Make or break chemistry

9th September 2005 at 01:00
John Morrison describes a memorable lesson for Year 10 students on exothermic and endothermic reactions

When a new chemical bond is being formed, does it release energy or require an input of energy? How many of your Year 10 students could confidently answer this question? If they can't, they will struggle to explain why some reactions are exothermic, but others are endothermic (see the national curriculum Sc33v: "Pupils should be taught how making and breaking chemical bonds in chemical reactions involves energy transfers").

A simple experiment with plaster of Paris will provide your students with a memorable demonstration of the energy changes involved in chemical reactions.

Breaking chemical bonds requires an input of energy. Making new chemical bonds releases energy, usually as heat. In most chemical reactions, existing bonds will be broken and new ones will be made. Whether a reaction is exothermic or endothermic depends on the relative amounts of energy used in breaking bonds and released in making bonds. Students often get confused about which process releases the energy.

When plaster of Paris sets it is making new bonds (all the particles are joining up to form a hard solid). As it sets it gives out a surprising amount of heat. A disposable plastic cupful will reach 55 degrees Celsius.

It's best to provide the students with pre-measured quantities.

Quarter-fill one cup with water at room temperature. Fill a second cup almost to the top with plaster of Paris. Add the powder to the water and stir. Pour the mixture into a small freezer bag. Fold the bag around a thermometer then wrap the whole thing in a piece of bubble wrap and leave it on the desk. It takes about 30 minutes for the temperature to rise. When students take off the bubble wrap they will be amazed by how hot the plaster has become. The temperature rise is not nearly as great or as memorable without the insulating bubble wrap.

Provide the students with some plaster of Paris that has already set. It is surprisingly hard. It is difficult to break the bonds that hold it together. It requires a significant input of energy to break it up. Make it memorable by giving the students hammers and allowing them to pulverise the plaster.

Hand out written notes that explain the energy changes in terms of bond-breaking and bond-making. To consolidate the learning, students should be given the opportunity to demonstrate their understanding. Using the plaster of Paris notes as a model, they should explain the energy changes observed in a number of other simple experiments. Three possible explanations, in order of increasing difficulty, would be the cooling effect of alcohol or surgical spirit evaporating from the back of your hand; the explosive combustion of hydrogen gas; and the drop in temperature when urea dissolves in water.

John Morrison teaches at Easingwold School, North Yorkshire Email:

Materials required

* 2kg plaster of Paris (enough for a class of 30 students working in pairs). Plaster of Paris is available from most craft shops and DIY stores and costs about pound;3.50 per 2kg bag.

* 30 plastic cups.

* 15 small freezer bags.

* 15 squares of bubble wrap.

* The total cost should be less than pound;4.

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