Question

Both \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) are amphoteric. Write an equation to show how each substance can act as an acid, and another equation to show how each can act as a base.

Short answer

As acids, H2O donates a proton to form OH- and H2PO4- donates a proton to form HPO4^2-. As bases, H2O accepts a proton to form H3O+ and H2PO4- accepts a proton to form H3PO4.

Step-by-step solution

Identify the Acidic Behavior of H2O

As an acid, a substance must be capable of donating a proton (H+). For water (H2O), we can show this behavior by writing H2O reacting with a generic base (B) to form hydroxide (OH-) and the conjugate acid of the base (BH+). The equation will be: H2O (l) + B (aq) → OH- (aq) + BH+ (aq).

Identify the Acidic Behavior of H2PO4-

The dihydrogen phosphate ion (H2PO4-) can act as an acid by donating a proton to yield the hydrogen phosphate ion ( HPO4^2-). The equation for this reaction is: H2PO4- (aq) → HPO4^2- (aq) + H+ (aq).

Identify the Basic Behavior of H2O

A substance acts as a base if it can accept a proton (H+). Water is capable of acting as a base by accepting a proton from an acid (HA) to form the hydronium ion (H3O+). The equation for this is: H2O (l) + HA (aq) → H3O+ (aq) + A- (aq).

Identify the Basic Behavior of H2PO4-

The dihydrogen phosphate ion (H2PO4-) can accept a proton to form a new species. As a base, it can react with a generic acid (HA) to form dihydrogen phosphate and the conjugate base of the acid (A-). The equation is: H2PO4- (aq) + HA (aq) → H3PO4 (aq) + A- (aq).

Key concepts

Acid-Base Reaction

An acid-base reaction is a chemical process where an acid and a base interact, often resulting in the formation of water and a salt. In simple terms, it's like a dance where protons are passed from one partner to another.

In the case of water (H₂O) acting as an acid, we can envision this reaction as water 'giving' a proton to a base (B), transforming into hydroxide (OH^-) and consequently turning B into its conjugate acid (BH⁺). This reaction can be represented as:H₂O (l) + B (aq) → OH^- (aq) + BH⁺ (aq).

When a substance like dihydrogen phosphate (H₂PO₄^-) donates a proton, it transforms into hydrogen phosphate (HPO₄^2-), showcasing its acidic side through the equation:H₂PO₄^- (aq) → HPO₄^2- (aq) + H⁺ (aq).The essential idea here is the transfer of protons, which is the core of acid-base chemistry.

Proton Donor and Acceptor

Understanding acids and bases is much easier when we view them as proton traders. A proton donor is an acid that 'throws' a proton into the chemical market, and a proton acceptor is a base willing to 'catch' it.

Take water (H₂O) for instance; when it accepts a proton from an acid (HA), it transforms into hydronium (H₃O⁺), indicating its role as a base. This is described by the equation:H₂O (l) + HA (aq) → H₃O⁺ (aq) + A^- (aq).

Similarly, dihydrogen phosphate (H₂PO₄^-) acts as a base when it grabs a proton from another acid to become phosphoric acid (H₃PO₄), leaving the other acid's conjugate base (A^-). The reaction looks like this:H₂PO₄^- (aq) + HA (aq) → H₃PO₄ (aq) + A^- (aq).Remember, the nature of an acid or a base in these situations is all about the presence and movement of that tiny, yet significant, H⁺ ion.

Conjugate Acid-Base Pairs

The idea of conjugate acid-base pairs is rooted in the notion that acids and bases come in pairs; when an acid loses a proton, it becomes a base, and vice versa. It's like two sides of the same coin.

For any acid-base reaction, such as when water (H₂O) acts as an acid and donates a proton to create hydroxide (OH^-), we have a pair. On the flip side, when water receives a proton to form hydronium (H₃O⁺), it's in a different pair. Each acid-base reaction introduces us to these pairs. Here's an important takeaway: every acid has a 'partner' base that forms when the acid gives up a proton, and each base has a 'partner' acid formed when the base gains a proton.

To visualize, consider the reaction of dihydrogen phosphate (H₂PO₄^-) as a base: it partners up with its conjugate acid, phosphoric acid (H₃PO₄), when it accepts a proton. Conversely, when it acts as an acid, its partner is the hydrogen phosphate (HPO₄^2-). These relationships are essential to understanding chemical reactivity and the strength of acids and bases in solution.