Energy changes
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I'm a little confused what the difference is between enthalpy and energy (topic 5 in Nuffield Chemistry). How do I work them both out? How are they related?
Ulex says
In A-level chemistry we are often concerned with energy transfers that heat thing up or cools things down. What we observe and measure are rises or falls in temperature as a result of these energy transfers.
This heat energy is stored in substances as movement of molecules, vibration and rotation. When reactions occur and new substances are formed, the amount of such energy stored in molecules is different from what was stored in the reactants. The difference often shows itself in a rise or fall in temperature.
In order to restore the temperature of the products to the temperature the reactants started at, heat energy has to be extracted (exothermic reaction) or added (endothermic reaction). This heat energy can be extracted/provided at constant volume, in which case it is called a change of ‘internal energy, DU’ or at constant pressure, ‘enthalpy change, DH’.
There is a small difference between the two, but the difference is usually of little practical significance. Since we mostly perform reactions at constant pressure, i.e. open to the atmosphere, the vast majority of the energy changes we measure are enthalpy changes.
At this level (and probably throughout most chemists’ professional lives) no one is expected to work out internal energies. All our calculations are of enthalpy changes.
You should be able to work out enthalpy changes based on the results of experiments, such as the ones you did in Nuffield Chemistry Topic 5, or on the basis of enthalpy changes of formation etc, obtained from lists of data.
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updated: 25 January 2006
