Reversible reactions and equilibrium-Chemistry chapter 10

• When wood is burnt, ash is formed. The ash cannot be converted back to wood. Thus, the burning (combustion) of wood cannot be undone. it is irreversible. In the same way, many chemical reactions are irreversible, but most physical reactions are reversible
• However, some chemical reactions can be reversed. The heating of a hydrated salt or adding water to an anhydrous salt is an example.
• The reverse of an exothermic reaction is always endothermic. For instance, when calcium carbonate reacts with hydrochloric acid to form calcium chloride and carbon dioxide, the reaction is exothermic. When carbon dioxide combines with limewater to produce calcium carbonate, this is an endothermic reaction.
• Heating the hydrated salt is endothermic as the salt loses water and absorbs the heat (endothermic means absorb heat). The reverse or backward reaction will be exothermic

  
• A chemical equilibrium is known as a dynamic equilibrium. Dynamic means undergoing constant change.
• In a dynamic equilibrium;

  • The rates of the forward reactions and the backward or reverse reactions are the same rate.
  • The concentration of reactant and the concentration of product remain the same (because the reactants and products are being used up and made at the same rate, so their concentrations do not change)
• When we make changes to the conditions of a reaction at equilibrium (such as changing the temperature, pressure or concentration) the position of the equilibrium is affected. The equilibrium can shift:

  • forward, to make more products
  • reverse/backwards, to reform more reactants
  • When we change the conditions, the equilibrium position always moves to reduce the effects of the change
• How does a change in temperature affect the position of the equilibrium?

  • Increasing the temperature of the reaction shifts the equilibrium in the direction in which energy is absorbed, which is towards the endothermic reaction
  • Lowering the temperature of the reaction shifts the equilibrium in the direction in which energy is released, which is towards the exothermic reaction.
• How does a change in pressure affect the position of the equilibrium?

  In reactions where some of the reactants and products are gases,

  • Increasing the total pressure shifts the equilibrium in the direction with fewer gas particles so that the pressure decreases.
  • Decreasing the total pressure shifts the equilibrium in the direction with more gas particles so that the pressure increases
• How does a change in concentration affect the position of the equilibrium?

  • Increasing the concentration (addition) of a substance shifts the equilibrium in the direction to reduce the amount of the substance that has been increased (eg when you add more product, equilibrium shift to left to reduce the amount of the product)
  • Decreasing the concentration (removal) of a substance shifts the equilibrium in the direction to increase the amount of the substance that has been removed. (eg when you remove reactant, equilibrium shifts to the right to make more of the reactant)
• How does adding a catalyst affect the position of the equilibrium?

  • A catalyst is a substance that increases the rate of a chemical reaction.
  • Catalysts increase the forward and reverse reactions equally so that the reaction reaches equilibrium faster.
  • They have no effect on the equilibrium position or the concentration of the product (the yield)
• Manufacturing ammonia by the Haber process: (reversible)

  • N2(g) + 3H2(g) ⇌ 2NH3(g)
  • Nitrogen+ Hydrogen makes Ammonia.
  • Nitrogen is obtained from air (fractional distillation of liquid air)
  • Hydrogen is produced from methane (cracking of petroleum)
  • The forward reaction is exothermic
  • Typical conditions in the Haber process: 450°C, 20000kPa /200atm and an iron catalyst
• Reason for the optimal conditions in Haber process:

  • The nitrogen molecule is generally unreactive. Hence there is no reaction at all between nitrogen and hydrogen at room temperature and pressure.
  • For nitrogen to react with hydrogen to form ammonia, a high pressure and a relatively high temperature are needed.
  • Iron is used as a catalyst to speed up the reaction.
  • Because the reaction is reversible, some of the ammonia formed may decompose and convert back to nitrogen and hydrogen. To achieve the maximum yield of ammonia at minimum cost, the temperature and pressure are controlled.
• How is the optimal pressure for the manufacture of ammonia selected?

  • The higher the pressure, the higher the yield of ammonia. High pressure also increases the speed of the reaction
  • However, maintaining high pressure is costly because expensive equipment (like special pumps an stronger pipes) is required.
  • There is also a safety risk of explosion at high pressures
  • Therefore, there is a compromise between higher rate of reaction with costs and safety to get 200atm or 20,000kPa as the optimal pressure.
• How is the optimal temperature for the manufacture of ammonia selected?

  • The lower the temperature, the higher the yield of ammonia. This is because the forward reaction is exothermic.
  • However, a lower temperature also results in a slower reaction.
  • Therefore, a relatively high temperature of 450 celsisus is used. (compromise between fast rate of reaction and high yield)
• Why is a catalyst used in Haber process?

  • Despite conditions like high pressure and a relatively high temperature, the reaction is still slow
  • Therefore, a catalyst is used to speed up the reaction
• Manufacturing Sulfur Trioxide by the Contact process: (reversible)

  • Symbol equation for the conversion of sulfur dioxide to sulfur trioxide in the Contact process, 2SO2(g) + O2(g) ⇌ 2SO3(g)
  • Sulfur dioxide + Oxygen makes Sulfur trioxide
  • Sulfur is sourced from burning sulfur (in air) or roasting sulfide ores. Oxygen is sourced from air
  • The forward reaction is exothermic.
  • Typical conditions for the conversion of sulfur dioxide to sulfur trioxide in the Contact process : 450°C, 200kPa /2atm and a vanadium(V) oxide catalyst
• Reasons for conditions required for manufacturing sulfur trioxide (contact process)

  • To increase the yield of the product sulfur trioxide, we need to decrease the temperature so that equilibrium moves to the right. (forward reaction is exothermic)
  • However, lowering the temperature will also slow down the rate of reaction.
  • So there is a compromise between high yield of sulfur trioxide and faster rate of reaction to get 450 celsius as optimal temperature.
• Reasons for conditions required for manufacturing sulfur trioxide (contact process)

  • To increase the yield of the product, we need to increase the pressure so that the equilibrium moves to the right. (as there are three moles of gaseous reactants and two moles of gaseous products)
  • Increasing the pressure can be expensive because special equipment and containers are required.
  • There is also a safety risk because of the danger of explosions
  • So there is a compromise between costs, safety and higher yield of sulfur trioxide to get optimal pressure of 2atm or 200kPa