Question

Equal molar quantities of hydrochloric acid and sodium hypochlorite (NaClO) 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?

Short answer

Balance equation: \( \text{H}^+ + \text{ClO}^- \rightarrow \text{HClO} \). Equilibrium lies to the right.

Step-by-step solution

Write the Components in Ionic Form

Identify the ions present in the solution. Hydrochloric acid (HCl) dissociates into \( \text{H}^+ \) and \( \text{Cl}^- \), while sodium hypochlorite (NaClO) dissociates into \( \text{Na}^+ \) and \( \text{ClO}^- \). Sodium ions (\( \text{Na}^+ \)) are spectator ions and can be ignored in the net ionic equation.

Formulate the Net Ionic Equation

The acid-base reaction involves \( \text{H}^+ \) from HCl reacting with \( \text{ClO}^- \) from NaClO to form hypochlorous acid (HClO). The balanced net ionic equation is:\[ \text{H}^+ + \text{ClO}^- \rightarrow \text{HClO} \]

Assess the Equilibrium Position

Consider the strengths of the acids and bases involved. Hypochlorous acid (HClO) is a weak acid, while hydrochloric acid (HCl) is a strong acid. The reaction favors the formation of the weaker acid, so the equilibrium lies to the right, favoring the formation of hypochlorous acid.

Key concepts

Net Ionic Equations

Net ionic equations simplify chemical reactions by showing only the species that actually participate in the reaction. They exclude substances that do not directly change in the reaction, known as spectator ions. This is particularly useful in many acid-base reactions, where strong acids or bases are involved. By focusing on only the essential components, you can concentrate on the core chemical changes that matter.
In the given problem, hydrochloric acid (HCl) dissociates to form hydrogen ions (\(\text{H}^+\)) and chloride ions (\(\text{Cl}^-\)), and sodium hypochlorite (NaClO) dissociates to form sodium ions (\(\text{Na}^+\)) and hypochlorite ions (\(\text{ClO}^-\)).
The net ionic equation will ignore the sodium ions, as they do not undergo any change, and focus on the reacting ions:\[\text{H}^+ + \text{ClO}^- \rightarrow \text{HClO}\] Notice how we excluded the \(\text{Na}^+\) and \(\text{Cl}^-\) ions as they do not participate in the actual chemical change.

Chemical Equilibrium

In chemical reactions, the concept of equilibrium refers to the state where the rate of the forward reaction equals the rate of the backward reaction. This balance can shift, depending on the conditions and the relative strengths of reactants and products.
For acid-base reactions, if one of the acids or bases is significantly stronger than the other, the equilibrium will usually favor the side forming the weaker acid or base.
In our scenario involving hydrochloric acid (a strong acid) and hypochlorous acid (a weak acid), the reaction favors the formation of hypochlorous acid. This means the reaction at equilibrium shifts to the right, producing more of the weaker acid, HClO.
It's the principle of chemical equilibrium that drives this preference: systems tend to arrange themselves in the state of lowest energy. Conversely, should conditions such as concentration or temperature change, it might prompt the equilibrium to shift to recover balance.

Weak and Strong Acids

Acids are classified into two main groups: strong acids and weak acids. This classification is based on their ability to dissociate in water:

• Strong acids dissociate completely in water. A common example is hydrochloric acid (HCl), which breaks down fully to hydrogen ions and chloride ions in a solution.
• Weak acids do not dissociate fully. Instead, they remain partially intact in solution. Hypochlorous acid (HClO) is a typical example, with a considerable portion of the acid remaining undissociated in the mixture.

The strength of an acid directly impacts its ability to react and dictates the direction of equilibrium in an acid-base reaction.
Remember, in our given exercise, HCl is the strong acid, prompting the reaction to favor the production of the weaker acid, HClO. Understanding these distinctions helps in predicting the behavior and outcome of various chemical processes, especially ionization and neutrality reactions.