reaction quotient
/ Q /
Suppose you are hiking toward a valley floor and you want to know which way is downhill from where you stand right now. The reaction quotient plays that role for a chemical reaction: it is a snapshot, taken at any moment, that tells you which way the reaction will move next to reach equilibrium. It is calculated exactly like the equilibrium constant, but using the amounts present right now rather than the amounts at equilibrium.
Formally, the reaction quotient Q has the same products-over-reactants form as K, with each substance raised to its coefficient — but you plug in the current concentrations or pressures, whether or not the system has settled. You then compare Q to K. If Q is less than K, there is too little product, so the reaction runs forward (toward products). If Q is greater than K, there is too much product, so it runs backward. If Q equals K, you are already at equilibrium and nothing net happens.
Q is genuinely useful because it turns a vague question — 'which way will this go?' — into a simple comparison of two numbers. The one thing to keep straight is the difference between Q and K: K is a fixed property of the reaction at a given temperature, while Q is a moving number that changes as the reaction proceeds, sliding until it finally meets K.
A reaction has K = 10. You measure the current mix and compute Q = 2. Because Q is less than K, the system has not made enough product yet, so it will run forward, producing more product until Q climbs to 10 and the reaction settles.
Q < K runs forward; Q > K runs backward; Q = K is equilibrium.
Q and K use the identical formula; the only difference is which concentrations go in. Q uses whatever you have now; K uses the equilibrium values. At equilibrium they are equal.