Question

Write the correct \(K_{\text {eq }}\) expression for this reaction. \(\mathrm{NaOH}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightleftarrows \mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)\)

Short answer

\(K_{eq} = \frac{[\text{NaCl}]}{[\text{NaOH}][\text{HCl}]}\)

Step-by-step solution

Identifying the Reaction

The given chemical equation represents a neutralization reaction. It involves an acid-base reaction between NaOH and HCl, forming NaCl and water.

Understanding Equilibrium Expression

The equilibrium constant expression, \(K_{eq}\), is based on the concentrations of the reactants and products at equilibrium. However, we only include aqueous species in the expression.

Excluding Pure Solids and Liquids

Since \(\text{H}_2\text{O}\) is a pure liquid, it is not included in the \(K_{eq}\) expression. Therefore, we only consider aqueous species.

Writing the \(K_{eq}\) Expression

The equilibrium constant expression is given by \( K_{eq} = \frac{[\text{NaCl}]}{[\text{NaOH}][\text{HCl}]} \). It includes the concentration of NaCl as the product over the concentrations of NaOH and HCl as the reactants.

Key concepts

Neutralization Reaction

A neutralization reaction is a chemical reaction where an acid and a base react together to form a salt and water. This type of reaction typically involves the combination of hydrogen ions from the acid and hydroxide ions from the base to produce water. In a neutralization reaction:

• The acid donates hydrogen ions (H⁺).
• The base donates hydroxide ions (OH^-).
• The resulting products are generally a salt (ionic compound) and water (H₂O).

The reaction mentioned in the exercise, \[\text{NaOH} (\text{aq}) + \text{HCl} (\text{aq}) \rightleftharpoons \text{NaCl} (\text{aq}) + \text{H}_2\text{O} (\ell),\]is a perfect example of a neutralization reaction. Here, sodium hydroxide (NaOH), a strong base, and hydrochloric acid (HCl), a strong acid, react to form sodium chloride (NaCl) and water.

Aqueous Solution

An aqueous solution is a solution in which the solvent is water. In the context of chemistry, substances dissolved in water are often denoted as "(aq)" in chemical equations. Water’s unique properties make it an excellent solvent for a variety of molecules, including acids, bases, and salts:

• It is a polar molecule, which means it can dissolve ionic compounds by surrounding and separating ions.
• Aqueous solutions can conduct electricity due to the presence of ions.

In the given reaction, both sodium hydroxide (NaOH) and hydrochloric acid (HCl) are in aqueous solution. This means they are dissolved in water and exist as ions. Strong acids and bases separate fully into their component ions when in aqueous solution, resulting in an efficient reaction towards forming products.

Acid-Base Reaction

An acid-base reaction is a fundamental chemical reaction that involves the exchange of protons between an acid and a base. Lewis and Brønsted-Lowry are two models that describe acid-base behavior: - The **Brønsted-Lowry model** defines acids as proton donors and bases as proton acceptors. This model is widely used in reactions like the one given in the exercise. - The **Lewis model** views acids as electron pair acceptors and bases as electron pair donors. For the reaction between NaOH and HCl:

• HCl acts as a Bronsted-Lowry acid by donating a proton (H⁺).
• NaOH acts as a Bronsted-Lowry base by accepting a proton to form water (H₂O) and releasing ions.

In this balanced reaction, each mole of NaOH reacts with one mole of HCl, demonstrating the stoichiometry typical of acid-base reactions. The formation of stable molecules like water meticulously drives the reaction forward.