In this relatively short topic, we review and develop the GCSE work on rates of reaction and build up to A-Level which prepares us for the year 13 topic about Rate Equations.
You must fully understand all aspects of collision theory and how the 4 main factors affect the rate of a reaction. In collision theory, we understand that two reactant particles must collide with sufficient energy (Activation energy -Ea) in order for them to react. The more collision per second that occur, the greater the likelihood there is of a successful collision so this gives rise to a greater rate.
- INCREASED TEMPERATURE: Increasing the temperature means that more of the particles will have Ea or greater. Also, as the average particle now has more energy, they will move faster and collide more frequently.
- INCREASED CONCENTRATION: This is a simple concept to get your head around, if you cram more particles into the same unit volume, there will be more collisions per second which makes the rate increase. Concentrations is all about solutions, however, the same effect is achieved by increasing the pressure in gases.
- INCREASED SURFACE AREA: When the rate is dependent on the number of collisions per second, having huge numbers of the particles buried inside and not in contact with other reactant particles, it will reduce the rate. Increasing the surface area allows more reactant particles to be in contact to collide.
- ADDING A CATALYST: Catalysts lower the Ea so more of the particles will have that minimum energy needed in order to react. Catalysts offer an alternative reaction mechanism.
Below is a Maxwell Boltzmann distribution. It shows the distribution of energy within a group of particles.
- The black line shows the distribution at 298K.
- The red line shows the effect of warming it up to 323K.
- The blue line shows the effect of cooling it down to 273K
- The purple shows the effect of adding a suitable catalyst.
Note that there are more particles with Ea or greater when you heat it up (red) or at a catalyst (purple) as the Ea is lowered. By cooling the reactants (blue) very few now have the Ea. The total area under each curve is constant as we have not changed the number of particles, just the distribution of energy.
Required practical 3 – Investigate how the rate of a reaction changes when the temperature is varied. This can be done with acid at different temperatures being added to marble chips whilst collecting the CO2 given off in a gas syringe. Better still, Sodium Thiosulfate and Hydrochloric Acid will cause the formation of elemental sulfur. Time how long it takes for the solution to become opaque enough so a cross is no longer visible beneath.