Concentration Effect on Rate – IGCSE Chemistry Definition

Increasing the concentration of a reactant in solution increases the rate of reaction. Concentration measures the number of moles of solute per unit volume of solution (mol/dm³). A higher concentration means more reactant particles are packed into the same volume, so collisions between reactant particles happen more often.

Collision theory explanation

According to collision theory, a reaction occurs when particles collide with sufficient energy (at least the activation energy) and with the correct orientation. When the concentration of a solution increases, there are more solute particles per unit volume. These particles are closer together on average, so they encounter each other more frequently. The increased collision frequency means more successful collisions occur per second, and the rate of reaction increases.

For gas-phase reactions, increasing the pressure has the same effect as increasing concentration, because the gas particles are forced into a smaller volume.

Graphical interpretation

On a graph of product formed against time, a reaction with higher concentration shows a steeper initial curve (faster rate) and reaches the final amount of product in less time. If the concentrated reactant is in excess, the final amount of product is the same as at lower concentration; it is just reached sooner.

Practical example

When magnesium ribbon is placed in dilute hydrochloric acid at two different concentrations (say 1.0 mol/dm³ and 2.0 mol/dm³), the more concentrated acid produces hydrogen gas faster. The effervescence is more vigorous and the magnesium dissolves more quickly.

Exam context

Paper 4 questions typically ask candidates to explain the effect of concentration on rate using collision theory. The explanation must include three elements: more particles per unit volume, more frequent collisions, and therefore a faster rate. Simply saying “more concentration means faster” without the particle explanation scores poorly.

Worked exam question

A student reacts zinc with 1.0 mol/dm³ HCl and then repeats the experiment with 2.0 mol/dm³ HCl. Explain why the second experiment has a faster rate. (2 marks)

With 2.0 mol/dm³ HCl, there are more H⁺ ions / acid particles per unit volume [1]. Collisions between H⁺ ions and zinc are more frequent, so there are more successful collisions per second and the rate increases [1].

Common mistakes

Candidates sometimes say “more acid” without specifying concentration. Adding more volume of the same concentration is different from increasing the concentration. The question is about particles per unit volume. Another error is forgetting to mention “per unit volume” --- the key idea is the density of particles, not the total number.