Collision Theory

For reaction to occur, we need a collision to occur, as well as sufficient energy for the reaction to take place (which is known as activation energy).

Factors that affect Rate of Reaction

1. Nature of Reactants

   \[CH_{4}\space+ 2O_{2}\space->\space CO_{2}\space+2H_{2}O\]
   • This reaction is relatively slow compared to the next reaction because it requires the breaking of the bonds + the reformation of the bonds.
   \[Ag^{+}\space+ Cl^{-}->\space AgCl\]
   • This reaction is relatively fast, because the ions are already there, but also the +ve and -ve charges are attracted to each other.
2. Concentration of Reactants
   • Only gases and solution have variable concentrations.
   • If we increase the concentration:
     • There are more particles in the same space
     • More chance for particles to collide
     • Therefore greater frequency of collisions (and successful collisions)
     • And hence greater rate of reaction
3. Subdivision of Reactants
   • Greater surface area
     • Solids: Block -> Granules -> Powder
       • Kindling catches fire much easier than large logs.
     • Liquid: Pour -> Drop -> Spray
   • Greater chance of collision
   • Greater rate of reaction
4. Temperature
   • Probably the most obvious one, because it is the most powerful way of increasing the rate of reaction.
   • Particles will move faster
     • More chance of collisions
   • But also provides sufficient energy for a larger proportion to react
5. Presence of Catalyst
   • Catalysts allow for a more energetically favourable pathway
   • This is done through lowering the activation energy required for a reaction to occur

Types of Catalysts

1. Homogenous Catalysts
   • These are in the same physical state as the reactants and products of the reaction.
   • E.g. Reaction between ethanol and ethanoic acid (very slow even when heat is applied)
     • Adding sulfuric acid greatly increases the rate of reaction.
     • Since sulfuric acid is a solution, and the reactants and products (ethyl ethanoate) are also solutions, it is very difficult to separate these into it’s constituents.
     • Difficult to separate and use sulfuric acid again as a catalyst for another reaction
2. Heterogenous Catalysts
   • These are in different physical state from the reactants and products of the reaction.
   • E.g. The decomposition of hydrogen peroxide (very slow)
     • Adding Manganese Oxide greatly increases the rate of reaction
     • As manganese oxide is a solid, while hydrogen peroxide is a liquid, we can easily filter out the manganese oxide to reuse.
     • This can be easily done through filtration of the products (or product because hydrogen gas and water)

Chemists prefer to use heterogenous catalysts for industrial processes because they are:

• More easily separated from the products of a reaction
• Much easier to reuse
• Able to be used at high temperature