Endothermic and Exothermic Reactions 1

Where the energy transferred in a reaction is heat, you can detect the type of reaction (endothermic or exothermic) by using a thermometer or your skin. Heat energy always flows from higher temperatures to lower temperatures. When you carry out an experiment in the lab at room temperature and the reaction vessel (beaker, conical flask or whatever) feels colder, then you have an endothermic reaction. Heat energy will flow from the warmer surrounings into the reaction mixture - it is taking in energy. If the vessel feels warmer, the reaction must be exothermic as the heat energy will flow from the reaction mixture into the surroundings. In practice, it isn't a good idea to just feel the temperature of a reaction vessel as you could end up with a hot or cold burn! It is better to use a thermometer

But sometimes, temperature can be misleading. When carrying out an electrolysis reaction, the temperature of the electrolyte will sometimes rise. So it must be exothermic? Wrong! Electrolysis reactions are endothermic as they absorb electrical energy. The rise in temperature is actually caused by the heating effect of the current.

For the exam, you need to know some examples of exothermic and endothermic reactions, most of which you should be familiar with from science lessons. Good examples of exothermic reactions are burning (some of the enrgy is released into the surroundings as light), the neutralization of a strong acid by a strong alkali and the reaction between calcium oxide and water to make calcium hydroxide (quicklime reacting to form slaked lime). Apart from electrolysis, good examples of endothermic reactions are any thermal decomposition (such as the decomposition of limestone in the blast furnace) and the reaction between ethanoic acid and sodium carbonate.