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  • In this lesson, we will learn about reversible reactions.

  • When we fry an egg, it is impossible to 'unfry' it.

  • A lot of reactions work in the same way, once it is done, it is irreversible.

  • A good example is the combustion of magnesium to form magnesium oxide.

  • Reversible reactions are reactions where formed products can revert, or go back, to their

  • original reactants. Since the reaction can proceed in either direction,

  • we use this reaction arrow to represent reversibility. An example of a reversible reaction is the

  • thermal decomposition of ammonium chloride to form ammonia and hydrogen chloride.

  • Place ammonium chloride in a boiling tube and heat using a Bunsen burner.

  • Ensure that this is conducted in a fume cupboard as ammonia and hydrogen chloride are both

  • corrosive gases. If we hold red litmus paper at the mouth of

  • the boiling tube, it turns blue, which confirms the presence of ammonia.

  • Blue litmus paper turns red, which confirms the presence of hydrogen chloride.

  • Remove the boiling tube from the heat and let cool.

  • Have a look near the mouth of the boiling tube.

  • You will see that there are white crystals, these are ammonium chloride crystals.

  • So what has happened here? Ammonium chloride has thermally decomposed

  • to ammonia and hydrogen chloride. The formed ammonia and hydrogen chloride have

  • reacted to produce ammonium chloride. Hence, ammonium chloride ammonia + hydrogen

  • chloride. Therefore, this is a reversible reaction.

  • Why is the thermal decomposition of ammonium chloride sometimes incorrectly called a 'sublimation

  • reaction'? Please pause the lesson to think about this

  • and resume when you are done. Here is a hint, think about phase changes

  • and what happens to a substance during a phase change.

  • When a substance goes from the solid phase to a gas phase, it is said that it has sublimed.

  • This phase change is not a 'reaction', as the substance itself has not changed.

  • In our example, solid ammonium chloride did not form gaseous ammonium chloride, but rather,

  • it decomposed under heat to form ammonia and hydrogen chloride.

  • Therefore, it is not a 'sublimation reaction'.

In this lesson, we will learn about reversible reactions.

Subtitles and vocabulary

B2 H-INT US chloride ammonium chloride ammonium ammonia hydrogen reaction

What are Reversible Reactions? | The Chemistry Journey | The Fuse School

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    Wayne Lin   posted on 2015/06/27
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