Chemguide: Core Chemistry 14 - 16

Some common endothermic reactions

This page gives a number of examples of simple endothermic reactions. An endothermic reaction is one in which heat is absorbed.

I am assuming that you have already read the page introducing energy changes during reactions.

An introduction to endothermic reactions

The first bit of video comes from the excellent University of Nottingham series of chemistry videos. It explains what an endothermic reaction is using the simple example of ammonium nitrate crystals dissolving in water.

Dissolving ammonium nitrate in water is endothermic because more energy is used to pull the ions in the solid apart than is produced when the ions form new bonds with water molecules.

Thermal decomposition reactions

Thermal decomposition is splitting up a compound into smaller pieces using heat. You have already come across an example of this in the effect of heat on copper(II) carbonate on the introductory page.

The thermal decomposition of carbonates is easy to do, and easy to write equations for. Here are a couple more.

Thermal decomposition of zinc carbonate

Zinc carbonate breaks up to form zinc oxide and carbon dioxide. You could see the effect of the gas moving the fine particles of zinc carbonate around at the beginning.

ZnCO3(s)   ZnO(s) + CO2(g)

Zinc oxide is a useful example of the effect of heat on a white carbonate because it gives a product which is coloured - at least when it is hot.

Thermal decomposition of lead(II) carbonate

Lead(II) oxide which is formed is red when it is hot, but cools to yellow. The carbon dioxide is again passed through lime water which turns milky.

PbCO3(s)   PbO(s) + CO2(g)

Thermal decomposition of copper(II) nitrate

Nitrates also decompose when you heat them. Most of them split up to give an oxide, brown nitrogen dioxide gas and oxygen.

Blue copper(II) nitrate crystals have the formula Cu(NO3)2.3H2O.

When you heat them, the first thing that happens is that the copper(II) nitrate dissolves in its own water of crystallisation to give a blue-green liquid. When the water is all driven off, the copper(II) nitrate decomposes to give black copper(II) oxide and a brown gas - a mixture of nitrogen dioxide and oxygen.

The fan noise you can hear in the video is because the reaction is being done in a fume cupboard. Nitrogen dioxide is poisonous.

After the water has all gone:

2Cu(NO3)2(s)   2CuO(s) + 4NO2(g) + O2(g)

The reaction between citric acid and sodium hydrogencarbonate

I am including this because I don't want you to get the idea that all endothermic reactions need heat to make them happen.

There is a rapid reaction between citric acid and sodium hydrogencarbonate giving off lots of carbon dioxide, but the temperature of the mixture falls.

The video you are about to see calls sodium hydrogencarbonate by one of its common names "baking soda".

Note:  I know the commentary is flawed - please don't tell me about it! She mis-remembers the original temperature, and 17.8°C isn't going to feel "really cold". It doesn't matter - all that matters is that the temperature has dropped.

The reaction is between hydrogen ions from the acid and hydrogencarbonate ions.

H+(aq) + HCO3-(s)   H2O(l) + CO2(g)

The reason it is endothermic is that citric acid is a weak acid, not producing many hydrogen ions in solution. Energy has to be used to ionise the citric acid, and the energy you get from the equation above isn't enough to compensate for that.

The reaction has to get that energy from the surroundings, and so the surroundings get colder.

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© Jim Clark 2020