## Concentration at equilibrium

An ICE Table can be used to find the concentrations of all aqueous and gaseous reactants and products when a chemical reaction achieves equilibrium. It is a method of organizing stoichiometric calculations, and its letters stand for the following:

• [I]nitial concentration
• [C]hange in concentration
• [E]quilibrium concentration

All three are measured in mol/L, and they are related by I + C = E.

### Example

In the reaction H2(g) + I2(g) ⇌ 2 HI(g), 2.00 mol of H2(g) and 3.00 mol of I2(g) are placed in a 1.00 L container. Calculate the other two equilibrium concentrations if I2(g) has an equilibrium concentration of 1.30 mol/L.

Here is the I calculation for H2(g) (you don’t need to show all of them):

c = n/V = (2.00\ "mol")/(1.00\ "L") = 2.00\ "mol/L".

Let x represent the absolute value of the change in concentration of H2(g). This can also be written more concisely like this: let x = |Delta ["H"_2]|.

When writing the let statement for x, always choose a reactant or product that has a coefficient of 1. This way, you can simply fill in all the C values with -ax for reactants and +ax for products, where a is the coefficient for that reactant or product.

 H2(g) I2(g) 2 HI(g) I 2.00 3.00 0 C -x -x +2x E 2.00 − x 3.00 − x 2x

The E value for I2(g) is known to be 1.30 mol/L, but our table tells us that it is also x subtracted from 3.00 mol/L, therefore we can set them equal:

1.30\ "mol/L" = 3.00\ "mol/L" - x,

x = 1.70\ "mol/L".

By substituting 1.70 mol/L for x into the E expressions for H2(g) and HI(g), we can easily find their concentrations at equilibrium as well (0.30 mol/L and 3.40 mol/L respectively).