This page explains how adding a catalyst affects the rate of a reaction. It assumes familiarity with basic concepts in the collision theory of reaction rates, and with the Maxwell-Boltzmann distribution of molecular energies in a gas. A catalyst is a substance which speeds up a reaction, but is chemically unchanged at its end.
When the reaction has finished, the mass of catalyst is the same as at the beginning. Several examples of catalyzed reactions and their respective catalysts are given below:. Collisions only result in a reaction if the particles collide with a certain minimum energy called the activation energy for the reaction. The position of activation energy can be determined from a on a Maxwell-Boltzmann distribution:.
Only those particles represented by the area to the right of the activation energy will react when they collide. Increasing the pressure for a reaction involving gases will increase the rate of reaction. Keep in mind this logic only works for gases, which are highly compressible; changing the pressure for a reaction that involves only solids or liquids has no effect on the reaction rate. The minimum energy needed for a reaction to proceed, known as the activation energy, stays the same with increasing temperature.
However, the average increase in particle kinetic energy caused by the absorbed heat means that a greater proportion of the reactant molecules now have the minimum energy necessary to collide and react.
An increase in temperature causes a rise in the energy levels of the molecules involved in the reaction, so the rate of the reaction increases.
Similarly, the rate of reaction will decrease with a decrease in temperature. Catalysts are substances that increase reaction rate by lowering the activation energy needed for the reaction to occur.
A catalyst is not destroyed or changed during a reaction, so it can be used again. For example, at ordinary conditions, H 2 and O 2 do not combine. However, they do combine in the presence of a small quantity of platinum, which acts as a catalyst, and the reaction then occurs rapidly. Substances differ markedly in the rates at which they undergo chemical change. The differences in reactivity between reactions may be attributed to the different structures of the materials involved; for example, whether the substances are in solution or in the solid state matters.
It is simply that the majority of particles will react via the easier catalysed route. If this is the first set of questions you have done, please read the introductory page before you start. These questions cover all the various factors which affect rates of reaction, not just catalysts. The facts What are catalysts?
Some examples Some common examples which you may need for other parts of your syllabus include: reaction catalyst Decomposition of hydrogen peroxide. Manufacture of ammonia by the Haber Process. The explanation The key importance of activation energy Collisions only result in a reaction if the particles collide with a certain minimum energy called the activation energy for the reaction. You can mark the position of activation energy on a Maxwell-Boltzmann distribution to get a diagram like this: Only those particles represented by the area to the right of the activation energy will react when they collide.
Catalysts and activation energy To increase the rate of a reaction you need to increase the number of successful collisions. In other words, to move the activation energy on the graph like this: As before, particles which don't have enough energy at a particular time will at some time in the future gain energy from random collisions, just as other particles will lose energy.
Showing this on an energy profile: A word of caution! Be very careful if you are asked about this in an exam. The correct form of words is "A catalyst provides an alternative route for the reaction with a lower activation energy.
Questions to test your understanding If this is the first set of questions you have done, please read the introductory page before you start. Catalysts only affect the rate of reaction - they do not affect the yield of the reaction. A catalysed reaction produces the same amount of product as an uncatalysed reaction but it produces the product at a faster rate.
Different substances catalyse different reactions. The table describes three common catalysts. Notice that these catalysts are transition metals or compounds of transition metals. A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions.
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