When you think about chemical reactions you need to consider two things, the system and the surroundings. The system is the chemicals that are reacting. The surrounding are the test-tubes or beakers in which the chemical reactions happen. The surroundings also include the classroom and everything else around it. Basically everything except the chemicals that are reacting are the surroundings. The image below shows some common chemical reactions, including: burning, displacement reactions, a neutralisation reaction and metal acid/water reactions, can you guess what all these reactions have in common?

Well all these reactions release heat energy to the surroudings. They are all exothermic reactions.

An exothermic reaction release heat energy to the surroundings. A thermometer (part of the surroundings) would measure an increase in temperature during an exothermic reaction.

The chemicals (the system) in a chemical reaction act as a store of chemical energy. During an exothermic reaction the system (chemicals which are reacting) lose energy to the surrounding, mainly as heat. The temperature of the surrounding will increase. Remember the law of conservation of energy, this states that energy cannot be created or destroyed, only changed from one form to another. So if the reacting chemical lose energy then the surrounding must gain the exact amount of energy lost by the reacting chemicals (the system).
Hand warmers use exothermic reactions to heat your hands on a cold day and similarily self-heating cans use exothermic reactions to warm your drink when your outside.

Have you ever sucked on a sherbet sweet and felt your mouth getting slightly cooler? Well this is because there is an endothermic reaction taking place in your mouth. Sour sherbet sweets contain citric acid and a base called sodium bicarbonate. In your mouth these chemical react to release carbon dioxide, which gives the fizzing sensation when you eat sherbet, but the reaction between citric acid and sodium bicarbonate is an endothermic one. It removes heat energy from the surrounding, and in this case the surroundings are your mouth! So you should feel a cooling sensation in your mouth as you enjoy your fizzy sherbet sweets.

Endothermic reactions are much less common than exothermic reactions. One of the most spectacular endothermic reactions to see is one where solid ammonium chloride is added to a beaker containing solid barium hydroxide. The beaker containing the solid barium hydroxide is sitting on top of a block of wood on which a small pool of water has been added. When the two solids are added together and stirred vigorously so much heat is removed from the surroundings that the pool of water under the beaker freezes and forms a solid block of ice. This ice is so thick that it will stick the beaker to the wooden block. The temperature changes are very dramatic. If the experiment is done in a lab at room temperature, 25⁰C, then after the experiment is over the temperature may have dropped to -10⁰C. A temperature drop of 35⁰C. In this experiment the chemicals, the system, has gained energy from the surrounding. As a consequence the temperature of the surroundings has dropped.

An endothermic reaction removes heat energy from the surroundings. A thermometer (part of the surroundings) would measure a decrease in temperature during an endothermic reaction.

Another example of an endothermic reaction is the thermal decomposition of a metal carbonate. When heated metal carbonates decompose to give metal oxides and carbon dioxide gas.

Metal carbonate → metal oxide + carbon dioxide

Check your understanding - Questions on exothermic and endothermic reactions

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