Chemical Equilibrium
Most reactions do not run to the end: they stop where the forward and backward reactions are equally fast. Type a reaction, its equilibrium constant K and the concentrations you have now. Compare the reaction quotient Q with K to see which way it goes and how strong the push is (ΔG), and get the concentrations it ends at, solved exactly, with the complete ICE table.
What is Chemical Equilibrium?
In a closed flask a reaction keeps going both ways. When the forward and backward reactions are equally fast, the concentrations stop changing: the mixture is at equilibrium, a dynamic balance rather than a standstill. Whatever you start with, the equilibrium concentrations always give the same equilibrium constant K at a given temperature.
The reaction quotient Q is built exactly like K, but with the concentrations as they are now. With too few products (Q < K) the reaction makes more; with too many (Q > K) it turns some back. That is Le Chatelier’s principle in numbers, and ΔG = RT ln(Q/K) tells how strong the push is. The ICE table (Initial, Change, Equilibrium) then tracks how far it moves: every species changes by its coefficient times the same amount x, until Q = K.
Where Q Stands Against K
Q/KNow and at Equilibrium
KICE Table
mol/L| Species | Initial | Change | Equilibrium |
|---|
Worked Techniques
Put the present concentrations into the K expression, with the coefficients as powers. Q = 4 is below K = 50.5, so more HI will form; ΔG < 0 says it happens on its own.
Write the start concentrations, let each species change by its coefficient times x, and put the result into K. Here a square root solves it; in general the equation is solved numerically, which this page always does.
When K is small, x is small compared with the start concentration and can be left out of the denominator. Check afterwards: if x is more than 5% of it, the shortcut is not good enough.
Sources & Verification
- S. S. Zumdahl & S. A. Zumdahl, Chemistry, 10th ed., Cengage Learning, 2018 — Ch. 13 (Chemical Equilibrium: K, Q, heterogeneous equilibria, ICE tables, Le Chatelier) and Ch. 17 (free energy and the equilibrium constant).
- P. Atkins, L. Jones & L. Laverman, Chemical Principles: The Quest for Insight, 7th ed., W. H. Freeman, 2016 — Focus 5 (Chemical Equilibrium: Q, K and ΔG = RT ln(Q/K)).