Let’s say we are cooking an egg, as well as seeing a change in colour we can feel some heat coming from the pan, so where does this heat come from? One law of energy is that it cannot be created or destroyed, but it can change to different types of energy. In this case, we see it in the form of heat and it is known as an exothermic reaction.
- We will be exploring two types of energy change: exothermic and endothermic.
- Then we will explore what formulas we can use to determine the different energy changes.
- Finally, we will explore what a battery is.
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
When we burn methane gas, which is a combustion reaction. Energy is released as heat and why we can feel the heat when something is burning.
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.
A key example is a thermal decomposition, where we take some calcium carbonate and heat it, to form carbon dioxide and calcium oxide. Energy is absorbed and heat is required for the reaction to take place.
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.
Bond energy: The energy needs to break the bond between two atoms. It is measured in kJ/mol.
To calculate the change in energy we need 2 things:
- The energy required to break bonds from the reactants.
- 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
Bond | Bond Energy (kJ/mol) |
N≡N | 940 |
N-H | 400 |
H-H | 440 |
Energy changes examples:
We will be using a table of bond energies to calculate our answer. These are made up values to go through an example.
For this example, lets us use the Haber Process, where nitrogen and hydrogen react to form ammonia. This reaction can go forwards and backwards, we will be calculating the forwards reaction.
Chemical equation:
$$N_{2(g)} + 3H_{2(g)}\rightleftharpoons 2NH_{3(g)}$$
1. First we need calculate the bond energy between the reactants, so for this reaction it is the one mole of nitrogen to nitrogen three moles of hydrogen to hydrogen single bonds.
1 x N≡N (940) = 820
3 x H-H (440) = 1600
So the total bond breaking energy is: 940 + 1320 = 2260 kJ/mol
2. Now we need to calculate the bond energy between the products. For this reaction it is two moles of nitrogen to hydrogen single bonds. For this bond, each nitrogen is bond to three hydrogens so it is 6 bonds.
6 · N-H (400) = 2400
3. Finally, we will calculate the energy change.
Energy change = 2260 - 2400 = -140 kJ/mol
Overall energy change = -140 kJ/mol
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.
Energy Changes - Key takeaways
- All chemical reactions have an energy change.
- There are two types of energy changes: exothermic and endothermic.
- In exothermic reactions, energy is released and the products have less energy than the reactants.
- In endothermic reactions, energy is absorbed and the products have more energy than the reactant.
- We can explore the different of energy in exothermic and endothermic reactions using reaction profiles.
- Energy change can be calculated by: total bond energy of the reactant – total bond energy of the products
- Reactive metals are used to produce batter.
- Two metals react so electrons can be transferred which produces an electric current.
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