Rate of Reaction & Collision Theory

The Collision Theory:

  • In a chemical reaction, the reactant particles can only react with each other when they bump into one another. According to collision theory when molecules collide bonds between their atoms can break, and then new bonds can form to make new molecules.
  • The molecules in gases and liquids are moving constantly, and millions of collisions take place every second. But only a small number of these collisions lead to the formation of product. For a collision to be ‘successful’, the particles involved must possess enough energy, called the activation energy, to break some of the existing bonds.

Measuring Rate of Reaction

  • We can measure the rate of reaction using the following equation-

1/Time taken

Factors Affecting the Rate of Reaction

Temperature- Increasing the temperature of a reaction speeds up the rate of reaction (increases the rate of reaction). The ions have more kinetic energy so they move about more and collisions occur more frequently. As there are more collisions, there are more successful collisions so the rate of reaction increases.

Concentration- Increasing the concentration of reactant liquids in a reaction also increases the rate of reaction. A high concentration means more particles per cm3 so there are more collisions per second which means there are more successful collisions so the reaction speeds up.

Surface Area- Increasing the surface area of a reactant solid increases the rate of reaction. A powdered substance always has a larger surface area than a block substance weighing the same amount. The more finely divided the solid is, the more of it is exposed to react with the other particles.

Catalyst- Use of a catalyst increases the rate of reaction. A catalyst is a substance that speeds up a reaction but is chemically unchanged. When the reaction finishes you have the same mass of catalyst. A catalyst provides an alternative route for the reaction, one with lower activation energy.