Factors affecting reaction rate

Nature of reactants

Reactions involving strong bonds are slower than those involving weak bonds. For instance, molecules often react slower at room temperature than ions in solution, so the combustion of methane at STP is typically slower than the precipitation of lead (II) iodide.

This is because strong bonds have high activation energies as more energy is needed to break the bonds.

Concentration of reactants

Increasing concentration of reactants by:

  • Increasing the solute in a solvent
  • Decreasing the solvent in a solution
  • Decreasing the volume of a gaseous mixture
  • Increasing the partial pressure of a gaseous mixture

results in an increase in the rate of reaction.

For a successful reaction, particles need to collide. If concentration is increased, more particles are present in a smaller space so there is a greater chance of the particles colliding and thus reacting.

Subdivision of reactants

The greater the surface area of a reactant that is exposed to the other reactant, the faster a reaction will take place. The powdered form would react more quickly than one in chips or in a solid block, and the gaseous form even more quickly.

If surface area is increased, more of it is exposed to collisions with the particles of the other reactant, increasing the chance of a successful collision.

Temperature

Increasing temperature will increase reaction rate.

Increasing temperature increases the average kinetic energy of the particles, meaning more of them would have sufficient activation energy for a successful collision. In addition, particles will move faster and so collide more often.

Catalyst

The presence of a catalyst is not used up in the reaction.

They provide an alternate reaction pathway by lowering the activation energy of the reaction.

catalysed reaction

Catalysts, such as enzymes, often do this by bonding with a reactant so that its particles are in a more favourable orientation so that more collisions are successful.

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