Endothermic and Exothermic Reactions

When reactions take place, they give out and take in energy, and nearly all reactions have an overall 'energy change'.  We will be looking at this energy in the form of heat.

During a reaction, bonds are formed and broken.  When they are formed, heat is taken in and when they are broken, heat is given out.  Each bond formed or broken can be given an energy value (bond enthalpy) and using this, we can find out the overall heat change.

If a reaction is exothermic it means that overall, the reaction has given out heat

If a reaction is endothermic it means that overall, the reaction has taken in heat

It is important to remember that, when studying heat changes in reactions, if the temperature of the substance which the reaction has taken place in is measured, the opposite heat change takes place,

e.g, 'ethanol is heated under a test tube of water, the temperature of the water changed from 10K to 20K' the reaction has given out heat to make the temperature warmer, so the reaction is exothermic, even though the temperature seems to have gone up.

We have predicted values for the heat changes of various reactions and bond enthalpies, these can be used to calculate the heat change of a reaction, but why do we need to know this?

• We can calculate the energy values of fuels
• We can see whether a reaction will take place or not
• We can calculate the energy values needed for reactions

The energy values of fuels

Currently, we use petrol which is derived from crude oil, which is running out.  Chemists are trying to find a new way to make fuel, current suggestions are ethanol, methanol (which can be made from plant material) and hydrogen.  The better fuels, the less their enthalpy of combustion will be (the amount of energy needed to burn a substance completely in oxygen under standard conditions).

Hydrogen's is really low (-242) compared to petrol (-5500) which seems amazing, but there is a problem, hydrogen is stored as a gas, which takes up a lot of space because of the low density of gases.