In the Arrhenius model, an acid is defined as a compound that dissociates when dissolved in water to produce a proton (H+) and a negatively-charged ion (an anion). In fact, naked protons (H+) do not roam around in solution. They always associate with at least one, and more likely multiple, water molecules.127 Generally, chemists use a shorthand for this situation, either referring to the H+ in aqueous solution as a hydronium ion (denoted as H3O+) or even more simply as H+, but do not forget, this is a short-hand. An example of an Arrhenius acid reaction is:
HCl(g) + H2O ⇄ H3O+ (aq) + Cl– (aq)
or, more simply (and truer to the original theory):
HCl(g) ⇄ H+ (aq) + Cl– (aq) or HCl(aq)
But this is really quite a weird way to present the actual situation, because the HCl molecule does not interact with a single water molecule, but rather interacts with water as a solvent. When hydrogen chloride (HCl) gas is dissolved in water, it dissociates into H+(aq) and Cl–(aq) almost completely. For all intents and purposes, there are no HCl molecules in the solution. An aqueous solution of HCl is known as hydrochloric acid, which distinguishes it from the gas, hydrogen chloride. This complete dissociation is a characteristic of strong acids, but not all acids are strong!
An Arrhenius base is defined as a compound that generates hydroxide (–OH) ions when dissolved in water. The most common examples of Arrhenius bases are the Group I (alkali metal) hydroxides, such as sodium hydroxide:
NaOH(s) + H2O ⇄ Na+(aq) + –OH(aq) or NaOH(aq)
Again, this is a reaction system that involves both NaOH and liquid water. The process of forming a solution of sodium hydroxide is just like the one involved in the interaction between sodium chloride (NaCl) and water: the ions (Na+ and –OH) separate and are solvated (surrounded) by the water molecules.
As we will see shortly, some acids (and bases) do not ionize completely; some of the acid molecules remain intact when they dissolve in water. When this occurs we use double-headed arrows ⇌ to indicate that the reaction is reversible, and both reactants and products are present in the same reaction mixture. We will have much more to say about the duration and direction of a reaction in the next chapter. For now, it is enough to understand that acid–base reactions (in fact, all reactions) are reversible at the molecular level. In the case of simple Arrhenius acids and bases, however, we can assume that the reaction proceeds almost exclusively to the right.
An Arrhenius acid–base reaction occurs when a dissolved (aqueous) acid and a dissolved (aqueous) base are mixed together. The product of such a reaction is usually said to be a salt plus water and the reaction is often called a neutralization reaction: the acid neutralizes the base, and vice versa. The equation can be written like this:
HCl(aq) + NaOH(aq) ⇄ H2O(l) + NaCl(aq)
When the reaction is written in this molecular form it is quite difficult to see what is actually happening. If we rewrite the equation to show all of the species involved, and assume that the number of HCl and NaOH molecules are equal, we get:
H+(aq) + Cl–(aq) + Na+(aq) + –OH(aq) ⇄ H2O(l) + Na+(aq) + Cl–(aq) Na+(aq) and Cl–(aq)
appear on both sides of the equation; they are unchanged and do not react (they are often called spectator ions because they do not participate in the reaction). The only actual reaction that occurs is the formation of water: H+(aq) + –OH(aq) ⇄ H2O(l)
The formation of water (not the formation of a salt) is the signature of an Arrhenius acid–base reaction. A number of common strong acids, including hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3), react with a strong base such as NaOH or KOH (which, like strong acids, dissociate completely in water) to produce water.
Such acid–base reactions are always exothermic and we can measure the temperature change and calculate the corresponding enthalpy change (ΔH) for the reaction. Regardless of which strong acid or strong base you choose, the enthalpy change is always the same (about 58 kJ/mol of H2O produced). This is because the only consistent net reaction that takes place in a solution of a strong acid and a strong base is:
H+ (aq) + –OH (aq) ⇄ H2O(l)
One other factor to note is that the overall reaction involves a new bond being formed between the proton (H+) and the oxygen of the hydroxide (–OH.) It makes sense that something with a positive charge would be attracted to (and bond with) a negatively-charged species (although you should recall why the Na+ and Cl– do not combine to form sodium chloride solid in aqueous solution.) Whether or not bonds form depends on the exact nature of the system, and the enthalpy and entropy changes that are associated with the change. We will return to this idea later in chapter 8.
Questions to Answer
- What would be the reaction if equal amounts of equimolar HNO3 and KOH were mixed?
- How about equal amounts of equimolar H2SO4 and KOH? What would the products be?
- How about equal amounts of equimolar H3PO4 and KOH?
- How many moles of NaOH would be needed to reactfully with one mole of H3PO4?
- Draw a molecular level picture of Arrhenius acid base reaction.