I wrote a few months ago, in response to a question from Felix, about the energy changes that occur when reactions happen. Some reactions are exothermic (that means they give out heat) whilst others are endothermic (which means that they absorb heat). Exothermic reactions are hot and endothermic ones are cold. If you have a hand warmer that you have to squeeze to create heat then you have experienced an exothermic reaction. Likewise a cold pack that you squeeze to become cold is an example of an endothermic reaction.
Combustion (burning in oxygen) is a classic example of an exothermic reaction. Hydrocarbon fuels burn in air to release heat. The hydrogen atoms react with oxygen to form water and the carbon atoms react to form carbon dioxide. The bonds between the carbon and hydrogen atoms have to be broken to make this happen – this process requires some energy which is why you usually have to ignite the fuel to get the reaction going. The new bonds that form between the hydrogen and oxygen or carbon and oxygen release heat. This is the heat you feel when you are near the burning fuel.
Because we know the energy required in breaking specific chemical bonds, and the energy released when new bonds form, we can calculate the energy given out or absorbed in different chemical reactions. Dima very sensibly asked how we know what energy is required for the forming or breaking of different bonds. It is an important question because ‘how we know what we know’ is at the core of understanding science. The energy released during a reaction can be measured experimentally. There are two main ways that this is done.
Firstly, atoms can be separated using electricity and then the energy that they release when they recombine can be measured. This is a simple exercise in principle but very pure samples need to be prepared and the actual experiments are difficult to do. Secondly, light of particular wavelengths can be used to break bonds and the effect that this has on the way that the light is absorbed can be measured. Although this is a much easier experiment to perform it only works for compounds capable of absorbing the light energy. By doing both kinds of experiment over and over, consistent average bond energies can be discovered. This can then be compared to the heat released during reactions and confirmed. As is often the way, it is only by repetition and careful recording of results that consistent, accurate data can be acquired.
That’s why you are always asked to repeat your experiments several times. It would be easy to record an anomaly and assume that it represented what normally happens.
Questions…
-
What is meant by the word
- (a) reaction
- (b) hydrocarbon
- (c) atom
- (d) compound
- (e) anomaly
- What property of light is controlled by its wavelength?
- What form of light is immediately (a) above and (b) below the frequencies of visible light?
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