HCN is an important but potentially dangerous industrial chemical. It can be produced in several ways, including by the reactions shown below. Assuming that each system is initially at equilibrium, predict the direction in which the reaction will shift to respond to the indicated stress.

(a) NH₃(g) + CH₄(g) ⇄ HCN(g) + 3 H₂(g); pressure is increased 

(b) 2 NH₃(g) + 2 CH₄(g) + 3 O₂(g) ⇄ 2 HCN(g) + 6 H₂O(g); pressure is decreased 

Strategy 

Count the total number of moles of gas on each side of the equation. If pressure is increased, the reaction that produces fewer moles of gas is favored; if pressure is decreased, the reaction that produces more moles of gas is favored.