(a) List three properties of a system that is in a state of chemical equilibrium. (b) Consider reaction represented by the following equation: 3H\(_{2(g)}\)...
(a) List three properties of a system that is in a state of chemical equilibrium.
(b) Consider reaction represented by the following equation: 3H\(_{2(g)}\) + N\(_{2(g)}\) \(\rightleftharpoons\) 2NH\(_{3(g)}\); H = 92KJ
(i) Explain the effect of increasing the temperature of the reaction on the yield of ammonia
(ii) Uses of energy profile diagram to illustrate the effect of a positive catalyst on the rate of either the forward reaction or the reverse reaction.
(c) In the extraction of aluminium from bauxite:
(i) outline the procedure used for purifying the ore;
(ii) write equation for the reaction at each electrode, during the electrolysis of the pure alumina;
(iii) state the function of molten cryolite in the electrolytic cell for the extraction.
(a) Three properties of a system in a state of chemical equilibrium
The change is dynamic: the forward and reverse reactions continue simultaneously and the rate of the forward reaction equals the rate of the reverse reaction.
The macroscopic properties remain constant with time: the concentrations of reactants and products (and observable properties such as colour and pressure) do not change once equilibrium is reached.
It is reached only in a closed system, and at equilibrium the free-energy change of the system is zero (\(\Delta G = 0\)).
(b) \(3H_{2(g)} + N_{2(g)} \rightleftharpoons 2NH_{3(g)};\ \Delta H = -92\ \text{kJ}\) (the forward reaction is exothermic).
(i) Effect of increasing the temperature on the yield of ammonia
Since the forward reaction is exothermic, by Le Chatelier's principle an increase in temperature favours the endothermic (reverse) direction. The equilibrium therefore shifts to the left, so the yield of ammonia decreases as the temperature is raised.
(ii) Effect of a positive catalyst (energy profile diagram)
A positive catalyst provides an alternative reaction pathway of lower activation energy. On the energy profile below, the peak of the catalysed curve (green, dashed) is lower than that of the uncatalysed curve (red, solid), while the energy levels of the reactants and of the products are unchanged. Because the activation-energy barrier is lowered by the same amount for both directions, the catalyst speeds up the forward and the reverse reactions equally; equilibrium is reached faster, but the position of equilibrium (and hence the yield) is unchanged.
Energy profile for 3H₂ + N₂ ⇌ 2NH₃ (exothermic forward reaction). The catalysed pathway (green, dashed) has a lower activation energy than the uncatalysed pathway (red, solid), while the reactant and product energy levels and ΔH are unchanged.
(c) Extraction of aluminium from bauxite
(i) Purifying the ore (Bayer process): the powdered bauxite is dissolved, under pressure, in hot concentrated sodium hydroxide solution. The amphoteric aluminium oxide dissolves as sodium aluminate, while insoluble impurities such as \(Fe_2O_3\) and \(SiO_2\) do not dissolve and are filtered off:
(iii) Function of molten cryolite: it acts as a solvent (flux) for the alumina and lowers the melting point of the electrolyte from about 2045 °C to about 950 °C, so that electrolysis can be carried out at a much lower temperature while also increasing the electrical conductivity of the melt.
(a) Three properties of a system in a state of chemical equilibrium
The change is dynamic: the forward and reverse reactions continue simultaneously and the rate of the forward reaction equals the rate of the reverse reaction.
The macroscopic properties remain constant with time: the concentrations of reactants and products (and observable properties such as colour and pressure) do not change once equilibrium is reached.
It is reached only in a closed system, and at equilibrium the free-energy change of the system is zero (\(\Delta G = 0\)).
(b) \(3H_{2(g)} + N_{2(g)} \rightleftharpoons 2NH_{3(g)};\ \Delta H = -92\ \text{kJ}\) (the forward reaction is exothermic).
(i) Effect of increasing the temperature on the yield of ammonia
Since the forward reaction is exothermic, by Le Chatelier's principle an increase in temperature favours the endothermic (reverse) direction. The equilibrium therefore shifts to the left, so the yield of ammonia decreases as the temperature is raised.
(ii) Effect of a positive catalyst (energy profile diagram)
A positive catalyst provides an alternative reaction pathway of lower activation energy. On the energy profile below, the peak of the catalysed curve (green, dashed) is lower than that of the uncatalysed curve (red, solid), while the energy levels of the reactants and of the products are unchanged. Because the activation-energy barrier is lowered by the same amount for both directions, the catalyst speeds up the forward and the reverse reactions equally; equilibrium is reached faster, but the position of equilibrium (and hence the yield) is unchanged.
Energy profile for 3H₂ + N₂ ⇌ 2NH₃ (exothermic forward reaction). The catalysed pathway (green, dashed) has a lower activation energy than the uncatalysed pathway (red, solid), while the reactant and product energy levels and ΔH are unchanged.
(c) Extraction of aluminium from bauxite
(i) Purifying the ore (Bayer process): the powdered bauxite is dissolved, under pressure, in hot concentrated sodium hydroxide solution. The amphoteric aluminium oxide dissolves as sodium aluminate, while insoluble impurities such as \(Fe_2O_3\) and \(SiO_2\) do not dissolve and are filtered off:
(iii) Function of molten cryolite: it acts as a solvent (flux) for the alumina and lowers the melting point of the electrolyte from about 2045 °C to about 950 °C, so that electrolysis can be carried out at a much lower temperature while also increasing the electrical conductivity of the melt.
