Proton Donors and Acceptors

Recall that hydronium ions (H₃O⁺) are produced when acids dissolve in water. When an acid and a base react in water, a proton from the hydronium ion from the acid combines with the hydroxide ion (OH⁻) from the base to form water (H₂O). Acids lose, or “donate,” protons. Bases “accept” protons, forming water, a neutral molecule. Acids can be defined as proton donors, and bases can be defined as proton acceptors. This definition allows you to classify a wider range of substances as acids or bases.

Figure 20 In the first reaction, water acts as a base, accepting a proton from hydrogen chloride. In the second reaction, water acts as an acid, donating a proton to the ammonia.

Applying Concepts What acts as the proton donor in the first reaction?

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Based on the definitions of acids and bases that you read earlier in this section, water is neither an acid nor a base. However, using the proton-donor or proton-acceptor definition, water can act as either an acid or a base depending on the compound with which it reacts.

Figure 20 shows the ionization of hydrogen chloride and ammonia as they form solutions. In the first reaction, water acts as a base. It accepts a proton from hydrogen chloride and becomes a hydronium ion. In the second reaction, water acts as an acid. It donates a proton to the ammonia, which acts as a base. The resulting solution contains hydroxide ions and ammonium ions, NH₄⁺.