How can concentration affect the rate of a reaction

This is due to the fact that it takes more energy to break the bonds of the strongly bonded molecules. Boundless vets and curates high-quality, openly licensed content from around the Internet.

This particular resource used the following sources:. Skip to main content. Chemical Kinetics. Search for:. Factors that Affect Reaction Rate. Learning Objective Explain how concentration, surface area, pressure, temperature, and the addition of catalysts affect reaction rate. Key Points When the concentrations of the reactants are raised, the reaction proceeds more quickly. This is due to an increase in the number of molecules that have the minimum required energy.

For gases, increasing pressure has the same effect as increasing concentration. When solids and liquids react, increasing the surface area of the solid will increase the reaction rate. This is due to an increase in the number of particles that have the minimum energy required. The reaction rate decreases with a decrease in temperature.

Catalysts can lower the activation energy and increase the reaction rate without being consumed in the reaction. Differences in the inherent structures of reactants can lead to differences in reaction rates. Some reactions can proceed at explosively fast rates like the detonation of fireworks Figure To understand the kinetics of chemical reactions, and the factors that affect kinetics, we should first examine what happens during a reaction on the molecular level.

During a molecular collision, molecules must also possess a minimum amount of kinetic energy for an effective collision to occur. This energy varies for each reaction, and is known as the activation energy E a Figure The rate of reaction therefore depends on the activation energy; a higher activation energy means that fewer molecules will have sufficient energy to undergo an effective collision. The proportionality constant k is called the rate constant , and its value is characteristic of the reaction and the reaction conditions.

A given reaction has a particular rate constant value under a given set of conditions, such as temperature, pressure, and solvent; varying the temperature or the solvent usually changes the value of the rate constant. The numerical value of k , however, does not change as the reaction progresses under a given set of conditions. The reaction rate thus depends on the rate constant for the given set of reaction conditions and the concentration of A and B raised to the powers m and n , respectively.

The values of m and n are derived from experimental measurements of the changes in reactant concentrations over time and indicate the reaction order , the degree to which the reaction rate depends on the concentration of each reactant; m and n need not be integers.

It is important to remember that n and m are not related to the stoichiometric coefficients a and b in the balanced chemical equation and must be determined experimentally. Under a given set of conditions, the value of the rate constant does not change as the reaction progresses. Although differential rate laws are generally used to describe what is occurring on a molecular level during a reaction, integrated rate laws are used to determine the reaction order and the value of the rate constant from experimental measurements.

Click the link for a presentation of the general forms for integrated rate laws. This reaction produces t -butanol according to the following equation:. Experiments to determine the rate law for the hydrolysis of t -butyl bromide show that the reaction rate is directly proportional to the concentration of CH 3 3 CBr but is independent of the concentration of water. Because the exponent for the reactant is 1, the reaction is first order in CH 3 3 CBr.

It is zeroth order in water because the exponent for [H 2 O] is 0. Recall that anything raised to the zeroth power equals 1. The reaction orders state in practical terms that doubling the concentration of CH 3 3 CBr doubles the reaction rate of the hydrolysis reaction, halving the concentration of CH 3 3 CBr halves the reaction rate, and so on. Conversely, increasing or decreasing the concentration of water has no effect on the reaction rate. Again, when working with rate laws, there is no simple correlation between the stoichiometry of the reaction and the rate law.

The values of k , m , and n in the rate law must be determined experimentally. Experimental data show that k has the value 5. The units of a rate constant depend on the rate law for a particular reaction. Under conditions identical to those for the t -butyl bromide reaction, the experimentally derived differential rate law for the hydrolysis of methyl bromide CH 3 Br is as follows:.

Thus, methyl bromide hydrolyzes about 1 million times more slowly than t -butyl bromide, and this information tells chemists how the reactions differ on a molecular level. Frequently, changes in reaction conditions also produce changes in a rate law. In fact, chemists often alter reaction conditions to study the mechanics of a reaction. Although the two reactions proceed similarly in neutral solution, they proceed very differently in the presence of a base, providing clues as to how the reactions differ on a molecular level.

Differential rate laws are generally used to describe what is occurring on a molecular level during a reaction, whereas integrated rate laws are used for determining the reaction order and the value of the rate constant from experimental measurements. Below are three reactions and their experimentally determined differential rate laws. For each reaction, give the units of the rate constant, give the reaction order with respect to each reactant, give the overall reaction order, and predict what happens to the reaction rate when the concentration of the first species in each chemical equation is doubled.

Asked for: units of rate constant, reaction orders, and effect of doubling reactant concentration. B The exponent in the rate law is 2, so the reaction is second order in HI. Because HI is the only reactant and the only species that appears in the rate law, the reaction is also second order overall.