C2bL2  Collision Theory

Key Words

Activation Energy  - energy needed to set a reaction going.

Catalyst - a substance that increases the rate of reaction, but does not react itself, nor does it affect the amount of product.

Concentration - how much substance there is dissolved in a volume of solvent.

Frequency - number of times something happens in a set period of time.

Kinetic Energy - energy in moving objects.

Pressure - force acting on a given area.

Successful collision - collision resulting in a reaction

Surface area - total area on the surface of an object.

Temperature - how hot something is.

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Grade E

Reactions happen only when particles of reactant collide.   The rate of reaction depends on the frequency of collisions, which can be increased by increasing :

  • the concentration;
  • the pressure of a gas;
  • the temperature;
  • the surface area.

Grade C

If we increase the concentration of a solution or pressure of a gas, we increase the frequency of the collisions, so it's much more likely that there will be a successful collision in which a reaction happens. 

If we increase the temperature, the particles move about much faster, so have more kinetic energy.  The higher the kinetic energy, the more likely it is that the energy will be sufficient for a reaction.

Reactions only happen at the surface, so grinding up the reactant into a fine powder increases the surface area available for a reaction.

Catalysts make the reaction go faster by:

  • giving an alternative pathway;
  • weakening bonds between atoms.

Enzymes are biological catalysts.

 

Grade A

Reactions rarely start spontaneously.  We usually need to give the reaction a push to get it going.  Petrol vapour and oxygen forms a very explosive mixture, but will not start reacting until a little bit of energy (from a spark) sets the reaction off.  The mixture then burns explosively.

We call this push the activation energy.  It is like pushing a car up a hill, then letting it roll down the other side.  The idea is shown below.

Catalysts can reduce the activation energy by a large amount. 

In effect, they enable the reaction to "tunnel" through the energy hill:

Many of the transition metals make good catalysts.