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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.
Molecules joined by stronger bonds will have lower reaction rates than will molecules joined by weaker bonds, due to the increased amount of energy required to break the stronger bonds. Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet.
How does concentration affect the rate of a reaction? Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react. If reaction is between a substance in solution and a solid, you just vary the concentration of the solution. The experiment is straightforward.
If the reaction is between two solutions, you have a slight problem. Do you vary the concentration of one of the reactants or vary the concentration of both? You might find that the rate of reaction is limited by the concentration of the weaker solution, and increasing the concentration of the other makes no difference.
What you need to do is fix the concentration of one of the reactants to excess. Now you can increase the concentration of the other solution to produce an increase in the rate of the reaction. How does surface area affect a chemical reaction? If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.
So the larger the surface area of the solid, the faster the reaction will be. Smaller particles have a bigger surface area than larger particle for the same mass of solid.
Rate laws are mathematical descriptions of experimentally verifiable data. Rate laws may be written from either of two different but related perspectives. In contrast, an integrated rate law describes the reaction rate in terms of the initial concentration [R] 0 and the measured concentration of one or more reactants [R] after a given amount of time t ; integrated rate laws are discussed in more detail later.
The integrated rate law is derived by using calculus to integrate the differential rate law. 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.
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