In which reaction will the point of equilibrium shift to the left when the pressure on the system is increased?
A. C(s) + O2(g) <-> CO2(g)
B. CaCO3(s) <-> CaO(s) + CO2(g)
C. 2Mg(s) + O2(g) <-> 2MgO(s)
D. 2H2(g) + O2(g) <-> 2H2O(g)
E. 2H3(g) + O2(g) <-> 2H2O(g)
A change in pressure affects equilibrium when gases are involved. An increase in pressure favors the formation of the smaller number of moles of gas (shifts to the side with the smaller number of moles of gas).
There is no effect on Choice A because each side has only 1 mole of gas. Choice C has 1 mole of gas on the left, none on the right so increased pressure shifts it to the right.
Choice D and E have 3 moles of gas (2H and 1 O) on the left, 2 moles on the right, so increased pressure shifts it to the right. Choice B has no gas moles on the left and 1 mole of gas on the right, so increased pressure shifts equilibrium to the left.