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Chemistry · University

Equilibrium, reaction quotient and Gibbs energy

A dynamic chemical equilibrium has balanced forward and reverse rates under the specified conditions, not necessarily equal reactant and product amounts.

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Prerequisites

Relevant school subject foundations and careful reasoning.

Study notes

Equilibrium

A dynamic chemical equilibrium has balanced forward and reverse rates under the specified conditions, not necessarily equal reactant and product amounts.

Quotient

The reaction quotient Q uses current activities in the equilibrium expression. Comparing Q and K predicts the thermodynamic direction toward equilibrium.

Energy

For a specified reaction, ΔG=ΔG°+RT ln Q. At equilibrium ΔG=0 and Q=K, yielding ΔG°=−RT ln K.

Conditions

Standard-state conventions and temperature matter. A thermodynamically favoured direction does not establish a rapid reaction rate.

Worked examples

At dynamic equilibrium, which quantities are equal?

Rates balance while nonzero molecular processes can continue.

Forward and reverse reaction rates

For A converting to B with K=4 and current activity ratio B/A=2, find Q.

The reaction quotient is the supplied current ratio.

2

Check your understanding

  1. For the reaction A ⇌ B at fixed temperature with Q<K, which net direction is thermodynamically favoured toward equilibrium?

    1. No conclusion even with conditions specified
    2. Forward
    3. Reverse
    4. Neither in all circumstances

  2. Use R=2 J/(mol K), T=300 K and ln K=3 as stated model values. Find ΔG°.

  3. Why does a negative ΔG not guarantee that a reaction is fast?

  4. Which quantities belong in a rigorous equilibrium expression?

    1. Activities under the stated convention
    2. Only molecular colours
    3. Only raw numbers with units ignored
    4. All concentrations added

Answers and marking guidance

Answers and marking notes
  1. Forward

    The quotient must rise toward K under the model.

  2. -1800 J/mol

    −RT ln K=−2×300×3.

  3. Thermodynamic driving force and kinetic barriers are different. A high activation barrier can make a favourable reaction slow.

    Credit thermodynamics versus kinetics.

  4. Activities under the stated convention

    Activities and standard states make the quotient dimensionless.

Further reading and review references

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