Equilibria

• What is the state that all reversible reactions reach?

  A dynamic equilibrium state
• What is the general form of a reversible reaction involving nitrogen and hydrogen?

  N2 + 3H2 ⇌ 2NH3
• What are the two features of dynamic equilibrium?

  1. Forward and backward reactions occur at equal rates.
  2. The concentrations of reactants and products stay constant.
• What does the term 'position of equilibrium' refer to?

  It describes the composition of the equilibrium mixture.
• What happens if the position of equilibrium favors the reactants?

  The equilibrium mixture will contain mostly reactants.
• What does Le Chatelier’s principle help us determine?

  How changing external conditions affect the position of equilibrium.
• What does Le Chatelier’s principle state about external changes?

  If an external condition is changed, the equilibrium will shift to oppose the change.
• What happens to the equilibrium when temperature is increased?

  The equilibrium will shift in the endothermic direction to reduce the temperature.
• What is the effect of increasing temperature on the yield of ammonia in the reaction N2 + 3H2 ⇌ 2NH3?

  The position of equilibrium will shift towards the left, giving a lower yield of ammonia.
• What happens if the temperature is decreased in a reaction?

  The equilibrium will shift in the exothermic direction to increase the temperature.
• What is the trade-off when using low temperatures in reactions?

  Low temperatures may give a higher yield of product but result in slow rates of reaction.
• What is the effect of increasing pressure on equilibrium?

  • The equilibrium will shift towards the side with fewer moles of gas.
  • This shift opposes the change and reduces the pressure.
• In the reaction CO (g) + 2H2(g) ⇌ CH3OH (g), what happens if pressure is increased?

  The position of equilibrium will shift towards the right, giving a higher yield of methanol.
• What happens if pressure is decreased in a reaction?

  The equilibrium will shift towards the side with more moles of gas to increase the pressure.
• What is the effect of equal moles of gas on both sides of a reaction when pressure changes?

  Changing pressure will have no effect on the position of equilibrium.
• What is the effect of catalysts on equilibrium?

  A catalyst speeds up the rate at which equilibrium is achieved but does not affect the position of equilibrium.
• What are the key factors affecting industrial processes involving equilibrium?

  • Temperature: Compromise temperature used for yield and rate.
  • Pressure: High pressure gives good yield but high energy costs.
  • Catalyst: Speeds up reaction without affecting equilibrium position.
• What are the conditions for the Haber process?

  T = 450°C, P = 200 – 1000 atm, catalyst = iron
• What are the conditions for the Contact process?

  T = 450°C, P = 1 or 2 atm, catalyst = V2O5
• What are the conditions for the hydration of ethene to produce ethanol?

  T = 300°C, P = 70 atm, catalyst = conc H3PO4
• What are the conditions for the production of methanol from CO?

  T = 400°C, P = 50 atm, catalyst = chromium and zinc oxides
• What does it mean for a process to be carbon neutral?


  An activity has no net annual carbon emissions to the atmosphere.
• What is necessary for a process to be considered carbon neutral?

  The energy required must not come from the combustion of fossil fuels.
• What is the importance of recycling unreacted reactants in industrial processes?

  • Improves overall yields.
  • Reduces waste and costs.
  • Enhances efficiency of the process.
• What is the general form of an equilibrium reaction?

  mA + nB ⇌ pC + qD

  • m, n, p, q are stoichiometric balancing numbers.
  • A, B, C, D are chemical formulas.
• What is the equilibrium constant expression Kc?

  Kc = $\frac{[C]^p[D]^q}{[A]^m[B]^n}$

  • [ ] indicates equilibrium concentration.
  • Kc varies based on the reaction.
• How do you calculate the moles of reactants at equilibrium?

  Initial moles – moles reacted
• How do you calculate the moles of products at equilibrium?

  Initial moles + moles formed
• In a container of volume 600 cm³, if there were initially 0.500 mol of H2 and 0.600 mol of Cl2, and at equilibrium there were 0.200 mol of HCl, how do you calculate Kc?

  Use the equilibrium concentrations in the Kc expression.
• If 30% of nitrogen reacted in the reaction N2 + 3H2 ⇌ 2NH3, how do you calculate the moles at equilibrium?

  Calculate the moles reacted and then use the balanced equation to find the moles of reactants and products at equilibrium.
• How do you find the equilibrium concentrations from moles?

  Divide the moles by the volume of the container (in dm³).
• What does it indicate if Kc has no unit?

  There are equal numbers of reactants and products.
• How do you write the Kc expression for the reaction H2 (g) + Cl2 (g) ⇌ 2HCl (g)?

  Kc = $\frac{[HCl]^2}{[H2][Cl2]}$
• How do you calculate Kc for the reaction N2 (g) + 3H2 (g) ⇌ 2NH3 (g)?

  Kc = $\frac{[NH3]^2}{[N2][H2]^3}$
• How do you calculate the unit of Kc for the reaction H2 (g) + Cl2 (g) ⇌ 2HCl (g)?


  The unit of Kc is no unit.
• How do you calculate the equilibrium concentrations for H2, Cl2, and HCl in the reaction H2 (g) + Cl2 (g) ⇌ 2HCl (g)?

  Use the moles at equilibrium divided by the volume of the container.
• If the initial moles of H2 are 0.5 and Cl2 are 0.6, and 0.2 moles of HCl are formed, what are the equilibrium moles of H2?

  0.4 moles of H2 at equilibrium.
• If 0.2 moles of HCl are formed, how many moles of Cl2 are used in the reaction H2 + Cl2 ⇌ 2HCl?

  0.1 moles of Cl2 are used.
• How do you calculate equilibrium concentration?

  Concentration = moles / volume (in dm$^3$)
• What does it mean if Kc has no unit?

  It indicates equal numbers of reactants and products.