How to write a chemical equation | Basics

Chemistry happens everywhere at any time: in our body chemical reactions allow to digest and break down fats, proteins, to transport hormones, to remove old cells, to carry the oxygen we breathe to our blood; combustion is a chemical reaction, photosynthesis is a chemical reaction as well. Chemistry is a fundamental part of science without which we could not explain many, and I say many things.

As a kid, you probably did the “volcano experiment” at least once: you simulated an eruption by mixing vinegar and baking soda. What happened? A lot of bubbles formed, and you got your homemade eruption. What is that? That’s a chemical reaction between the acetic acid contained in the vinegar and the sodium hydrogen carbonate (or sodium bicarbonate), commonly known as baking soda. This reaction produces a lot of carbon dioxide, the gas that comes out of your lungs when you exhale. Another product, which stays dissolved in the solution, is sodium acetate. Water is also produced.

How to write a chemical reaction

Let’s take another example: water. Water is a chemical substance, a solvent to be precise. As you may know, its chemical formula is H₂O. Thanks to this formula, we can see that it’s made up of two atoms of hydrogen and one of oxygen. Water is generated by the combustion of hydrogen in an atmosphere of oxygen. We can think of the reaction as

Well, that’s the concept, but there are many errors in this equation. First of all, oxygen exists in nature as O₂, this is why we call it a biatomic gas: O is an atom of oxygen, O₂ is two atoms. The same is for hydrogen, which is always H₂. Also, in a chemical reaction, we don’t use the sign “=”; instead, an arrow is used:

This is almost correct, except we MUST have the same number of atoms of each element on both sides of the equation.

Balancing an equation

We need two atoms of hydrogen and one of oxygen, not two, but as I said before, O alone does not exist, so we must write O₂. That small number can’t be changed. For example, He is helium. It is a monoatomic gas. There’s not such a thing as He₇. If you want 7 atoms of helium, you must write 7He. So if you need 6 atoms of oxygen you will write 3O₂. So, in order to have one atom of oxygen, you might think of writing 1/2 O₂, but atoms can’t be divided in a chemical equation. The coefficients must be integers. So in the reaction between hydrogen and oxygen the only way to solve our problem is to “multiply” H₂O by two. When you write a coefficient next to a chemical formula, that coefficient is multiplied by each element that makes up that formula. So, 2H₂O means two molecules of water and therefore 4 atoms of hydrogen and two of oxygen.

Let’s check this out: on the left-hand side are two atoms of hydrogen and two of oxygen; on the right-hand side are 4 atoms of hydrogen and two of oxygen. So now the problem is the hydrogen, but we can fix it by multiplying H₂ by 2 in order to have four atoms.

Now everything is perfect. This is the chemical reaction between hydrogen and oxygen, which yields water. This procedure is called balancing. So, when you hear “balance an equation”, it means “adjust the equation so that the number of atoms of each element is the same on both sides”. The chemical formulas on the left side are the reagents, the ones on the right are the products. In this case, hydrogen and oxygen are the reagents, water is the product.

A fizzy reaction

Let’s take the first example, the reaction between acetic acid (C2H4O2) and sodium bicarbonate (NaHCO3) which gives sodium acetate (C2H3NaO2), water (H2O) and carbon dioxide (CO2).

Let’s see if this equation is balanced:

On the left-hand side we have 3 atoms of carbon, 5 of hydrogen, 5 of oxygen and 1 of sodium. On the other side we have 3 atoms of carbon, 5 atoms of hydrogen, 5 of oxygen and one of sodium. That’s great! We don’t have to add anything else.

Reaction between carbon and oxygen

Carbon and oxygen can react to form two different products, carbon dioxide and carbon monoxide (the one in this equation). The latter is formed when there is not enough oxygen. As you can seen the equation is not balanced. We need one more atom of oxygen on the right side, and to fix this we can add a 2;

Ok, but now we need one more atom of carbon on the left side. So guess what? We’ll write a 2 next to C:

That’s it! Now you should have the basics to explore chemical equations more deeply. If something is not clear, leave a comment and I’ll be happy to help you!

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