Introduction

In AP Chemistry, both physical and chemical changes in matter can be represented using chemical equations. This topic will go over the different types of chemical equations and how they can be used to represent and interpret reactions.

Chemical Equations

Reactions and processes in the real world can be represented through chemical equations. In a chemical equation, you have the reactants, or the atoms/molecules you have before the reaction, and the products, the atoms/molecules you have after the reaction. An example of this is $\mathrm{C}{}_3\mathrm{H}{}_8(\mathrm{g})+5\mathrm{O}{}_2(\mathrm{g})\to 3\mathrm{CO}{}_2(\mathrm{g})+4\mathrm{H}{}_2\mathrm{O}(\mathrm{g})$. The molecules of $\mathrm{C}{}_3\mathrm{H}{}_8$ and $\mathrm{O}{}_2$ are the reactants, and the molecules of $\mathrm{CO}{}_2$ and $\mathrm{H}{}_2\mathrm{O}$ are the products. The coefficients next to each molecule show how many of that molecule there are. This is how chemical equations also represent the Law of Conservation of Mass, which says that in a chemical reaction, mass is not created or destroyed but simply displaced or transfered to other molecules. If you add up all the carbon, hydrogen, and oxygen atoms in the chemical equation above, you will see that there are equal numbers of each on the reactants and products side. 

Charge is also conserved in a chemical reaction, and the chemical equation must show that. In the equation $\mathrm{Ca}{}^{2+}(\mathrm{aq})+\mathrm{CO}{}_3{}^{2-}(\mathrm{aq})\to \mathrm{CaCO}{}_3(\mathrm{s})$, we can see that the reactants have a total charge of $0$ ($2+(-2)=0$) and the products also have a charge of $0$.

Types of Chemical Equations

There are multiple different types of chemical equations in chemistry, and it is important to know the difference, as oftentimes the same reaction can be represented by multiple different types of chemical equations. Take the precipitation reaction between silver nitrate ($\mathrm{AgNO}{}_3$) and sodium chloride ($\mathrm{NaCl}$) to form solid silver chloride ($\mathrm{AgCl}$). In a molecular equation or balanced equation, you show each of the ions in their ionic compounds, which would look like $\mathrm{AgNO}{}_3(\mathrm{aq})+\mathrm{NaCl}(\mathrm{aq})\to \mathrm{AgCl}(\mathrm{s})+\mathrm{NaNO}{}_3(\mathrm{aq})$. This equation can be useful for understanding every part of the reaction, what type of reaction it is, and where each of the ions came from. In a complete ionic equation, all the ions are separated from their original compounds, which would be $\mathrm{Ag}{}^{+}(\mathrm{aq}+NO3-(aq)+Na+(aq)+Cl-(aq)->AgCl(s)+Na+(aq)+NO3-(aq)$. This type of equation can be useful for seeing how each component of the reaction really is appearing and reacting. The ionic compounds are already dissolved in water, and this type of equation better shows how the sodium and nitrate ions aren’t joining together, but rather both just staying as independent ions. In a net ionic equation, we take out the ions that appear on both the reactant and product sides, or spectator ions. These spectator ions are not actually part of the reaction, but rather are “watching” the reaction take place. An example of this would be $\mathrm{Ag}{}^{-}(\mathrm{aq})+\mathrm{Cl}{}^{-}(\mathrm{aq})\to \mathrm{AgCl}(\mathrm{s})$. Net ionic equations are useful for seeing what is really changing in a reaction.

Practice