Why does rate of reaction slow down

Some reaction mechanisms are very simple, involving only one step. But most reaction mechanisms involve two or more steps to make it from the starting materials to the finished products. Just as there can be a very slow, difficult step when putting together a piece of furniture or building a model, there is usually one step in each reaction mechanism that is more complicated and slower than all of the other steps in the mechanism.

This slow step, officially known as the rate-determining step , holds up the completion of the entire reaction because it requires more energy to occur than any other step. In other words, the slow step of the mechanism has a higher activation energy E a than any other step in the mechanism.

This fact is important to chemists, because if chemists want to change the speed of the reaction, they need to make changes that affect the slow step, because if they can make the slow step of the mechanism go faster, the entire reaction will go faster. The video below gives a silly, but clear example of a process a mechanism that has several steps. While watching the video, see if you can determine the rate-determining step.

Hint : The rate-determining step changes in the middle of the video! The overall process involves starting with candies and wrappers, and ending with wrapped candies. We can state that the overall process involves two steps, with each step occurring at its own speed:. At the start of the process, the slow step is the number of candies coming down the belt, because the two women are able to wrap up each candy and have some time to wait.

But soon, the number of candies coming down the belt increases, and the women cannot wrap each candy; they have become the slow step. By carrying out several experiments, it was determined that this reaction has a two-step mechanism:.

The first step is the slow step. Let's look at how each of the four factors that affect the rate of a reaction affect the slow step. Remember that chemical reactions usually involve the collision of particles. So, if there is a higher concentration of particles, then there should be more collisions in a given amount of time, and the reaction should go faster.

We need to be careful, though, because if we want to speed up a reaction that occurs in several steps, we need to specifically speed up the slow step to make the entire reaction go faster. In other words, the reaction will go faster only if we increase the concentration of particles that take part in the slow step. The solid molecules trapped within the body of the solid cannot react.

Therefore, increasing the surface area of the solid will expose more solid molecules to the liquid, which allows for a faster reaction. For example, consider a 6 x 6 x 2 inch brick. This shows that the total exposed surface area will increase when a larger body is divided into smaller pieces.

Therefore, since a reaction takes place on the surface of a substance, increasing the surface area should increase the quantity of the substance that is available to react, and will thus increase the rate of the reaction as well.

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. At the start of the reaction, the maximum amount of Lithium is exposed to the Hydrochloric Acid. As a result, there is more surface area for successful collisions between the reactant particles - the result of these collisions is the products namely, Lithium Chloride and Hydrogen.

As the reaction progresses, less surface area becomes available and, thus, the reaction slows down until eventually there is nothing left to react. Of course, there are ways to increase the reaction rate; namely by adding a catalyst such as liver. Why does the rate of reaction decrease with time? Chemistry Chemical Kinetics Rate of Reactions. May 30, This can be answered by the collision theory : Let's take an example: Say I added some Lithium to Hydrochloric Acid and I decided to measure the rate of the reaction.