Chemical equilibria, Le Chatelier's principle and Kc common mistakes

Common mistakes

• Confusing static and dynamic equilibrium

  Students often think that equilibrium means no reaction is occurring.

  Fix itRemember that in dynamic equilibrium, reactions are still occurring, but the rates are equal, maintaining constant concentrations.

• Confusing the effects of concentration changes

  Students often think that increasing the concentration of products will shift the equilibrium to the right.

  Fix itRemember that increasing the concentration of reactants shifts the equilibrium to the right, while increasing products shifts it to the left.

• Ignoring the number of moles of gas

  Students often forget to consider the total number of moles of gas on each side of the equilibrium when predicting shifts.

  Fix itAlways count the moles of gas on both sides of the equation to apply Le Chatelier's principle correctly.

• Confusing Endothermic and Exothermic

  Students often confuse the effects of temperature changes on endothermic and exothermic reactions.

  Fix itRemember that increasing temperature favors endothermic reactions (shifts right) and decreases Kc for exothermic reactions.

• Confusing the effects of catalysts and changes in concentration.

  Students may incorrectly state that a catalyst shifts the position of equilibrium.

  Fix itRemember that catalysts do not change the position of equilibrium; they only speed up the rate at which equilibrium is reached.

• Incorrect Kc expression

  Students often forget to raise the concentrations to the power of their coefficients in the balanced equation.

  Fix itAlways ensure that the Kc expression reflects the stoichiometry of the balanced equation.

• Incorrectly applying Kc expression

  Students often forget to raise the concentrations to the power of their coefficients when calculating Kc.

  Fix itAlways ensure that each concentration in the Kc expression is raised to the power of its respective coefficient from the balanced equation.

• Ignoring changes in concentration

  Students often forget to consider how changes in concentration of reactants or products affect the equilibrium position.

  Fix itAlways apply Le Chatelier's principle to predict the direction of the shift when concentrations change.

• Incorrectly determining units for Kc

  Students often forget to account for the stoichiometric coefficients when calculating the units of Kc.

  Fix itAlways derive the units based on the balanced equation, ensuring to raise the concentrations to the power of their respective coefficients.

• Confusing Kc with Kp

  Students often confuse Kc (concentration-based) with Kp (pressure-based) equilibrium constants.

  Fix itRemember that Kc is used for reactions involving concentrations, while Kp is used for reactions involving gases and partial pressures.