Energy Changes

Types of energy change

There are so many different chemical reactions that can take place in so many different places. Even in our own bodies, we have a variety of reactions happening all at the same time. All of these reactions are categorised as either exothermic or endothermic. So let us explore what they are and some examples that we see in our day-to-day life.

Exothermic

Exothermic reactions are those in which energy is transferred from the reaction to the surrounding. This means energy is released and a lot of the time it can take place in the form of heat and so the temperature increases. As energy is released it generally means the products of said reactants will have less energy than the reactants at the start.

Wood Fire, Wikimedia Commons

Endothermic

Endothermic reactions are completely opposite to exothermic reactions. Energy is transferred from the surroundings to the reaction. This means energy is absorbed and so the temperature decreases. In endothermic reactions, the products will have more energy than the reactants.

Reaction profiles

So now we have explored that there are two different types of energy changes we can use reaction profiles to present both of these and explore the change in energy is the reactants and products.

The first diagram shows the reaction profile of an exothermic reaction. The reactants A + B are higher than the product C. This shows that energy has been released into the surroundings and as discussed before it is usually in the form of heat energy.

Fig. 2: Endothermic Reaction, Wikimedia Commons

The second diagram shows the reaction profiles of an endothermic reaction. The reactants A + B is lower than the product C. This shows that energy has been absorbed and the temperature decreases.

This is the basis of energy reaction profiles. We will be exploring these diagrams further in a later article.

Change in energy formula

We need to be able to calculate the overall energy change. To do this we need to know the different bond energies.

To calculate the change in energy we need 2 things:

1. The energy required to break bonds from the reactants.
2. The energy released when bonds are formed to produce products.

Together we can form this equation to calculate the overall energy change:

$$Energy\ change=total\ bond\ energy\ of\ the\ reactant\ -\ total\ bond\ energy\ of\ the\ products $$

Calculating energy change

We now know the equation to calculate energy, so let’s go through an example

Cells and batteries

All devices have batteries, we usually see these are small metal blocks that can be removable or permanent. But how do they provide energy to our devices? This is done using reactive metals that produce energy for them to work. We can use the order of reactivity of metals, we can use two to form an electrical cell. These cells allow the flow of electrons to form an electric current which powers the device.

Fig. 3: Electrical Cell, Wikimedia Commons

If we look at this diagram we can see that zinc (Zn) donates electrons to copper (Cu). The electrons travel from one end to another which forms an electric current and this reaction takes place within our devices to power them.

The energy current can be recorded using a voltmeter, which detects the difference in reactivity between the two metals and forms a voltage.

The bigger the difference, the higher the voltage.

Another type of cell is a fuel cell. This is done through oxidising hydrogen, whereby hydrogen and oxygen react to produce water and is presented in the following equation:

$$2H_{2} + O_{2}\rightarrow 2H_{2}O$$

Two moles of hydrogen react with one mole of oxygen to form two moles of water.

Some cars run using this type of fuel cell, however, a constant supply of hydrogen is required and is therefore not commonly used.