16.2 Reactions of Acids and Bases
Learning Objectives
By the end of this section, you will be able to:
- Summarize general reactions of acids and bases
In this section, the more common types of acid-base reactions that take place in aqueous solutions will be summarized. In continuation with the context learned earlier in this chapter, a strong acid is a substance that will dissolve in water to yield hydronium ions, H3O+. Whereas, a strong base is a substance that will dissolve in water to yield hydroxide ions, OH−. We will learn other definitions of acids and bases in the following sections. The most common bases are ionic compounds composed of alkali or alkaline earth metal cations (groups 1 and 2) combined with the hydroxide ion. For example, NaOH and Ca(OH)2. When these compounds dissolve in water, hydroxide ions are released directly into the solution. These ions are responsible for the characteristic behaviour of acids and bases in aqueous solution.
Neutralization Reactions
The reaction between an acid and a base is called an acid-base reaction or a neutralization reaction. Later in this chapter, a section is dedicated to discussing neutralization reactions and introducing calculations relating to neutralization reactions. However, a brief introduction into this type of reaction is provided. In reactions where the acid is a hydrogen-ion-containing compound and the base is a hydroxide-ion-containing compound, the two react chemically to form water and a salt. The general form for a neutralization reaction is
acid + base → water + salt
where the term salt is used to define any ionic compound (soluble or insoluble) that is formed from a reaction between an acid and a base, as outlined in chapter 15. In chemistry, the word salt refers to more than just table salt. For example, the balanced chemical equation for the reaction between HCl(aq) and KOH(aq) is
HCl(aq) + KOH(aq) → H2O(l) + KCl(aq)
where in this instance, the salt is soluble potassium chloride, KCl. By counting the number of atoms of each element, we find that only one water molecule is formed as a product. However, in the reaction between HCl(aq) and Mg(OH)2(aq), additional molecules of HCl and H2O are required to balance the chemical equation:
2HCl(aq) + Mg(OH)2(aq) → 2H2O(l) + MgCl2(aq)
Here, the soluble salt is MgCl2. (This is one of several reactions that take place when a type of antacid—a base—is used to treat stomach acid.)
There are acid-base reactions that do not follow the “general acid-base” equation given above. For example, the balanced chemical equation for the reaction between HCl(aq) and NH3(aq) is
HCl(aq) + NH3(aq) → NH4Cl(aq)
Neutralization reactions are one type of chemical reaction that proceeds even if one reactant is not in the aqueous phase. For example, the chemical reaction between HCl(aq) and Fe(OH)3(s) still proceeds according to the equation
3 HCl(aq) + Fe(OH)3(s) → 3H2O(l) + FeCl3(aq)
even though Fe(OH)3 is not soluble. When one realizes that Fe(OH)3(s) is