Compare exothermic and endothermic reactions

The Science Behind the Science
It is often noticed that when a chemical reaction takes place, a energy change occurs. The most obvious example of this is during the burning of a fuel, such a methane. The methane is ignited and reacts with the oxygen to produce carbon dioxide and water. During this reaction, we can observe a release of energy in the form of thermal energy and light energy (a flame). This is known as an exothermic reaction, as energy is being released into the surroundings. Exothermic literally means giving out heat (exo -  outside or 'out of' and therm - heat). These exothermic reactions are very common and are something that we utilise in much of our daily lives. 

Something which is seen less commonly are chemical reactions that see the temperature drop. These are known as endothermic reactions, which literally means to take in heat (endo - in or 'in to' and therm - heat). These reactions take in energy from the surroundings which causes them to feel cold. The portable, disposable cold pack is a very good example of this. When the two chemicals inside are mixed, they react together and take in energy, causing the pack to feel very cold. Very convenient for when an athlete gets injured. 

The cause of the energy change is very interesting. All of the atoms within a molecule are connected to each other, or bonded together. For the new substances to be made, these bonds must all be broken. The bonds taken energy to break, much like when you snap a pencil. When the bonds between the atoms in the products are made, energy is released. 

If more energy is needed to break the bonds in the reactants than is released when the products form, the reaction is endothermic, as it has taken more energy from the surroundings than it has released. This is why they feel cold, as they take in the energy from the surroundings - including your fingers. 

If more energy is released when the bonds in the products are formed than is needed to break the bonds in the reactants, the reaction is exothermic, as it has released more energy into the surroundings than it took in. This is why they feel hot, as they released energy into the surroundings - including your fingers. 

If completing this practical at home, it is important to make up the solution of sodium hydroxide before students are allowed to complete it. To make up the sodium hydroxide solution, dissolve 4 g of sodium hydroxide in 100 ml of water. This will give a 1 mol - 3 solution which should generate enough heat when reacting with vinegar. Sodium hydroxide is also know as lye or caustic soda and is readily available from many DIY stores.