Problem 37 - Chemistry and Chemical Reactivity

Question

Equal molar quantities of sodium hydroxide and sodium hydrogen phosphate \(\left(\mathrm{Na}_{2} \mathrm{HPO}_{4}\right)\) are mixed. (a) Write the balanced, net ionic equation for the acid-base reaction that can, in principle, occur. (b) Does the equilibrium lie to the right or left?

Step-by-Step Solution

Answer

(a) \( \mathrm{HPO}_4^{2-} + \mathrm{OH}^- \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H_2O} \); (b) Equilibrium lies to the right.

Step 1: Identify Reactants and Products

Begin by identifying the reactants in the solution. We are given sodium hydroxide (NaOH), a strong base, and sodium hydrogen phosphate (Na2HPO4), which can act both as a weak acid and a base. When these two are mixed, an acid-base reaction may occur where sodium hydrogen phosphate acts as an acid.

Step 2: Write Chemical Equations

Sodium hydroxide dissociates completely in solution: \( \mathrm{NaOH} \rightarrow \mathrm{Na}^+ + \mathrm{OH}^- \). Sodium hydrogen phosphate can ionize to produce \( \mathrm{HPO}_4^{2-} \), so the reaction in principle is: \( \mathrm{HPO}_4^{2-} + \mathrm{OH}^- \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H_2O} \).

Step 3: Determine Net Ionic Equation

To write the net ionic equation, we remove the spectator ions (sodium ions, \( \mathrm{Na}^+ \)). The net ionic equation for this reaction is: \( \mathrm{HPO}_4^{2-} + \mathrm{OH}^- \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H_2O} \).

Step 4: Evaluate Equilibrium Position

Consider the acid-base reactions of the principal players: \( \mathrm{HPO}_4^{2-} \) (which can donate \( \mathrm{H}^+ \) being a weaker acid) reacts with \( \mathrm{OH}^- \) (a strong base). Since a strong base is present, the reaction is driven forward with the formation of \( \mathrm{H_2O} \) and \( \mathrm{PO}_4^{3-} \). Thus, the equilibrium lies to the right, favoring the formation of product.

Key Concepts

Net Ionic Equation

In an acid-base reaction, not every ion participates actively in the chemical transformation. Some ions remain unchanged in the solution and are known as spectator ions. To focus on the actual chemical change, we write what is called the net ionic equation. This equation only includes the ions and molecules directly involved in the reaction, omitting spectator ions.

• For the reaction between sodium hydroxide and sodium hydrogen phosphate, the net ionic equation is: \[ \mathrm{HPO}_4^{2-} + \mathrm{OH}^- \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H_2O} \]
• Here, the sodium ions \((\mathrm{Na}^+)\) do not partake in the reaction and are therefore omitted.

Understanding the net ionic equation helps focus on what exactly occurs in the chemical reaction without the distraction of spectator ions.

Equilibrium Position

An important concept in chemical reactions is understanding the equilibrium position. In the context of an acid-base reaction, the equilibrium position indicates the direction in which the reaction favors forming either reactants or products over time.

• For the reaction between sodium hydroxide and sodium hydrogen phosphate, the strong base \((\mathrm{OH}^-)\) drives the reaction forward.
• This forms water \((\mathrm{H_2O})\) and the phosphate ion \((\mathrm{PO}_4^{3-})\).

Because the reaction involves a strong base with a weaker acid, the equilibrium lies to the right, suggesting that products are favored. This means more products will form compared to the reactants being reformed, under the given conditions.

Sodium Hydroxide

Sodium hydroxide, chemically represented as \(\mathrm{NaOH}\), is a strong base commonly used in chemical reactions and industrial applications. It dissociates completely in water, breaking down into sodium ions \((\mathrm{Na}^+)\) and hydroxide ions \((\mathrm{OH}^-)\).

• The full dissociation of \(\mathrm{NaOH}\) is what makes it a strong base.
  • This ability allows it to readily react with acids.

In an acid-base reaction, \(\mathrm{NaOH}\) donates \(\mathrm{OH}^-\) ions, driving the reaction forward. Its role is crucial in determining the direction of equilibrium in reactions involving weak acids.

Sodium Hydrogen Phosphate

Sodium hydrogen phosphate, \(\mathrm{Na}_2\mathrm{HPO}_4\), is an important compound that can act both as a base and a weak acid, due to its ability to donate and accept protons. This makes it amphiprotic.

• In the described reaction, \(\mathrm{Na}_2\mathrm{HPO}_4\) acts as an acid.
• It donates a proton to the \(\mathrm{OH}^-\) ion, forming water and phosphate ions.

Due to its amphiprotic nature, solving reactions with \(\mathrm{Na}_2\mathrm{HPO}_4\) involves understanding which role (acid or base) it plays, depending on the reacting partner.