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Effect of concentration on reaction rate

Concentration influences the rate of reaction in solutions.

At a lower concentration (left), there are fewer collisions while at a higher concentration (right), there are more collisions.
At a lower concentration (left), there are fewer collisions while at a higher concentration (right), there are more collisions.

As the concentration (measured in moles of a chemical per unit volume) of reactant molecules increases, collisions between reactant molecules become more frequent.

This increases the likelihood of having many effective collisions. The rate of reaction thus increases with concentration.

Acid rain corrodes statues more quickly than normal rainwater because acid rain has higher concentration of $$\ce{H^+}$$ ions.

Pressure influences the rate of reaction in gases.

For gases, pressure is directly related to concentration.

As the pressure increases, collisions between molecules become more frequent. The rate of reaction thus increases with pressure.

When ammonia is produced from hydrogen and nitrogen gas, the pressure is set to 200 atmospheres because the reaction is too slow at lower pressures.