Temperature: An increase in temperature increases the rate of reaction.

Transcription

1 9 Kinetics I Factors affecting the rate of reaction Temperature: An increase in temperature increases the rate of reaction. Concentration: An increase in the concentration of a solution of reactants increases the rate of reaction. Pressure: Increasing the pressure of a gas involved in a reaction increases the rate of reaction. Increasing a gas pressure is like increasing the concentration of the gas. Surface area of reactants: An increase in the surface area of a solid reactant increases rate of reaction. The surface area of a solid is increased if it is broken into smaller pieces. Catalyst: The use of a catalyst increases the rate of reaction (including photocatalysis).

2 Theories of Kinetics Collision theory* the basics For a reaction to occur, particles must collide but few collisions result in reaction. There are 2 additional requirements: The molecules must hit at the correct angle and /or orientation. This is a steric factor. Sufficient kinetic energy must exist to overcome the activation energy. What is steric hindrance? What is the definition of activation energy? *Although the theory originally referred to gases, it is extended to cover liquids and solids These 2 factors combine to determine whether a collision is successful. Particles may collide in the correct orientation but with insufficient ke, or with ample ke but at the wrong angle. Read pp252/3 & do Qs

3 Surface area The rate of reaction depends on the rate of collisions or collision frequency, and the proportion of molecules with enough ke. Thus, we can explain the factors that affect rate of reaction based on these ideas: Increasing concentration, pressure or surface area results in an increase in the number of collisions. Read pp254/5 & do Qs

4 Increasing temperature increases the number of particles with sufficient ke to react and the collision frequency. The first factor is more significant, but why? The Maxwell-Boltzmann Distribution In any system, the particles present will have a very wide range of energies. For gases, the Maxwell-Boltzmann Distribution represents the number of particles having each particular energy. The area under the curve is a measure of the total number of particles. The green area represents the number of particles with E>Eact Read pp256/7 & do Qs

5 The Maxwell-Boltzmann Distribution and activation energy If the temperature is increased, there is a large increase in the number of molecules with sufficient energy to successfully collide, therefore the rate of reaction increases rapidly. NB. The total area remains the same(!!) and the peak is lower and shifts to the right.

6 Catalysts To be precise, a catalyst does not lower the activation energy of a reaction - it provides an alternative route for the reaction with a lower activation energy. In this way, the number of successful collisions and therefore, the rate of a reaction are increased.

7 We can show this on a Boltzmann Distribution: The new activation energy refers to that of the new route. Read pp258/9 & do Qs

8 HETEROGENEOUS CATALYST THEORY: The catalyst and reactants are in different phases (usually solid catalyst and liquid/gaseous reactant) The reaction occurs on the catalyst surface which may be the transition metal or one of its compounds, examples quoted above. The reactants must be adsorbed onto the catalyst surface at the 'active sites. This can be physical adsorbed or 'weakly' chemically bonded to the catalyst surface. Either way, it has the effect of concentrating the reactants close to each other and weakening the original intra-molecular bonds within the reactant molecules and so allows a greater chance of 'fruitful' collision. Why are heterogeneous catalysts so important to the chemical industry?

9 Homogeneous catalysis theory The catalyst and reactants are in the same phase (usually a solution), and so the catalysed reaction can happen throughout the bulk of the reaction medium. Catalysis can be due to temporary changes in the oxidation state and ligand(s) of a transition metal ion and results in a 'catalytic cycle'. In other words, the reaction occurs via some intermediate species and the original catalyst is reformed Read pp258/9 & do Qs