Writing Chemical Reactions

Turning a Reaction Description into an Equation

A chemical reaction equation is a compact way to record starting substances, products, and their relative stoichiometric amounts. Reactants are written on the left and products on the right. Reaction symbols such as arrows connect the two sides.

The basic pattern looks like this.

Read the arrow as "yields". A plus sign means there is more than one substance on the same side.

Methane Combustion Equation

Start from an ordinary sentence: methane reacts with oxygen to produce carbon dioxide and water. Reactants go on the left, products go on the right.

The starting formulas are not balanced because the left side has only $2$ oxygen atoms, while the right side has $3$. After the coefficients are added, each element has the same atom count on both sides.

The coefficients read as the ratio $1:2:1:2$. Because all four substances in this example are molecular, the ratio may be pictured as $1$ methane molecule reacting with $2$ oxygen molecules to produce $1$ carbon dioxide molecule and $2$ water molecules. On a laboratory scale, the same coefficients give the mole ratio.

The molecule model is specific to molecular substances. For an ionic or network solid, the coefficient gives a stoichiometric amount or formula-unit ratio. In every case, the coefficients give relative mole amounts for a macroscopic reaction.

Coefficients Do Not Change Formulas

Atom counts come from multiplying coefficients by subscripts. Coefficients can change because they state relative amounts.

The coefficient $2$ before $\mathrm{O_2}$ means two oxygen molecules. The subscript in $\mathrm{O_2}$ is part of the oxygen formula, so it must not be changed into $\mathrm{O_4}$.

State Symbols Come After Formulas

When the physical states are known, write the state symbol after each formula. At the high temperature of a methane flame, the water product is vapor, so the equation can be written as:

State symbols do not change atom counts. They tell the reader the physical form of each substance under the stated conditions. If the products cool enough for water to condense, the water would instead be written as $\mathrm{H_2O(l)}$.

Silver Chloride Precipitation Example

Two clear solutions can form a new solid. For example, silver nitrate solution reacts with sodium chloride solution to produce a silver chloride precipitate.

This equation is already balanced with the coefficient ratio $1:1:1:1$. The $\mathrm{(s)}$ after $\mathrm{AgCl(s)}$ identifies silver chloride as the solid product.

Steps for Writing a Reaction Equation

1. Write reactant names on the left and product names on the right.
2. Convert each substance name into the correct chemical formula.
3. Count each element on the left and right.
4. Adjust coefficients until the atoms balance.
5. Add state symbols when the information is available.

A wrong chemical formula cannot be fixed by balancing the equation. Check each formula before adjusting the coefficients.