What will slow down a chemical reaction

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. Let's go back to the video to clarify this point.

When reactants get used up, their concentration decreases, and so less successful collision between the desired particles occur, therefore slowing down the rate of reaction. A simple lab experiment to do would be reacting hydrochloric acid with a metal, such as magnesium, and then use a gas syringe to calculate how much gas evolved. Time the experiment for around five minutes. As the reaction progresses, you see less and less bubbles being formed over time until the reaction stops, due to the concentrations of reactants decreasing, and you can see that clearly when the magnesium ribbon slowly gets used up.

What change will slow the rate of a chemical reaction? Chemistry Chemical Kinetics Rate of Reactions. Nam D. More collisions afford more opportunities for reaction. How do catalysts affect the rate of a reaction? Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction.

This is really because the reaction proceeds by a different pathway when the catalyst is present essentially lowering the activation energy required for the reaction to take place.

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. When collisions occur, they do not always result in a reaction atoms misaligned or insufficient energy, etc. Higher concentrations mean more collisions and more opportunities for reaction.

What affect does pressure have on the reaction between two gasses? You should already know that the atoms or molecules in a gas are very spread out. For the two chemicals to react, there must be collisions between their molecules. By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between them.

You can easily increase the pressure by simply reducing the volume of the reaction vessel the gases are in. 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. But you can increase the surface area of a solid by cutting it up. Think of it this way, if you have a loaf of bread you have 6 sides of surface area, correct?