woodchurch science

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this podcast gives the user an introduction to energy level diagrams for year 11 GCSE chemistry coursework.
the presentation lasts approximately 4 minutes and you will need to refer to the diagrams in the text.

most collisions do not result in chemical change. before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the activation energy. this is shown on the energy level diagrams below (sometimes called reaction profiles).

going up to the top hump represents bond breaking on reacting particle collision. the purple arrow up represents this minimum energy needed to break bonds to start the reaction, that is the activation energy. going down the other side represents new bonds forming in the reaction products. the red arrow down represents the energy released in an exothermic reaction. however, it does not matter whether the reaction is an exothermic or an endothermic in terms of energy change, it is the activation energy which is important in terms of temperature and reaction speed.

now heated molecules have a greater average kinetic energy, and so at higher temperatures, a greater proportion of them have the required activation energy to react. this means that the increased chance of successful higher energy collision greatly increases the speed of the reaction, depending on the fraction of molecules with enough energy to react.

for this reason, generally speaking, a low activation energy reaction is likely to be fast, and a high activation energy reaction much slower, reflecting the trend that the lower the energy barrier to a reaction, the more molecules are likely to have sufficient energy to react on collision.

now i will try to resolve an apparent confusion for some gcse students!

with increase in temperature, there is an increased chance of collisions, due to the more energetic situation – but this is a minor factor when considering why rate of a reaction increases with temperature. the minimum energy needed for reaction, the activation energy, stays the same when temperature increases.

however, the average increase in particle kinetic energy caused by the absorbed heat means that a much greater proportion of the molecules now have the minimum or activation energy to react. shown as the blue shaded segment for the low temperature and the blue+red segments for the higher temperature.

it is this increased chance of a successful higher energy collision leading to product formation, that is the major factor, and this effect increases more than the increased frequency of particle collisions, for a similar rise in temperature.

this is usually only fully discussed at a-level, but it may impress your teacher for gcse coursework if you look up the maxwell-boltzmann distribution of kinetic energies.

it is often quite difficult to get over some of these ideas without considering the diagrams and graphs of probability versus particle kinetic energy, but that is up to you!

thanks to doc brown for the diagrams and to you for listening – look out for more podcasts from woodchurch science.

This entry was posted on Wednesday, September 20th, 2006 at 3:25 pm and is filed under chemistry, coursework, ks4, podcasts, warren, y11. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.