Q.Write the conditions to maximise the yield of H2SO4 by Contact process.

Step 1 — The key equilibrium.

Step 2 — Le Chatelier-based conditions.

• Temperature: being exothermic, low temperature favours the forward reaction, but the rate becomes too slow at very low T; an optimum working temperature of about 720 K (~450 °C) is used, aided by the $V_2O_5$ catalyst.
• Pressure: since moles of gas decrease in the forward direction, higher pressure favours $SO_3$ formation; a moderate pressure of about 2 atm is used industrially (very high pressure is not economically needed given already-high conversion).
• Catalyst: $V_2O_5$ (vanadium pentoxide) increases the rate of attaining equilibrium without changing its position.
• Excess oxygen (air): using excess $O_2$ shifts the equilibrium further toward $SO_3$ formation (Le Chatelier, increasing a reactant's concentration).

Step 3 — Downstream step. …

Apply Le Chatelier's principle to the $2SO_2 + O_2 \rightleftharpoons 2SO_3$ equilibrium for temperature, pressure, and excess reactant, and separately note the catalyst's role …

• Don't say very high pressure is used industrially — because the SO₂→SO₃ conversion is already highly efficient (~97%) at moderate pressure with the $V_2O_5$ catalyst, only a modest pressure (~2 atm) is economically used, unlike the Haber process's much higher pressures. …