Reversible Reactions C2 Higher Tier

• Open system: when a chemical reaction happens in a container where one or more of the reactants or products can escape.
• Closed system: when a chemical reaction happens in a container where none of the reactants or products can escape.
• Reversible reactions in a closed system eventually reach equilibrium - the forward and reverse reactions are still happening but at the same rate, so the concentrations of reactants and products don't change.
• The point of equilibrium can be changed by temperature, and also pressure for gases.
• If the products are removed from an equilibrium mixture, more reactants are converted into products (i.e. the forward reaction happens more). This is because the concentrations of reactants and products are upset, so more products are made to get the balance back again.
• If a catalyst is used the reaction reaches equilibrium faster because it speeds up the forward and reverse reactions by the same amount. The concentration of reactants and products is the same at equilibrium.

• Forward reaction exothermic (so reverse endothermic) and temperature increased, yield of products decreases because position of equilibrium moves in direction of reverse reaction.
• Forward reaction exothermic and temperature decreased, yield of products increases because position of equilibrium moves in direction of forward reaction.
• Forward reaction endothermic (so reverse exothermic) and temperature increased, yield of products increases because position of equilibrium moves in direction of forward reaction.
• Forward reaction endothermic and temperature decreased, yield of products decreases because position of equilibrium moves in direction of reverse (exothermic) reaction.
• Because: with exothermic reactions, the lower the temperature the higher the yield, but with endothermic reactions, the higher the temperature the higher the yield.
• However, at lower temperatures reactions are slower, whether exothermic endothermic, so in industrial processes such as the Haber process (making ammonia) a compromise needs to be made between yield and rate of reaction.

• Changing pressure usually only has an effect on equilibrium in reactions with gases.
• If pressure increased, position of equilibrium moves in direction of fewest moles.

• E.g. the Haber process, which makes ammonia (NH₃):

N₂(g) + 3H₂(g) 2NH₃(g)

Left - 1 + 3 = 4 moles Right - 2 moles Pressure increased,position of equilibrium moves right and more ammonia made. Pressure reduced, position of equilibrium moves left and less ammonia made.

• Temperature - 450*C: compromise between rate of reaction and yield (higher temp. - faster rate but lower yield, lower temp. - higher yield but slower rate)
• If rate of reaction too slow - uneconomical.
• Iron catalyst used - speeds up reaction but doesn't affect position of equilibrium so yield not affected. Ammonia produced in shorter time - cheaper. Iron - cheap catalyst.
• Pressure - 200atm: high pressure increases yield but very high pressures require very strong and expensive equipment and can be dangerous. 200atm high enough to get good yield of ammonia but not so high that major costs are added.