As the title would suggest, in this section we’re going to examine the role that energy plays in chemical reactions and how a reaction is able to impact energy.
First thing to learn, energy cannot be made or destroyed – it simply transforms from one use to another. And all chemical reactions can be either endothermic or exothermic.
The balance between the energy needed to break the bonds of the reactants and that needed to form the bonds in the product determines whether a reaction is exothermic or endothermic.
In an endothermic reaction the energy required to break the bonds in the reactants is greater than the energy released when bonds form in the products. This means that, overall, energy is taken from the surroundings.
Energy level diagrams show us the relative amounts of energy of the reactants and products, measured in KJ/mole. When the reactants are at a higher energy level than the products the reaction is exothermic, because energy is released when reactants form products.
When the reactants are at a lower energy level than the products, energy is taken from the surroundings during the reaction, so the reaction is endothermic. The activation energy is the minimum amount of energy colliding particles must have for a reaction to happen.
Now we know the basics about endothermic and exothermic reactions, let’s take a look at some other energy changes.
Catalysts increase the rate of a reaction by providing an alternative pathway to the products with a lower activation energy.
The collision theory tells us that there is a minimum amount of energy needed before colliding particles can react – this is known as the activation energy. By lowering the activation energy required for a reaction to happen, catalysts increase the proportion of reactant particles with enough energy to react.
We would use the units KJ/mole of fuel burnt, or joules/mole to compare the energy content of different fuels. When comparing the energy released by foods we sometimes use calories instead of joules, however the joule is the usual unit of energy.
It is important that we can compare the energy released by different fuels so we know how to get the most energy from the smallest amount of an expensive resource.
We can also use a calorimeter to make energy measurements. The amount of energy released when a fuel is burnt is related to the rise in temperature of the water in a calorimeter.
This technique can be used to compare the energy released by different fuels but is not accurate enough for calculating the exact amount of energy released during combustion of a fuel.
Finally, when burning some elements, it is important to be aware of the environmental impact. For example, it would be cleaner to burn hydrogen as a fuel rather than burning a fossil fuel as carbon dioxide is not a waste product.
Hydrogen burns well and produces no pollutants, however safe storage is still a problem. Another issue is supplying the hydrogen gas. Obtaining it from electrolysis of water would require electricity generated from burning fossil fuels, which would use resources and generate carbon dioxide.