The equilibrium constant for the formation of phosphorus pentachloride is 1.9. If the partial pressure of each gas is 0.5 atm, will the reaction (as written) proceed forward or backward to reach equilibrium?

PCl3(g) + Cl2(g) PCl5(g)

A. Forward, because K is greater than Q
B. Backward, because K is less than Q
C. Forward, because K is less than Q
D. Backward, because K is greater than Q

Is that Kc or Kp? I will assume it is Kp since the pressures are given but the problem is worked the same way for each (Kp is used with partial pressures; Kc with concentrations in mols/L)

K = 1.9
Q = p(PCl5)/p(PCl3)*p(Cl2)
Q = (0.5)/(0.5)(0.5) = 2

Looking at Q = 2, that is > Kp of 1.9. That means the numerator is too large and the denominator too small. So the reaction must shift to the left (backwards).

I know that the answer is not A... can anyone help me any further?

To determine whether the reaction will proceed forward or backward to reach equilibrium, we need to compare the equilibrium constant (K) with the reaction quotient (Q).

The equilibrium constant (K) is a measure of the extent to which a reaction proceeds at equilibrium. It is calculated using the concentrations of the reactants and products at equilibrium.

The reaction quotient (Q) is similar to the equilibrium constant, but it is calculated using the concentrations of the reactants and products at any given point in the reaction, not just at equilibrium.

In this case, the question mentions that the partial pressure of each gas is 0.5 atm. Since partial pressure is directly proportional to concentration, we can consider this as the concentration of the gases.

For the reaction PCl3(g) + Cl2(g) ↔ PCl5(g), the equilibrium constant expression is K = [PCl5]/[PCl3][Cl2] where the brackets denote the concentrations.

Since the equilibrium constant (K) is given as 1.9, we can compare it to the reaction quotient (Q) which is calculated using the concentrations (in this case, partial pressures) of the reactants and products at a given point in the reaction.

If K > Q, it means the reaction has not yet reached equilibrium and will proceed forward to reach equilibrium. In this case, the reaction will proceed forward because K (1.9) is greater than Q (which we don't know its value as it is not given).

Therefore, the answer is:

A. Forward, because K is greater than Q.