Deduce the extent of the reaction at 200 °C and 1 atm.

The Contact process operates at a temperature of 450 °C and a pressure of 2 atm as optimum conditions for the production of \({\text{S}}{{\text{O}}_{\text{3}}}\). Outline the reasons for choosing these conditions.

Temperature:

Pressure:

An engineer at a Contact process plant hypothesized that using pure oxygen, instead of air, would increase the profits. Comment on whether or not her hypothesis is valid, giving your reasons.

concentration of products is much higher than the concentration of reactants / reaction nearly/almost goes to completion / position of equilibrium lies very far to the right / OWTTE;

Response must indicate the position of equilibrium is far to the right, but not complete conversion.

Temperature:

rate of reaction/production is slow at low temperature/fast at high temperature / OWTTE;

forward reaction is exothermic/backward reaction is endothermic

and

high temperature shifts equilibrium to left/reactants/favours reverse reaction / low temperature shifts equilibrium to right/products/favours forward reaction / OWTTE;

450 °C is a compromise temperature / produces a relatively good equilibrium yield at a reasonably fast rate;

Pressure:

rate of reaction/production is slow at low pressure/fast at high pressure / OWTTE;

more moles of gaseous reactants/less moles of gaseous products

and

high pressure shifts equilibrium to right/products/favours forward reaction / low pressure shifts equilibrium to left/reactants/favours reverse reaction / OWTTE;

a high yield/good reaction rate is obtained even at low pressure;

high pressure is expensive/dangerous;

(hypothesis is not valid as) equilibrium already nearly goes to completion / OWTTE;

(hypothesis is not valid as increase in yield may not be worth) expense of using pure oxygen / OWTTE;

(hypothesis is valid as pure oxygen) increases the rate of (the forward) reaction / more \({\text{S}}{{\text{O}}_{\text{3}}}\) produced per hour/day;

(hypothesis is valid as pure oxygen) shifts equilibrium to the right/products/\({\text{S}}{{\text{O}}_{\text{3}}}\) / increases the equilibrium concentration of \({\text{S}}{{\text{O}}_{\text{3}}}\);

Award [1 max] if no reference to “hypothesis”.

A number of candidates seemed confused as to what Part (a) required, but most students could relate completeness of reaction to the value of \({K_{\text{c}}}\), a common error being to declare the reaction “complete” rather than “almost complete”. Obviously some candidates had covered the “compromise” conditions for the reaction in some detail and could give a thorough answer to Part (b), though candidates often failed to give reasons (forward reaction exothermic and decreases moles of gas) for equilibrium shifts. Candidates seemed less at ease with the hypothesis question in Part (c), with many stating opinion without any reference to the hypothesis, in addition quite a few failed to realise that two separate factors were required to gain full marks.

A number of candidates seemed confused as to what Part (a) required, but most students could relate completeness of reaction to the value of \({K_{\text{c}}}\), a common error being to declare the reaction “complete” rather than “almost complete”. Obviously some candidates had covered the “compromise” conditions for the reaction in some detail and could give a thorough answer to Part (b), though candidates often failed to give reasons (forward reaction exothermic and decreases moles of gas) for equilibrium shifts. Candidates seemed less at ease with the hypothesis question in Part (c), with many stating opinion without any reference to the hypothesis, in addition quite a few failed to realise that two separate factors were required to gain full marks.

A number of candidates seemed confused as to what Part (a) required, but most students could relate completeness of reaction to the value of \({K_{\text{c}}}\), a common error being to declare the reaction “complete” rather than “almost complete”. Obviously some candidates had covered the “compromise” conditions for the reaction in some detail and could give a thorough answer to Part (b), though candidates often failed to give reasons (forward reaction exothermic and decreases moles of gas) for equilibrium shifts. Candidates seemed less at ease with the hypothesis question in Part (c), with many stating opinion without any reference to the hypothesis, in addition quite a few failed to realise that two separate factors were required to gain full marks.