Understanding enthalpy is straightforward. It's the measure of the total amount of energy stored in a chemical system (for dummies, it's just heat energy!). The enthalpy change would be the change of enthalpy throughout a reaction, which you can get two main types: (exothermic and endothermic). For those special literature/scientists would be able to work it out for themselves: exo- prefix meaning outside, and -therm- meaning heat. An exothermic reaction is one which throws heat out from the chemical system (the reaction) into the surroundings (air, water etc.) And by common sense, endothermic reactions take in energy.
Well, all reactions take in and let out heat energy (through the breaking and making of bonds), but what determines an exothermic/endothermic reaction is the comparison between the input and outputs of energy.
We rely on these reactions everyday. Respiration is an exothermic reaction produced by our cells; as we take in glucose and oxygen a reaction occurs which converts the orientation of the atoms into carbon dioxide and water. Photosynthesis is the opposite (endothermic), which produces our vital ingredients.
Every reaction has an enthalpy change (as you know), and it's called the Enthalpy Change of Reaction (no way!) which for my spec, you couldn't walk into the exam without knowing the definition - so by default - I know them off the dome. Here I go:
The enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation. Jheeze.
Since combustion (reacting a substance with oxygen) is another important reaction, scientists have decided to exclude combustion as it's own enthalpy change called the enthalpy change of combustion (no seriously?) where most organic combustion reactions form products such as carbon dioxide and water.
The third most important (for you, anyway) is the enthalpy change of formation, which is the change made when a single mole of a compound is formed straight from it's broken down elements.
There are loads more important enthalpy changes including:
-enthalpy change of vaporisation
-enthalpy change of atomization
-enthalpy of hydration, work function enthalpies. Yeah, we could be here for a long time. But the last thing I want is my readers to walk out here not understanding enthalpy.
There are a couple ways of working out enthalpy changes, some being more useful than others. You can work it out using direct determination, where you work out the heat exchange by using the formula Q=mcT (J) and the amount in mol of the substance not in excess (mol).
Then, you scientists can use your brilliant mathematical brains to get in the form (kJ/mol) which enthalpy is measured in. Seriously though, you might want to up your math game to help you be the world's best scientists, my potential readers.
Another way is to use bond enthalpies, which is the difference between the average bond enthalpies broken to start the reaction and the ones made by the reaction (I think this method is the most straight forward)
The last method is using Hess' Law (named after Germain Hess, one of the scientists that if he was alive today, I could never disrespect him due to how much he devoted his life to chemistry). He worked out that you can follow a reaction through different routes (different intermediate product made) but as long as the initial and final conditions are the same, the enthalpy change will be the same for every one of those routes. Examiners love to ask students about these in exams to test their mathematical abilities for chemistry, so watch out my fellow readers.
Note to readers: If you would like me to expand on this, mabye if you still don't understand this and you want me to demonstrate the comment post is just below -_-
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