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
Enthalpy? Isn't that from Tenet? I guess yeah, as it is a sci-fi, and not fantasy. But did Tenet mention anything about Hess cycles? Thankfully not, as that would have decreased my enjoyment of the film. But enthalpy is real, and something we need to learn about. Overall the topic is mostly about concepts, and there isn't much to memorise like definitions and experiments and stuff. Unfortunately that does make it harder to write about, but better to remember. We'll see how it goes at the end. You can be the judge of whether Tenet or this topic is more confusing.
Enthalpy is basically to do with heat energy. The enthalpy of a system is the heat energy of it. It is also sometimes seen as the energy stored in the bonds of a molecule, which is useful for later on. A common use of enthalpies is to work out the enthalpy change (∆H) of a reaction. Here we have a simple equation, which luckily isn't too hard:
∆H=H(products)-H(reactants)
This basically shows the resultant amount of heat energy left in the system, whether is is positive or negative. That leads us nicely onto exothermic and endothermic. This was covered at GCSE, but it is important to know, and was confusing to learn at first (I have the hang of it now). Exothermic is when energy is released from the system (exits). This results in the reaction feeling hot, because energy is transferred to the surroundings. However, the ∆H is negative, as the system loses energy to the surroundings, so the products have less enthalpy than the reactants. Most reactions are exothermic. Endo on the other hand is the opposite, so energy is absorbed by the system. This results in the reaction feeling cold, as energy is transferred from the surroundings. ∆H is positive, as the system takes energy in, so the products have more energy than the reactants.
A way this enthalpy change can be shown is with a energy profile. Remember those? The concept of these is basically what I explained before, it is just a nice visualised way of seeing the enthalpy change. One aspect of an energy profile which doesn't affect ∆H is the activation energy. This is the minimum energy required for the reactions to happen, as it is the min energy for bonds to be broken.
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Here we take a small tangent from enthalpies. Don't worry though- it is all relevant, and in fact necessary to know for this topic. For enthalpies measurements, the results could vary depending on variables outside the experiment. Because of this, standard conditions are used, to get more accurate results, and results in line with other people's. The symbol to show a standard enthalpy change is ∆H°(the circle is supposed to be ⦵, which there isn't a superscript character for). The units are kjmol⁻¹.Here are the conditions:
Standard pressure- 100kPa. Also one atmosphere is 101kPa, which is quite close.
Standard temperature- room temperature, or 25°. Of course, in SI units this would be 298K
