UNIT 2.8 - KINETICS CROWE2009

2.8.1 – Rate of reaction Connector: a. recall the factors that influence the rate of chemical reaction, including concentration, temperature, pressure, surface area and catalysts b. explain the changes in rate based on a qualitative understanding of collision theory c. use, in a qualitative way, the Maxwell-Boltzmann model of the distribution of molecular energies to relate changes of concentration and temperature to the alteration in the rate of a reaction d. demonstrate an understanding of the concept of activation energy and its qualitative relationship to the effect of temperature changes on the rate of reaction Connector: What three things must happen for a chemical reaction to take place? Draw an energy level diagram for an exothermic reaction, then use arrows to show the activation energy, and the energy released by the reaction. Explain the term activation energy.

Note: 1,2 & 3 are all necessary for a successful collision to occur For a chemical reaction to happen: The reacting particles have to collide They have to have at least the Activation Energy 3. They have to have the correct orientation e.g HX + C=C Activation energy Energy released Activation energy is the minimum amount of energy needed for a reaction to happen. Note: 1,2 & 3 are all necessary for a successful collision to occur

The importance of orientation of collision

Factors affecting the rate of a reaction Use collision theory to explain how the following factors can influence the rate of a reaction: concentration pressure temperature surface area catalysts

Effect of concentration on rate of reaction Effect of pressure on rate of reaction An increase, increases the number of reactive particles and so increases the chance of effective collisions.

Effect of temperature on rate of reaction Increasing the temperature: Increases the kinetic energy of the particles so the particles move faster this increases the number of collisions also since the particles are moving faster, each collision will involve more energy, and so the chances of an effective/successful collision is increased.

Surface area and rate of reaction Solids can only react at their surface, increasing the surface area of a solid increases the rate of reaction. The smaller the pieces the greater the surface area

Catalysts A catalyst is a substance that speeds up a chemical reaction. A catalyst is not used up in the reaction it catalyses, and so can be used over and over again. "A catalyst provides an alternative route for the reaction with a lower activation energy."

Reaction Profiles Potential energy Note: when particles collide they slow down, stop then fly apart again, so KE converted to PE that then becomes KE ∆H ∆H = Enthalpy change of reaction (Reaction coordinate)

The Maxwell-Boltzmann Distribution This is a plot of the number of particles having each particular energy The area under the curve is a measure of the total number of particles present.

The Maxwell-Boltzmann Distribution and activation energy

The Maxwell-Boltzmann Distribution and an increase in temperature

Summary exercise Sketch a Maxwell-Boltzmann distribution curve to show the effect of using a catalyst. Plus questions from work sheets or past papers