Chemguide: Core Chemistry 14 - 16

Some common exothermic reactions

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

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

Combustion reactions

Combustion involves something burning. It is pretty obvious that a combustion reaction must give out heat. Here are a few random examples.

Burning sulfur in oxygen

S(s) + O2(g)   SO2(g)

Note:  Actually, it is a bit debatable whether we shouldn't be using the state symbol (l) for sulfur. It is liquid sulfur burning, not solid sulfur. I have used (s) because that's what the video used.

Burning a hydrocarbon in air

Hydrocarbons are compounds made from carbon and hydrogen only. They range from simple ones like methane (natural gas), CH4, to much more complicated ones like those in candles. Candles contain mixtures of big hydrocarbons.

In each case when hydrocarbons burn energy is given out and carbon dioxide and water are produced.

Burning magnesium in carbon dioxide

You mustn't assume that combustion reactions must involve air or oxygen. This bit of video shows magnesium burning in carbon dioxide.

2Mg(s) + CO2(g)   2MgO(s) + C(s)

Burning sodium in chlorine

Here is another example which doesn't involve oxygen anywhere.

2Na(s) + Cl2(g)   2NaCl(s)

The reaction between calcium oxide and water

If limestone, calcium carbonate, is heated very strongly, it undergoes thermal decomposition to form calcium oxide and carbon dioxide. Calcium oxide is also known as quicklime.

If you add water to quicklime, you get calcium hydroxide - known as slaked lime. This is a strongly exothermic reaction.

The best bit of video I could find on this is actually in Spanish, but has English subtitles. It also rather annoyingly translates temperatures from Celcius to Fahrenheit.

CaO(s) + H2O(l)   Ca(OH)2(s)

Calcium hydroxide is only sparingly soluble, but some of it will dissolve to form lime water.

Reactions between Group1 metals and water

This is one of my favourite bits of video, and I have used it elsewhere on the site.

The general equation (where M is any of these metals) is:

2M(s) + 2H2O(l)     2MOH(aq) + H2(g)

Neutralisation reactions

A neutralisation reaction involves the neutralisation of an acid by reaction with a base - for example hydrochloric acid reacting with sodium hydroxide solution to make sodium chloride and water.

HCl(aq) + NaOH(aq)   NaCl(aq) + H2O(l)

These are always quoted as examples of exothermic reactions, but visually they are really boring! You mix colourless solutions of hydrochloric acid and sodium hydroxide, and get a colourless solution of sodium chloride.

All that happens is that if you put a thermometer in, the temperature is found to have gone up a few degrees.

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