Compilation of my Chemistry revision
This is for the 2021 Chemistry AS level OCR course. The chapters follow the same chapters in the OCR Oxford textbook A, up to about half-way, as I only went up to AS-level
First published 4th December 2021
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Of course catalysts have to be special, and have a bunch more info. One handy aspect is that we haven't covered this part in lessons, which isn't especially helpful. Catalysts act as an intermediate in a reaction, and speed up the RoR without being used up. This is because they provide an alternate pathway for the reaction to happen, which has a lower activation energy. Going back to the last topic, this can be seen on the energy profile, as with a catalyst the curve is smaller. There are 2 types of catalyst, homogeneous and heterogeneous. Homogeneous catalysts have the same physical state as the reactants, and the catalyst reacts with the reactants, forming an intermediate. This intermediate breaks down, and the products are formed, plus the catalyst is regenerated. On the other hand, heterogeneous catalysts are a different state to the reactants, which is very often a solid catalyst in gas reactants. This means the reactants are absorbed by the catalyst, the reaction happens, and then the products are desorbed (apparently that's a word). As a last little catalyst fact, they are quite environmentally friendly, as they provide the need for less energy being used up. That's nice of them.
New Scientist incoming. This section was inspired by Boltzmann, who was named after the Boltzmann distribution, which coincidentally he invented. The Boltzmann distribution is a graph, of energy (x) against number of molecules (y). Also these molecules are specified to have energy. The thing that's special about this graph is that we need to know the shape of it, and be able to sketch it. This means there aren't any values on the axis, except a marker on the x-axis we need to know, which is the activation energy. The area under the graph is the total number of molecules. With this graph, you can basically see the comparison of the amount of molecules with each energy. The graph starts at the origin, as no molecules have 0 energy, but it doesn't meet the x-axis at this end.
At the standard temp, the graph curves up, and then curves down. The graph curves up first, showing that at these conditions, there are lots of molecules with a moderate amount of energy. Then, you go down, so there are less molecules with lots of energy. Somewhere near-ish the end, you have a marker on the x-axis, which is the activation energy, Eₐ. This shows that most the molecules are below the activation, but after that point those molecules are above Eₐ, and these molecules (which is only a small fraction of the overall population) can react.
This is just the standard distribution, so the graph differs for different circumstances. If temperature is increased, then the graph height lowers, but as there are still the same number of molecules (so the area must be the same), the graph width fattens. This means that at and above the activation energy there are more molecules, so the reaction is faster. If temperature is decreased, the graph increases near the start, so more molecules have less energy. This also means there are less molecules with energy ≥ Eₐ.
The next factor that could be changed is concentration. This one seems quite simple, as a change in the amount of molecules changes the graph size by that, e.g halving the concentration of molecules, means the graph area is half the size, so the graph is half the size. This graph looks different to the temperature ones as with those the area is the same.
