Questions: Reaction Quotient (Q) and Equilibrium Direction

Question 1

For the reaction A ⇌ 2B, Kc = 10. A student measures [A] = 0.5 M and [B] = 3.0 M, calculates Q = (3.0)²/(0.5) = 18, and concludes 'Q > K, but since we need more products, the reaction will proceed forward.' What is wrong with this reasoning?

• A. The student calculated Q incorrectly; the correct value is less than 10
• B. The student's conclusion is backwards: Q > K means there are too many products relative to equilibrium, so the reaction proceeds in reverse
• C. The student is correct — when Q > K, there is room to make more products
• D. The comparison Q vs K only applies at standard conditions; at arbitrary concentrations you must use ΔG directly

Answer: (B)

When Q > K, the ratio of products to reactants is already too high compared to what equilibrium demands. The reaction must shift in reverse — converting products back to reactants — until Q falls to match K. The student confused the direction: Q > K does NOT mean 'need more products,' it means 'already have too many products.' The K value is the target; Q is where you are. If your coordinates (Q) are higher than the destination (K), you must move backward, not forward.

Question 2

A reaction has Kc = 0.001 at a given temperature. You mix only reactants (no products) and measure Q = 0. What does this tell you about the direction the reaction will proceed?

• A. The reaction will not proceed because Kc is very small, meaning products are heavily disfavored
• B. The reaction will proceed in the forward direction because Q < K, regardless of how small K is
• C. The reaction will proceed in reverse because Kc < 1 means reactants are favored
• D. The comparison is undefined when Q = 0 because you cannot divide by zero

Answer: (B)

When Q < K, the reaction always proceeds forward — regardless of how small K is. Q = 0 (no products present) is always less than any positive K, so the forward reaction will occur. It will produce very little product (since K is tiny, only a small amount of products will be present at equilibrium), but it will proceed forward nonetheless. The magnitude of K tells you where equilibrium lies (favoring reactants or products), but the direction of approach is determined by comparing Q to K, and Q < K always means forward.

Question 3

The equilibrium constant K changes whenever you add more reactant to a reaction mixture at constant temperature.

• T. True
• F. False

Answer: False

K is determined solely by temperature — it is a property of the reaction at a given temperature, not of the specific amounts of reactants and products present. Adding more reactant changes Q (the reaction quotient) immediately, but K stays fixed. The new Q < K comparison then drives the reaction forward until Q rises back to K. This is the power of the Q vs K framework: K is the destination, and adding or removing substances changes Q (your current position) while K (the destination) remains constant. Only changing the temperature changes K.

Question 4

Calculating Q requires knowing which concentrations or pressures the reaction mixture has right now, and gives the same numerical value as K only when the system is at equilibrium.

• T. True
• F. False

Answer: True

Correct. Q and K use identical mathematical expressions — products over reactants, raised to stoichiometric powers — but Q uses whatever concentrations or pressures the system has at the moment of measurement, while K uses only the equilibrium concentrations. At any non-equilibrium state, Q ≠ K. Q = K is the definition of equilibrium: it is the one concentration ratio where no net forward or reverse reaction occurs. Before equilibrium, Q ≠ K, and the sign of (K − Q) tells you the direction the system will shift.

Question 5

Explain why K is called the 'destination' and Q is called the 'current position' of a reaction, and how this comparison predicts the direction of reaction.

Model answer: K is fixed at a given temperature — it represents the unique ratio of products to reactants that the system will always settle into at equilibrium, no matter what you start with. It is the destination because every reaction mixture, regardless of initial composition, moves toward Q = K. Q is calculated from the current (non-equilibrium) concentrations, so it tells you where the system is right now on its journey toward K. If Q < K, the current product-to-reactant ratio is too low — the system must make more products to reach K, so the forward reaction dominates. If Q > K, the ratio is too high — the system must consume products and regenerate reactants, so the reverse reaction dominates. When Q = K, the system has arrived at its destination and no net change occurs.

The GPS analogy captures the practical power of Q: you don't need to run the experiment to know which way it will go. Mix any arbitrary amounts of reactants and products, calculate Q from those concentrations, compare to K, and the direction follows immediately. This is especially useful in industrial chemistry (where conditions change constantly), biochemistry (where reactions are rarely at equilibrium in living cells), and analytical chemistry (where predicting precipitation or dissolution requires knowing whether Q exceeds the solubility product K_sp).