Difference between revisions of "ICE table"

Latest revision as of 19:16, 21 September 2010

An ICE table or ICE chart is a tabular system of keeping track of changing concentrations in an equilibrium reaction. ICE stands for "Initial, Change, Equilibrium"^[1]. It is used in chemistry to keep track of the changes in amount of substance of the reactants and also organize a set of conditions that one wants to solve with. Occasionally, the chemical reaction is written at the top of the table, resulting in a RICE table.

Example

To illustrate the processes, consider the case of dissolving a weak acid, HA, in water. How can the pH be calculated? First write down the equilibrium expression.

HA <math>\rightleftharpoons</math> A^- + H⁺

The columns of the table correspond to the three species in equilibrium.

The first row, labelled I, has the initial conditions: the concentration of acid is c₀ and it is initially undissociated so the concentrations of A^- and H⁺ are zero.

The second row, labelled C, specifies that when the acid dissociates, its concentration changes by an amount -x and the concentrations of A^- and H⁺ both change by an amount +x. This follows from the equilibrium expression. The columns which have initial values of zero will always be added to (augmented). Note that the coefficients in front of the "x" correlate to the mole ratios of the reactants to the product. For example, if the equation worked out to have 2 H as a product, the "change" would be "2x"

The third row, labelled E, is the sum of the first two rows and shows the concentrations at equilibrium.

It can be seen from the table that at equilibrium [H⁺] = x.

To find x the equilibrium constant must be specified.

<math>K_a = \frac{[H^+][A^-]}{[HA]}</math>

Substitute the concentrations with the values found in the last row of the ICE table.

<math>K_a = \frac{x.x}{c_0 - x}; x^2-K_ac_0 +K_a x = 0</math>

With specific values for c₀ and K_a this equation can be solved for x. Assuming^[2] that pH = -log₁₀[H⁺] the pH can be calculated as pH = -log₁₀x.

When the degree of dissociation is quite small, c₀ >> x and the expression simplifies to

<math>K_a = \frac{x.x}{c_0}</math>

and pH= 1/2( pK_a - log c₀). This approximate expression is good for pK_a values larger than about 2.

References

1. ↑ http://www.youtube.com/user/genchemconcepts#p/a/u/5/LZtVQnILdrE
2. ↑ Strictly speaking pH is equal to -log₁₀{H⁺} where {H⁺} is the activity of the hydrogen ion. In dilute solution concentration is almost equal to activity