Writing Chemical Equations — Parts & Examples

Importance of Chemical Equations

We've been studying the different varieties of chemical reactions. So far, we've investigated synthesis, decomposition, combustion, single-replacement, and double-replacement reactions. We also saw some common subtypes like precipitation and acid-base neutralization. But, how do describe chemical reactions? We use a chemical equation!

Chemical equations describe a reaction using...

• molecular formulas.

• states of matter.

• the number of moles.

The balanced equations show us...

• what species are in the reaction.

• how the compounds experience phase changes.

• how each species are related to each other through mole ratios.

General Chemical Equation

A general chemical equation has reactants (starting materials), products (species made), states, moles, and an arrow to indicate the reaction's direction. Sometimes, you will see double arrows ($\rightleftharpoons$). This indicates the reaction is reversible. So, it's an equilibrium reaction.

$XReactant\ A\ (s,l,g)\ +\ YReactant\ B\ (s,l,g)\ \rightarrow\ ZProduct\ C\ (s,l,g)$

X, Y, and Z denote the number of moles. They are added to balance the equation. Balancing entails making sure that both sides of the equation have equal numbers of each atom. The state of each compound is either solid, liquid, or gas. It is important to include states because changes in state can indicate a reaction is happening.

Example of a Reaction

As an example, let's consider the reaction between nitrogen, oxygen, and water. They combine to form nitric acid. Write the equation including the state and moles.

First, we consider the reactants and determine the molecular formulas. These formulas can be found by looking up the elements or compounds.

• $N_2$ - nitrogen

• $O_2$ - oxygen

• $H_2O$ - water

• $HNO_3$ - nitric acid

Next, determine the states. $N_2$ and $O_2$ are easy! They are always gaseous ($g$). At standard conditions, water is always a liquid ($l$). Nitric acid is a liquid. Some acids exist as solids. But most strong acids, like nitric acid, are liquid. But, if you were unsure, you can look up the compound.

• $N_{2(g)}$ - nitrogen gas

• $O_{2(g)}$ - oxygen gas

• $H_{2}O_{(l)}$ - liquid water

• $HNO_{3(l)}$ - liquid nitric acid

Now, we can combine these to get an unbalanced equation:

$N_{2(g)} + O_{2(g)} + H_2O_{(l)} \rightarrow HNO_{3(l)}$

The final step is to balance the equation. A simple way is to count the number of atoms on each side:

$Number\ of\ Atoms$

$N:2 \rightarrow 1$

$O: 3 \rightarrow 3$

$H: 2 \rightarrow 1$

Then add moles in front of each compound to balance. Sometimes, this can be trial and error. But often balancing oxygens is a good place to start. The final equation should be:

$2N_2\ (g)\ +\ 5O_2\ (g)\ +\ 2H_2O\ (l)\ \rightarrow\ 4HNO_3\ (l)$

(Video) Writing Chemical Equations in Words

By Professor Dave Explains

In this video, professor Dave mentions several ways to describe chemical reactions. First, you can use chemical formulas, which abbreviate all the elements in each compound. Additionally, you can write out the phases of the compounds in chemical formulas.

On the other hand, you can also describe chemical reactions using words, which can be achieved by translating chemical formulas into words. You will start with the phase of the chemical compound. Starting from the far left on the reactant side, you can add “reacts with” between the reactants. And, you can add “to produce” between the reactants and the products to complete the description of a chemical reaction.

Writing Chemical Equations: How to write chemical equations in equation form and in words?