The reversible reaction A(g) ⇌ B(g) has ΔH = –75 kJ mol⁻¹
CHEMISTRY · Single correct · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The reversible reaction A(g) ⇌ B(g) has ΔH = –75 kJ mol⁻¹. At 298 K the system is at equilibrium with K = 4.0 and concentrations [A] = 1.0 M and [B] = 4.0 M in a 1.0-L vessel. The temperature is rapidly raised to 350 K (where K = 1.0), and at that same instant 1.0 mol of B is injected into the vessel, raising [B] to 5.0 M. Which statement best predicts the direction in which the reaction will shift as it re-establishes equilibrium?
(a) No net shift will occur because, after the changes, Q already equals the new K.
(b) The reaction will shift toward reactants (formation of A).
(c) The reaction will shift toward products (formation of B) because adding product drives the reaction forward.
(d) The reaction will shift toward products until [B]/[A] returns to 4.0, the value of the original equilibrium constant at 298 K.