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A reaction system adjusts itself when its equilibrium is disturbed by changing the reaction parameters. These adjustments counteract the effect of change and are explained using Le-Chatelier's principle.

Using Le-Chatelier's principle, the behaviour of a reaction system at different temperatures and pressures can be studied, and the conditions at which maximum yield is obtained can be determined.

The direction favoured by a reaction depends on its enthalpy and the nature of reactants and products.

The manufacture of sulfuric acid from sulfur by oxidation is called the contact process. T is the most common method used for sulfuric acid manufacture and uses different catalysts. In this process, the sulfur is oxidized using oxygen to sulfur dioxide.

The sulfur dioxide undergoes catalytic oxidation to sulfur trioxide which forms oleum with sulfuric acid. Oleum undergoes hydrolysis to form 2 moles of sulfuric acid. The catalysts used include nitric oxide, vanadium pentoxide, etc.

(a) The enthalpy of the reaction is -99.2 kJ. Therefore, it is a negative value, and the reaction is exothermic. As the temperature increases, the heat content of the system increases. Therefore, the rate of exothermic reactions decreases at higher temperatures. Since oxidation of sulfur dioxide is exothermic, it is favoured at lower temperatures.

The reactant side contains1 a mole of sulfur dioxide and half a mole of oxygen, while the product side contains 1 mole of sulfur trioxide. Therefore, the total number of moles on the reactant side is 1.5, and on the product side is 1. The total pressure is directly proportional to the number of moles, and at higher pressure, the system decreases the pressure by favouring the reaction forming a lesser number of moles. Therefore, the formation of sulfur trioxide is favoured at higher pressures as that side has a lower number of moles.

Therefore, catalytic oxidation of sulfur dioxide is favoured at low temperatures and high-pressure conditions.

(b) Oxygen is on the reactant side. Therefore, the addition of oxygen to the mixture increases the reactant concentration. The reaction quotient can be calculated by the equation:

$$\begin{gathered} \mathbf{Q}\mathbf{}\mathbf{=}\frac{\left[\mathrm{Product}\right]}{\left[\mathrm{Reactant}\right]} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\frac{\left[{\mathrm{SO}}_3\right]}{\left[{\mathrm{SO}}_2\right]{\left[O_2\right]}^{\mathbf{1}\mathbf{/}\mathbf{2}}} \end{gathered}$$

Therefore, the addition of oxygen decreases the reaction quotient.

Equilibrium constant K is independent of the reactant concentration. When oxygen is added, the system's equilibrium is disturbed and the value of the reaction quotient changes. The system then adjusts itself such that the reaction quotient value becomes equal to the equilibrium constant.

The equilibrium constant can be changed only by changing the reaction temperature.

(c) The catalyst increases the rate of both the forward and backward reactions to the same extent. Catalysts used in the oxidation of sulfur dioxide provide a surface for the adsorption of the reactants so that the collision between the reactants increases. Catalysts also help in carrying out reactions at a lower temperature.

Therefore, catalysts are used to rapidly oxidize sulfur dioxide at lower temperatures.