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C8 - Rates of Reactions

What Factors Affect the Speed of Chemical Reactions?

Kinetics is the study of the speed of a chemical reaction. Some chemical reactions are fast; others are slow. Sometimes chemists want to speed the slow ones up and slow the fast ones down.

There are several factors that affect the speed of a reaction:

  • Surface area of reactants
Reaction depends on collisions. The more surface area on which collisions can occur, the faster the reaction. You can hold a burning match to a large chunk of coal and nothing will happen. But if you take that same piece of coal, grind it up very, very fine, throw it up into the air, and strike a match, you’ll get an explosion because of the increased surface area of the coal.

  


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  • Pressure on the reactants:
Increasing the pressure of a gas is exactly the same as increasing its concentration. If you have a given mass of gas, the way you increase its pressure is to squeeze it into a smaller volume. If you have the same mass in a smaller volume, then its concentration is higher.

Increasing the pressure on a reaction involving reacting gases increases the rate of reaction by reducing the overall volume which increases the concentration. Changing the pressure on a reaction which involves only solids or liquids has no effect on the rate of reaction.

  • Concentration of the reactants
Increasing the number of collisions speeds up the reaction rate. The more reactant molecules there are colliding, the faster the reaction will be. For example, a wood splint burns okay in air (20 percent oxygen), but it burns much faster in pure oxygen
In most simple cases, increasing the concentration of the reactants increases the speed of the reaction. However, if the reaction is complex and has a complex mechanism (series of steps in the reaction), this may not be the case. Determining the concentration effect on the rate of reaction can give you clues as to which reactant is involved in the rate-determining step of the mechanism.


  • Temperature of the reactants
Increasing the temperature causes molecules to move faster, so there’s an increased chance of them colliding with each other and reacting. But increasing the temperature also increases the average kinetic energy of the molecules.

The following figure shows an example of how increasing the temperature affects the kinetic energy of the reactants and increases the reaction rate.

  • Agitation :
Agitating or mixing the reactants  increases the chance of collision by forcing the atoms together.  
  • Catalysts:
Catalysts are substances that increase the reaction rate without themselves being changed at the end of the reaction. They increase the reaction rate by lowering the activation energy for the reaction.

In the preceding figure, if you shift to the left that dotted line representing the minimum amount of kinetic energy needed to provide the activation energy, then many more molecules will have the minimum energy needed, and the reaction will be faster.

Catalysts lower the activation energy of a reaction in one of two ways:

1) Providing a surface and orientation

2)Providing an alternative mechanism (series of steps for the reaction to go through) with a lower activation energy
  • inhibitors
There is also something called an inhibitor that works exactly the opposite of catalysts. Inhibitors slow the rate of reaction. Sometimes they even stop the reaction completely. You might be asking, "Why would anyone need those?" You could use an inhibitor to make the reaction slower and more controllable. Without them, some reactions could keep going and going and going. If they did, all of the molecules would be used up. That would be bad, especially in your body. 

If you want to produce as much of a product as possible as fast as possible in a chemical reaction, you must consider the kinetics of the reaction.