Chemical Equilibria

• What is the definition of dynamic equilibrium?

  The rate of the forward reaction is equal to the rate of the reverse reaction.
• What happens to the concentrations of reactants and products in dynamic equilibrium?

  The concentrations of reactant and product do not change.
• What is an essential condition for an equilibrium mixture?

  Equilibrium occurs in a closed system.
• What does it mean for macroscopic properties to not change with time in an equilibrium mixture?

  It means that observable properties like pressure and concentration remain constant.
• What is Le Chatelier’s principle?

  If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.
• In the reaction CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) with ΔH°= +210 kJ mol^-1, what effect does increasing the temperature have on the position of equilibrium?

  The equilibrium position shifts to the right, increasing the yield of hydrogen.
• Why does the equilibrium position shift to the right when the temperature is increased in an endothermic reaction?

  Because the forward reaction absorbs heat, favoring product formation.
• In the same reaction, what effect does increasing the pressure have on the position of equilibrium?

  The equilibrium position shifts to the left, decreasing the yield of hydrogen.
• Why does the equilibrium position shift to the left when pressure is increased in the reaction CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g)?

  Because the forward reaction produces more moles of gas than the reverse reaction.
• Why might an industrial chemist use high pressure for the production of hydrogen in the reaction CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g)?

  High pressure increases collision frequency, increasing the rate of reaction.
• What is the compromise pressure used by industrial chemists for the production of hydrogen?

  It is a balance between an economically viable rate of reaction and a slightly lower yield of hydrogen.
• What effect does a catalyst have on the position of equilibrium?

  A catalyst has no effect on the position of equilibrium.
• What conditions affect the value of Kc?

  Concentration, pressure, and temperature affect the value of Kc.
• For the reaction 2[A] + 3[B] + [C] ⇌ [D] + 4[E], what is the expression for Kc?

  Kc = $\frac{[D][E]^4}{[A]^2[B]^3[C]}$
• What are the units for the value of Kc in the expression Kc = $\frac{[D][E]^4}{[A]^2[B]^3[C]}$?


  The units for Kc are mol^-1 dm³.
• What type of system is Kc relevant for?

  Kc is relevant for homogeneous systems in equilibrium.
• What does it suggest if Kc is greater than 1?

  It suggests that the equilibrium position is over to the right.
• What does it suggest if Kc is less than 1?

  It suggests that the equilibrium position is over to the left.
• What effect does decreasing the temperature in an endothermic reaction have on Kc?

  Kc decreases.
• What effect does increasing the temperature in an endothermic reaction have on Kc?

  Kc increases.
• What effect does decreasing the temperature in an exothermic reaction have on Kc?

  Kc increases.
• What effect does increasing the temperature in an exothermic reaction have on Kc?

  Kc decreases.