Problem 39 - Chemistry and Chemical Reactivity

Question

Equal molar quantities of acetic acid and sodium hydrogen phosphate (Na_HPO,) are mixed. (a) Write a 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

The equation is: CH 3 COOH + HPO 4 2- → CH 3 COO- + H 2 PO 4 -. Equilibrium lies to the left.

Step 1: Identify Acid and Base

The reaction involves acetic acid ( CH 3 COOH) and sodium hydrogen phosphate ( NaHPO 4 ). Acetic acid is a weak acid, and the hydrogen phosphate ion ( HPO 4 2- ) can act as a weak base.

Step 2: Write the Reaction Equation

Since this is an acid-base reaction, hydrogen ions ( H+ ) from acetic acid will react with hydrogen phosphate ions: CH 3 COOH + HPO 4 2- → CH 3 COO- + H 2 PO 4 -.

Step 3: Write the Net Ionic Equation

Remove the spectator ions (in this particular reaction, there are none in terms of ions that do not change state), focusing on the species that take part in the reaction: CH 3 COOH(aq) + HPO 4 2-(aq) → CH 3 COO- (aq) + H 2 PO 4 −(aq).

Step 4: Determine Direction of Equilibrium

The direction of the equilibrium depends on the acid and base strength. Since CH 3 COOH is a weaker acid than H 2 PO 4 − and CH 3 COO- is a stronger base than HPO 4 2-, the equilibrium lies to the left, favoring the reactants.

Key Concepts

Acetic Acid

Acetic acid, also known as ethanoic acid, is a common weak acid found in vinegar. Its chemical formula is \( ext{CH}_3 ext{COOH} \). Being a weak acid, it does not fully dissociate in water, which means only a small fraction of acetic acid molecules donate protons to form the acetate ion \( ext{CH}_3 ext{COO}^- \) and hydrogen ion \( ext{H}^+ \).

This partial dissociation is what characterizes acetic acid's behavior in solution:

• Releases hydrogen ions, but to a limited extent.
• The solution conducts electricity weakly due to fewer ions.

Understanding acetic acid's properties is essential for predicting how it behaves in acid-base reactions. When mixed with a base like sodium hydrogen phosphate, acetic acid participates in complex equilibria determined by its weak acid properties.

Sodium Hydrogen Phosphate

Sodium hydrogen phosphate is often found as \( ext{Na}_2 ext{HPO}_4 \), which contains the hydrogen phosphate ion \( ext{HPO}_4^{2-} \). This compound serves as a weak base, capable of accepting hydrogen ions to form \( ext{H}_2 ext{PO}_4^- \).

Key aspects of sodium hydrogen phosphate involve:

• Acts as a buffer component, maintaining pH stability in solutions.
• Forms part of the phosphate buffer system that regulates pH in biological and chemical systems.

Sodium hydrogen phosphate's dual ability to act as both a proton donor and acceptor makes it versatile in participating in acid-base equilibria. In reactions with acids like acetic acid, it tends to form more stable conjugate species, further demonstrating its buffering capacity.

Equilibrium Direction

In chemical reactions, particularly acid-base reactions, the equilibrium direction indicates which side of the reaction is favored once the system reaches balance.

Several factors influence the direction of equilibrium:

• Strengths of the acid and base involved.
• The relative stabilities of the reactants and products.

For the acetic acid and sodium hydrogen phosphate reaction, the equilibrium lies to the left, favoring the reactants. This is because acetic acid is a weaker acid compared to \( ext{H}_2 ext{PO}_4^- \), and acetate \( ext{CH}_3 ext{COO}^- \) is a stronger base than \( ext{HPO}_4^{2-} \). Thus, the reactants are more stable, and the system naturally shifts towards them.

Net Ionic Equation

A net ionic equation simplifies chemical reactions by showing only the species that actually undergo chemical change, omitting spectator ions that do not participate directly in the reaction.

To construct a net ionic equation:

• Identify the ions that participate in the reaction.
• Balance the charge as well as the atoms involved.

For the reaction between acetic acid \( ext{CH}_3 ext{COOH} \) and sodium hydrogen phosphate \( ext{Na}_2 ext{HPO}_4 \), the net ionic equation is:\[\text{CH}_3 ext{COOH(aq)} + ext{HPO}_4^{2-} ext{(aq)} \rightarrow \text{CH}_3 ext{COO}^- ext{(aq)} + ext{H}_2 ext{PO}_4^- ext{(aq)}\]This equation highlights the acid-base interaction where acetic acid donates a proton to the hydrogen phosphate ion, forming acetate and dihydrogen phosphate ions while accurately depicting the fundamental species at play.