Reactions are fundamental to the study of chemistry, as you will know. Chemical equations are how we show every chemical reaction simply and easily.
- This article is about chemical equations.
- We are going to learn about the two different kinds of chemical equations you might find.
- We will consider what state symbols are.
- We'll also learn how to balance chemical equations.
What is a chemical equation?
Before we delve into topics like balancing chemical equations, we need to know what they are.
A chemical equation is a symbolic representation of a chemical reaction in the form of symbols and formulae, showing the reactants and products of the reaction.
Chemical equations serve to represent reactions in a standardised and simple way that allows us to get the important information at a glance, that is, the reactants and products and their state, and the stoichiometry and direction of the reaction.
Law of conservation of mass
The Law of conservation of mass is one of the most important laws in chemistry. It states that atoms cannot be created or destroyed in a chemical reaction.
In chemical equations, this means that there must be the same number of every type of atom on the reactant (left) side of the equation as on the product (right) side of the equation.
When this is the case, we say that the equation is balanced. Given that this law can never be broken, we have to make sure that every chemical equation we write and find must be balanced!
Another thing we need to know about this law is that it also applies to mass. Remember that everything has a mass, even gases! So, if the total mass of all the reactants put together is 10g, the total mass of the products put together must also equal 10g. You might find some reactions that seem to break this rule. But remember, that isn’t 'possible'; there must always be an explanation. In most cases, it’s because there is a gas involved.
Some examples:
One of the products is a gas and the reaction is done in an open container. It will seem like the mass has decreased.
Heating in air - it might seem like suddenly the mass of the substance you were heating has increased.
State symbols
Sometimes, you will find state symbols in a balanced symbol equation. They tell us some specific information about the substance they are next to.
Table 1. State symbols |
|---|
State symbol | Meaning |
(s) | solid |
(l) | liquid |
(g) | gas |
(aq) | aqueous solution/dissolved in water |
In an equation, they go immediately after the substance they are describing and so look like this:
$$Fe_{(s)} + CuSO_{4\ (aq)} \rightarrow FeSO4_{(aq)}+ Cu_{(s)}$$
In this example, by looking at the state symbols, we know that iron (Fe) is in its solid state, copper sulfate (CuSO4) is in aqueous solution, iron sulfate (FeSO4) is also in aqueous solution, and copper (Cu) is in its solid state.
Why do we need chemical equations?
You can’t escape reactions in chemistry. They’re integral to understanding how things work and finding out the properties of a substance. We use chemical equations to represent every single reaction, so it’s pretty important that you understand them!
Chemical equations formula
In any reaction, you start off with substances that, after some time and changes, become different substances. These substances can be molecules of elements e.g. H2, or of compounds e.g. NH3.
The substances at the beginning of a chemical reaction are called reactants, and the ones at the end are called products.
$$N_{2}+3H_{2}\rightarrow 2NH_{3}$$
To represent the changes during the reaction, the simplified formula above is usually used. This is a chemical equation formula. It just gives information about the products and reactants, so any intermediaries that are consumed during the chemical reaction will not appear in a basic chemical equation.
A little number after an element shows how many atoms of the element are found in one unit of that substance.
For example, next to the N for nitrogen on the reactant side here, there is a little ‘₂’ after the element; this means there are 2 nitrogen atoms!
What are word equations?
One way we can represent a reaction is by using a word equation. That is to say by writing out the names of all the substances, instead of using symbols.
E.g. the chemical word equation for photosynthesis is:
$$Carbon\ dioxide + water \rightarrow glucose + oxygen$$
The balanced chemical formula for photosynthesis, though, is:
$$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$
Or,
$$Hydrogen+ oxygen \rightarrow water$$
This equation tells us that if we add hydrogen and oxygen together, we make water.
As you can see, the reactants go on the left, and the products go on the right. If there are multiple substances, we separate them using a ‘+’ sign.
It is exactly the same as a sum in maths, e.g. 2+3 = 5, just replace the ‘=’ into a ‘→’!
What are symbol equations?
Word equations are great, as they tell us what substances are being used and what they become in a reaction. But what if you wanted to know how much of everything is reacting? As in the example of hydrogen and oxygen making water, is there an equal amount of hydrogen and oxygen reacting, or is there more of one or the other? Using symbol equations can help us understand that!
The symbol equation for the same reaction is:
$$2H_{2} + O_{2}\rightarrow 2H_{2}O$$
You will have noticed that there is a number ‘2’ in front of some of the substances. If there isn’t a number in front of a substance, we always assume that it is ‘1’. We put numbers in front of every substance involved in a reaction to show the ratio of units of each substance. If there isn't a number there, we assume there is a '1'.
In this example, 2 units of hydrogen react with one unit of oxygen to produce 2 units of water. It doesn't matter how big each unit is, only that we have twice as many hydrogen units as oxygen units.
This means that in this reaction, there are twice as many hydrogen and water molecules as there are oxygen molecules.
Don’t worry if you don’t know where these numbers came from or why they are 2s and not 3s or 4s! We will learn all about that later when we balance chemical equations.
Chemical equation examples
Here is a list of chemical equations to help you recognise when you have one!
$$N_{2\ (g)} + 3H_{2\ (g)} \rightarrow 2NH_{3\ (g)}$$
Here, we have 1 unit of nitrogen and 3 units of hydrogen, which react to form 2 units of ammonia.
Here are some others. Can you understand what each equation is showing?
$$Mg(OH)_{2\ (s)} + SO_{2\ (g)}\rightarrow MgSO_{3\ (s)} + H$$
$$C_{6}H_{12}O_{6} \rightarrow 2 C_{2}H_{5}OH + 2 CO_{2}$$
Balancing chemical equations
We have mentioned balancing chemical equations a lot through this article. Now is the time for you to learn how to do it!
Learning to do this is important because you will have to do this hundreds of times during the entire course. There is always a question asking you to do this in the GCSE Chemistry exam.
To balance a chemical equation, we need to have the same number of atoms of each element involved on either side of the equation. If we don't, we would be breaking the Law of the Conservation of Mass! We do this by changing the quantities of each substance involved. However, we can't add in any different substances, because this would create an entirely new reaction.
The steps to balancing an equation are:
- Write out the reaction as a symbol equation, and count the number of atoms of each element on the reactant side, and the number of atoms of each element on the product side.
- Multiply the quantity of one substance, to have the same number of atoms of one element on both sides of the equation.
- Recount the number of atoms of each element on the reactant and product sides of the equation.
- Repeat steps 2 and 3 until there are equal numbers of each element atom on both sides of the equation. Then, the equation is balanced.
Let’s go through these steps together with an example - hydrogen reacting with oxygen to make water.
1. Write out the reaction as a symbol equation
Hydrogen is usually represented as H₂, oxygen as O₂ and water as H₂O. As we know what the reactants and products are, let’s demonstrate this as a symbol equation which we are going to balance. This step might already be done for you.
$$H_{2}+O_{2}\rightarrow H_{2}O$$
2. Below the reactant and product side of the equation, write a list of all the elements involved in the reaction
Now count the number of atoms of each element on the reactant side, and then on the product side.
This is to keep track of how many of each element there are on either side of the equation. in this case, the elements involved are hydrogen (H), and oxygen (O).
Make sure to count every single atom, not forgetting about the little numbers - like a little ‘₂’ after an element! Therefore ‘H₂’ means there are 2 H atoms in that molecule, so we can write 2 beside the H in the element list we wrote in step 2.
Check hydrogen (H) doesn’t appear in any of the other substances on that side!
Once you’re sure, repeat this counting for all of the other elements.
Remember if there isn’t a little number after an element, we always assume there is only 1 - as in the case of O in H₂O.
$$H_{2}+O_{2}\rightarrow H_{2}O$$
H: 2 H: 2
O: 2 O: 1
3. Multiply the quantity of one substance, to have the same number of atoms of one element on both sides of the equation
Look at each row individually in the list you wrote for step 2 which shows the number of atoms of each element on both sides of the equation. You can see that there are some elements with different numbers of atoms on either side of the equation. In this case, there are two oxygen atoms on the left-hand side, but only one on the right-hand side.
Remember, according to the law of conservation of mass, we can't create or destroy atoms; there has to be the same number of atoms of each element on both sides of the equation. We need to add more oxygen atoms to the left-hand side of the equation, as right now there aren't enough.
Looking at oxygen (O) though, we see that on the reactant side there are 2 oxygen atoms, and only 1 on the product side. So, this means we need to add one more oxygen atom to the product side.
So how can we add an oxygen atom? We can’t add a different substance or change the formula of water as that would change the equation completely - it would be a whole new reaction! So we add one more water molecule, and have 2 water molecules in total in the equation (1 originally + 1 added now). We represent this by writing a 2 in front of the water.
So we have:
H₂ + O₂ → 2H₂O
4. Recount the number of atoms of each element on the reactant and product sides of the equation. Update your list!
Because we have added another molecule, the number of atoms of one, some, or all the elements will change. So, we need to update our list to check!
$$H_{2}+O_{2}\rightarrow 2H_{2}O$$
H: 2 H: 4
O: 2 O: 2
Previously, we had 1 water molecule. This gave us 2 hydrogen atoms and 1 oxygen atom. We now have 2 water molecules, so we have twice as many hydrogen and oxygen atoms.
5. Repeat steps 3 and 4 until the equation is balanced!
We can see that by adding that water molecule, the number of oxygen atoms on both sides of the equation is the same, but the number of hydrogen atoms is now not the same!
Thankfully, we can do the same thing that we did last time and look for a reactant that contains hydrogen atoms to try and make the number of hydrogen atoms the same on both sides. In this case, we need 2 more hydrogen atoms on the reactant side, as the difference between 4 (product side) and 2 (reactant side) is 2!
We can add 1 more H₂ molecule, which contains 2 hydrogen atoms to make the number of hydrogen atoms the same on both sides.
$$2H_{2}+O_{2}\rightarrow 2H_{2}O$$
And repeat step 4, updating the list again:
$$2H_{2}+O_{2}\rightarrow 2H_{2}O$$
H: 4 H: 4
O: 2 O: 2
We can see that we have finally balanced this chemical equation!
6. Count the atoms on each side of the equation for the last time, like in step 2, just to check!
$$2H_{2}+O_{2}\rightarrow 2H_{2}O$$
H: 4 H: 4
O: 2 O: 2
Chemical Equations - Key takeaways
- In a reaction, the substances you have at the beginning are called reactants, and the ones you have at the end are called products.
- In a word equation, you write out the names of each substance. For example, hydrogen + oxygen → water.
- Symbol equations let us know how much of each substance is actually reacting.
- The law of conservation of mass states that atoms cannot be created or destroyed in a chemical reaction. So, the total mass of reactants always equals the total mass of products
- State symbols give us extra information about substances: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution.
- In a balanced chemical equation, there is the same number of each type of atom on either side of the equation.
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