# 3.9: Chemical Equations

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Learning Objectives

• Identify the reactants and products in any chemical reaction.
• Convert word equations into chemical equations.
• Use the common symbols, $$\left( s \right)$$, $$\left( l \right)$$, $$\left( g \right)$$, $$\left( aq \right)$$, and $$\rightarrow$$ appropriately when writing a chemical reaction.

The combustion of fossil fuels results in the formation of carbon dioxide, a greenhouse gas, and other pollutants.  This is an example of chemical change, in which new substances are formed. In order for this to occur, the chemical bonds of the substances break, and the atoms that compose them separate and rearrange themselves into new substances with new chemical bonds. When this process occurs, we call it a chemical reaction. A chemical reaction is the process in which one or more substances are changed into one or more new substances.

## Reactants and Products

To describe a chemical reaction, we need to indicate what substances are present at the beginning and what substances are present at the end. The substances that are present at the beginning are called reactants and the substances present at the end are called products.

## Writing Chemical Equations

When sulfur dioxide is added to oxygen, sulfur trioxide is produced. Sulfur dioxide and oxygen, $$\ce{SO_2} + \ce{O_2}$$, are reactants and sulfur trioxide, $$\ce{SO_3}$$, is the product.

$\underbrace{\ce{2 SO2(g) + O2(g) }}_{\text{Reactants}} \rightarrow \underbrace{\ce{2SO3(g)}}_{\text{Products}} \nonumber$

In chemical reactions, the reactants are found before the symbol "$$\rightarrow$$" and the products are found after the symbol "$$\rightarrow$$". The general equation for a reaction is:

$\text{Reactants } \rightarrow \text{Products} \nonumber$

There are a few special symbols that we need to know in order to "talk" in chemical shorthand. In the table below is the summary of the major symbols used in chemical equations. Table $$\PageIndex{1}$$ shows a listing of symbols used in chemical equations.

Table $$\PageIndex{1}$$: Symbols Used in Chemical Equations
Symbol Description Symbol Description
$$+$$ used to separate multiple reactants or products $$\left( s \right)$$ reactant or product in the solid state
$$\rightarrow$$ yield sign; separates reactants from products $$\left( l \right)$$ reactant or product in the liquid state
$$\rightleftharpoons$$ replaces the yield sign for reversible reactions that reach equilibrium $$\left( g \right)$$ reactant or product in the gas state
$$\overset{\ce{Pt}}{\rightarrow}$$ formula written above the arrow is used as a catalyst in the reaction $$\left( aq \right)$$ reactant or product in an aqueous solution (dissolved in water)
$$\overset{\Delta}{\rightarrow}$$ triangle indicates that the reaction is being heated

Chemists have a choice of methods for describing a chemical reaction.

1. You can draw a picture of the chemical reaction showing the submicroscopic particles. 2. You can write a word equation for the chemical reaction:
"Two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water vapor."

3. You can write the equation with chemical symbols

$2 \ce{H_2} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{H_2O} \left( g \right)$

In the symbolic equation, chemical formulas are used instead of chemical names for reactants and products, while symbols are used to indicate the phase of each substance.

To turn word equations into symbolic equations, we need to follow the given steps:

1. Identify the reactants and products. This will help you know which symbols go on each side of the arrow and where the $$+$$ signs go.
2. Write the correct formulas for all compounds.
3. Sometimes a reactant or product is only an element (like hydrogen gas or oxygen gas). There are seven elements that are considered diatomic, meaning that they are always found in pairs in nature. They include those elements listed in the table.
 Element Name Formula Hydrogen Nitrogen Oxygen Fluorine Chlorine Bromine Iodine $$H_2$$ $$N_2$$ $$O_2$$ $$F_2$$ $$Cl_2$$ $$Br_2$$ $$I_2$$

Example $$\PageIndex{1}$$

Transfer the following symbolic equations into word equations or word equations into symbolic equations.

1. $$\ce{HCl} \left( aq \right) + \ce{NaOH} \left( aq \right) \rightarrow \ce{NaCl} \left( aq \right) + \ce{H_2O} \left( l \right)$$
2. Gaseous propane, $$\ce{C_3H_8}$$, burns in oxygen gas to produce gaseous carbon dioxide and liquid water.
3. Hydrogen fluoride gas reacts with an aqueous solution of potassium carbonate to produce an aqueous solution of potassium fluoride, liquid water, and gaseous carbon dioxide.

Solution

a. An aqueous solution of hydrochloric acid reacts with an aqueous solution of sodium hydroxide to produce an aqueous solution of sodium chloride and liquid water.

b. Reactants: propane ($$\ce{C_3H_8}$$) and oxygen ($$\ce{O_2}$$)

Product: carbon dioxide ($$\ce{CO_2}$$) and water ($$\ce{H_2O}$$)

$\ce{C_3H_8} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + \ce{H_2O} \left( l \right) \nonumber$

c. Reactants: hydrogen fluoride and potassium carbonate

Products: potassium fluoride, water, and carbon dioxide

$\ce{HF} \left( g \right) + \ce{K_2CO_3} \left( aq \right) \rightarrow \ce{KF} \left( aq \right) + \ce{H_2O} \left( l \right) + \ce{CO_2} \left( g \right) \nonumber$

Exercise $$\PageIndex{1}$$

Transfer the following symbolic equations into word equations or word equations into symbolic equations.

1. Hydrogen gas reacts with nitrogen gas to produce gaseous ammonia.
$$H_2 (g) + N_2 (g) \rightarrow NH_3 (g)$$

## Summary

• A chemical reaction is the process by which one or more substances are changed into one or more new substances.
• Chemical reactions are represented by chemical equations.
• Chemical equations have reactants on the left, an arrow that is read as "yields", and the products on the right.