Final answer:
In an ICE table problem, use '+x' when the reaction moves towards products and '-x' when it moves towards reactants. Discard any physically impossible values, like negative concentrations, to find the single feasible solution.
Step-by-step explanation:
When using an ICE table in a chemistry problem, the "C" stage involves identifying the change in concentrations or pressures as the system moves towards equilibrium. To determine whether to use "+x" or "-x", first identify the direction in which the reaction will proceed to reach equilibrium. If the reaction is moving towards the products (forward reaction), you subtract "-x" from the reactants and add "+x" to the products. Conversely, if the reaction is moving towards the reactants (reverse reaction), you add "+x" to the reactants and subtract "-x" from the products. Additionally, once the changes have been applied, it is necessary to calculate the concentration changes, and then calculate the equilibrium concentrations using the equilibrium constant expression.
When evaluating for both signs in the numerator (“+x” and “-x”), it is possible to obtain two values for "x". Generally, the physically impossible value (like negative concentrations), should be discarded, leaving a single feasible solution. If a quadratic equation arises during the calculations, two possible values for "x" will be found, and again, the nonsensical or negative value is discarded.