Effect of Concentration on Equilibrium

$$\text{reactants} \rightleftharpoons \text{products}$$

Increasing a reactant’s concentration makes the system respond by using up some of the added reactant. Overall, more reactants react and more products form until equilibrium is re-established. We describe this as the position of equilibrium shifting to the right, towards the products.

This is an application of Le Chatelier’s principle: the response counteracts the change. It does not mean that all the extra reactant is used up or that its concentration must return to its original value.

Removing a product also favours the forward reaction

Decreasing a product’s concentration makes the system respond by forming more of that product. More reactants react, so the equilibrium again shifts to the right.

For example, ethanol can be formed by the reversible reaction:

$$\mathrm{C_2H_4(g) + H_2O(g) \rightleftharpoons C_2H_5OH(g)}$$

The equation identifies ethene and steam as the reactants and ethanol as the product. If ethanol is condensed and removed from the reacting gas mixture, its concentration in that mixture decreases. More ethene and steam then react to form ethanol, counteracting the removal.

A single removal is followed by a new equilibrium if conditions are then left unchanged. Continuing to remove product keeps encouraging further product formation. This is useful when the aim is to obtain more of the desired substance.

Predicting all four concentration changes

The same reasoning works whichever side of the equation is changed: adding a substance favours the direction that uses it up; removing a substance favours the direction that replaces it.

These directions refer to the equation as written: reactants on the left and products on the right.

Using given information to explain a prediction

Start with the supplied equation and identify whether the changed substance is a reactant or a product. Then decide which direction would counteract its increase or decrease, and connect that direction to the substance the question asks about.

For the ethanol equation above, suppose the given information is that steam concentration increases. Steam is a reactant, so the forward reaction uses up some of the extra steam. The equilibrium shifts right and more ethanol forms. If instead ethanol concentration increases, the reverse reaction uses up some of the extra ethanol: the equilibrium shifts left, forming more ethene and steam.

Concentration–time data can show this response too. An addition or removal produces an immediate change in the concentration of the affected substance. The subsequent changes show the reaction mixture adjusting. When the concentrations become constant again—shown by horizontal sections on a graph—the new equilibrium has been reached.

• Add reactant: right; more products form.
• Remove reactant: left; more reactants form.
• Add product: left; more reactants form.
• Remove product: right; more reactants react to replace the product.

The response counteracts the change; it need not restore the original concentrations.

Read given data

Use the equation to identify the changed substance, choose the direction that uses it up or replaces it, then state the effect on the requested product. Constant concentrations indicate that equilibrium has been re-established.