Kc)

  • So, the reaction is not at equilibrium and will move to the left side to reach equilibrium

Answer 2:

  • K subscript c space equals space fraction numerator open square brackets straight N subscript 2 close square brackets squared open square brackets straight O subscript 2 close square brackets over denominator open square brackets straight N subscript 2 straight O close square brackets squared end fraction

  • begin mathsize 14px style Q space equals space fraction numerator stretchy left parenthesis 0.0510 stretchy right parenthesis squared stretchy left parenthesis 0.0100 stretchy right parenthesis over denominator stretchy left parenthesis 1.4 space cross times space 10 to the power of negative 18 end exponent stretchy right parenthesis end fraction equals space 1.33 space cross times space 10 to the power of 31 end style

  • The value of Q is less than that of Kc (Q < Kc)

  • So, the reaction is not at equilibrium and will move to the right side to reach equilibrium

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    Reviewer
    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.