Question

Explain how the two acid salts of phosphoric acids are formed from the reaction of \(\mathrm{H}_{3} \mathrm{PO}_{4}\) with \(\mathrm{NaOH},\) and name them.

Short answer

The two acid salts formed are sodium dihydrogen phosphate (\(\text{NaH}_2\text{PO}_4\)) and disodium hydrogen phosphate (\(\text{Na}_2\text{HPO}_4\)). The first is formed by the neutralization of one proton, and the second by the neutralization of two protons from phosphoric acid by sodium hydroxide.

Step-by-step solution

Understanding the Reaction Between \( \textrm{H}_3\textrm{PO}_4 \) and \( \textrm{NaOH} \)

Phosphoric acid (\(\text{H}_3\text{PO}_4\)) can react with sodium hydroxide (\(\text{NaOH}\)) to produce acid salts. Depending on the ratio of the reactants, different acid salts can be formed. Phosphoric acid is a triprotic acid, meaning it can lose three protons (\text{H}^+), and depending on how many are replaced by sodium ions (\text{Na}^+), different salts are formed.

Formation of the First Acid Salt

In the first reaction, one mole of \(\text{NaOH}\) neutralizes one mole of \(\text{H}_3\text{PO}_4\), replacing one hydrogen ion to form sodium dihydrogen phosphate (\(\text{NaH}_2\text{PO}_4\)). The chemical equation is: \(\text{H}_3\text{PO}_4 + \text{NaOH} \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O}\).

Formation of the Second Acid Salt

In a further reaction, two moles of \(\text{NaOH}\) can react with one mole of \(\text{H}_3\text{PO}_4\) to replace two hydrogen ions and form disodium hydrogen phosphate (\(\text{Na}_2\text{HPO}_4\)) as a second acid salt. The chemical equation for this reaction is: \(\text{H}_3\text{PO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{HPO}_4 + 2\text{H}_2\text{O}\).

Key concepts

Triprotic Acid Reactions

Triprotic acids, such as phosphoric acid ($H\_3PO\_4$), are unique because they have the ability to lose not just one but three protons ($H^+$). As a result of this characteristic, they can form different types of salts, based on how many protons are released and subsequently replaced by other ions like sodium ($N^+$).

For a student trying to understand this process, it's essential to grasp the concept of stepwise dissociation. Each time phosphoric acid loses a proton, it steps down to a different form. The first proton loss turns it into dihydrogen phosphate ($H\_2PO\_4^-$), the second to hydrogen phosphate ($HPO\_4^\{2-\}$), and finally, to phosphate ($PO\_4^\{3-\}$) once all protons are lost. Each of these steps represents a potential stage for salt formation with sodium hydroxide ($NaOH$), leading to different acid salts.

Sodium Dihydrogen Phosphate

Sodium dihydrogen phosphate ($NaH\_2PO\_4$) is the product of the first step in a triprotic acid reaction when phosphoric acid reacts with sodium hydroxide. Here, it is important to note that only one proton is replaced with a sodium ion.

To see this more clearly, imagine pouring one cup of sodium hydroxide into a container of phosphoric acid. This first 'cup' removes one 'proton bead' from the 'necklace' of phosphoric acid, turning it into sodium dihydrogen phosphate - a compound with one sodium ion and the acid residue of two hydrogens and one phosphate group remaining together.

In lab terms, this is often a common buffer solution used in biochemistry due to its stable pH. So, if a student can remember that one 'bead' (proton) has been replaced by sodium, the compound's name—sodium dihydrogen phosphate—tells them exactly what the structure is: one sodium ($Na^+$), connected to a phosphate ($PO\_4$) that still holds two hydrogens ($H$).

Disodium Hydrogen Phosphate

When you double the 'recipe' and mix two 'cups' of sodium hydroxide with phosphoric acid, a second proton is replaced, and thus enters disodium hydrogen phosphate ($Na\_2HPO\_4$). This is a salt where two sodium ions have stepped in, and only one 'original' hydrogen remains from the phosphoric acid molecule.

This salt is often used to prepare buffer solutions with slightly basic pH, a common requirement in many biochemical reactions. The naming here again is intuitive - the 'di' in disodium tells the student there are two sodium ions, and the remaining hydrogen phosphate indicates the presence of a single hydrogen within the phosphate group.

For students, a helpful perspective is to think of sodium dihydrogen phosphate and disodium hydrogen phosphate as siblings in a family, where they share a common lineage from phosphoric acid but differ in how many 'proton beads' they've kept. The first retains two, while the latter keeps just one, each with their sodium counterparts corresponding to the number of protons replaced.