Chemical equilibrium

• All reversible reactions reach a dynamic equilibrium state.
• N2 + 3H2 → 2NH3 is a reversible reaction.
• Forward and backward reactions are occurring at equal rates in dynamic equilibrium.
• The concentrations of reactants and products stay constant in dynamic equilibrium.
• The expression 'position of equilibrium' is used to describe the composition of the equilibrium mixture.
• If the position of equilibrium favours the reactants, the equilibrium mixture will contain mostly reactants.
• Le Chatelier’s Principle states that if an external condition is changed, the equilibrium will shift to oppose the change and try to reverse it.
• If temperature is increased, the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
• If temperature is decreased, the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
• Low temperatures may give a higher yield of product but will also result in slow rates of reaction.
• Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure.
• If pressure is increased, the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure.
• The term carbon neutral is only applicable if the energy required to carry out the reaction is not made by combustion of fossil fuels.
• Carbon neutral refers to an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere.
• Catalysts speed up the rate, allowing a lower temperature to be used (and hence lower energy costs), but have no effect on the position of equilibrium.
• For a generalised reaction, mA + nB pC + qD, m,n,p,q are the stoichiometric balancing numbers, A,B,C,D stand for the chemical formula, and K c is the equilibrium constant.
• Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes.
• High pressure leads to too high energy costs for pumps to produce the pressure and too high equipment costs to have equipment that can withstand high pressures.
• The hydration of ethene to produce ethanol involves the reaction of ethene and water, with the equilibrium shifting to the right at a temperature of 300 o C and a pressure of 70 atm, with a catalyst of conc H3PO4.
• A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the equilibrium is achieved.
• The contact process involves the reaction of sulphur dioxide and oxygen, with the equilibrium shifting to the right at a temperature of 450 o C and a pressure of 1 or 2 atm, with a catalyst of V2O5.
• Both methanol and ethanol can be used as fuels.
• Common examples of industrial processes that use equilibrium include the Haber process, the contact process, and the hydration of ethene to produce ethanol.
• Industrially high pressures are expensive to produce due to high electrical energy costs for pumping the gases to make a high pressure and the equipment is expensive to contain the high pressures.
• The Haber process is used to produce ammonia at a temperature of 450 o C and a pressure of 200 - 1000 atm, with a catalyst of iron.
• Increasing the concentration of OH - ions causes the equilibrium to shift to oppose this and move in the forward direction to remove and decrease the concentration of OH - ions.
• The term dynamic means both forward and backward reactions are occurring simultaneously.
• Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure.
• If the number of moles of gas is the same on both sides of the equation, changing pressure will have no effect on the position of equilibrium.
• Increasing pressure may give a higher yield of product and will produce a faster rate.
• Catalysts have no effect on the value of K c or the position of equilibrium as they speed up both forward and backward rates by the same amount.
• If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat
• Typical exam question: What effect would increasing temperature have on the yield of ammonia?

  N 2 + 3H2 R= 2NH3

  H = -ve exo

  Exam level answer : must include bold points

  If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.
• If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium
• Typical exam question: What effect would increasing pressure have on the yield of methanol?

  CO (g) + 2H 2(g) ⇌ CH3 OH (g)

  And its reverse

  Exam level answer : must include bold points

  if pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.
• Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the equipment is expensive (to contain the high pressures)
• Increasing the concentration OH ions causes the equilibrium to shift to oppose this and move in the forward direction to remove and decrease the concentration of OH ions. The position of equilibrium will shift towards the right, giving a higher yield of I-and IO: (The colour would change from brown to colourless)

  Adding H+ ions reacts with the OH ions and reduces their concentration so the equilibrium shifts back to the left giving brown colour.

  
• Hydration of ethene to produce ethanol

  대,대2(0)+H20g)=대,COmAH=-ve

  T= 300°C, P= 70 atm, catalyst = concH,PO.

  Low temp gives good yield but slow rate: compromise temp used

  High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure

  High pressure also leads to unwanted polymerisation of ethene to poly(ethene)

  
• Production of methanol from CO

  CO (8) + 2H2(8) = CH,OH (8) 4H = -ve exo

  T= 400°C, P= 50 atm, catalyst = chromium and zinc oxides

  Low temp gives good yield but slow rate: compromise temp used

  High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure

  
• Haber process

  N2 + 3H2 = 2NH; AH = -ve exo

  T= 450°C, P= 200 - 1000 atm, catalyst = iron

  Low temp gives good yield but slow rate: compromise temp used

  High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure