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Pressure effects on reversible gas-phase equilibria

The rate and extent of chemical change • Reversible reactions and equilibrium

Key concepts

What you'll likely be quizzed about

  • Equilibrium occurs when the rates of the forward and reverse reactions are equal, leading to constant concentrations or partial pressures for gases.
  • Le Chatelier’s principle predicts the shift direction when external conditions change.
  • Increasing pressure, typically by reducing volume, causes the equilibrium to shift toward the side with fewer gas molecules to counteract the change.
  • Conversely, decreasing pressure, usually by increasing volume, results in a shift toward the side with more gas molecules.

Flashcards

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Cause→effect: increasing pressure by reducing volume

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Reducing volume raises collision frequency and partial pressures; equilibrium shifts to the side with fewer gaseous moles to oppose this change.

Key notes

Important points to keep in mind

Count only gaseous species when deciding direction of shift.

Increased pressure favors the side with fewer gas moles; decreased pressure favors the side with more gas moles.

Solids and liquids do not change equilibrium position in response to pressure changes.

Addition of inert gas at constant volume does not alter the equilibrium position.

Kp remains constant at fixed temperature; use Qp to assess direction after a pressure change.

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