An Exothermic Reaction

Right, this is going to be a really quick post for a pretty short topic!

Here’s the basics: when chemical reactions occur, energy is transferred (because energy is required to break and make bonds). Two things can happen in this case: either the reaction ‘gives out’ heat (energy) to its surroundings, or it ‘takes’ heat (energy) from its surroundings. We have two special terms for these events:

  • Exothermic – This is when the reaction ‘gives out’ energy, in the form of heat. Think of it as heat exiting the reaction (or alternatively, just learn the two different definitions – there are only two after all…
  • Endothermic – You guessed it – this is when the reaction ‘takes’ (usually heat) energy from the surroundings, for its own devious purposes. You could think of it as energy entering the reaction.

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reversible reaction symbol

This is a quick one, believe it or not. A reversible reaction is simply what it says on the tin – a reaction that can go both ways.

So if the products of a chemical reaction can react to produce the reactants, we can say that this is a reversible reaction – meaning that the reaction can go in both directions.

Want an example? Here’s one:

ammonium chloride ⇌ ammonia + hydrogen chloride

Here, you can see that the ammonium chloride can be thermally decomposed in order to form ammonia and hydrogen chloride. However, when cooled, this products react to form ammonium chloride again – a perfect example of a reversible reaction


And that’s about it for reversible reactions – a quick topic, to be sure. You won’t need any specific examples in the exam, but you may be given information about other types of reversible reaction or other specific reversible reactions in the exam. As ever, it’s a case of applying knowledge where you can.

Hope this helped!


The yield of a chemical process is how much of a product that you make from the reactants, so you can find out how efficient your reaction is (in terms of producing useful products). Here’s a quick guide on how to calculate them and on what they mean.

Note: Before reading this article, it is ideal that you read the article on moles here:

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There are two types of chromatography. Today, we will be discussing paper chromatography, also known as ‘simple’ chromatography – for that very reason: it’s simple! As you may have guessed, paper chromatography is carried out on paper. Chromatography is used to separate different mixtures into their separate components – for example: you can separate inks, food colourings and dyes.

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