Problem 42 - Chemistry and Chemical Reactivity

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?

Step-by-Step Solution

Answer

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

Step 1: Identify the Reactants and Products

When hydrochloric acid (HCl) and sodium hypochlorite (NaClO) are mixed, hydrochloric acid (HCl) can dissociate into hydrogen ions (H⁺) and chloride ions (Cl⁻), and sodium hypochlorite (NaClO) can dissociate into sodium ions (Na⁺) and hypochlorite ions (ClO⁻). The reactants involved in the acid-base reaction are H⁺ from HCl and ClO⁻ from NaClO. The product will be hypochlorous acid (HClO).

Step 2: Write the Balanced Net Ionic Equation

To write the net ionic equation, only include the species that change during the reaction. Therefore, the net ionic equation for the reaction between H⁺ and ClO⁻ is: \[ \text{H}^+_{(aq)} + \text{ClO}^-_{(aq)} \rightarrow \text{HClO}_{(aq)} \]

Step 3: Determine the Direction of Equilibrium

To determine the direction of equilibrium, compare the strengths of the acid and conjugate acid. Hypochlorous acid (HClO) is a weak acid, whereas hydrochloric acid (HCl) from which H⁺ ions come is a strong acid. This means the reaction will favor the formation of the weaker acid HClO, causing the equilibrium to lie to the right (towards the formation of HClO).

Key Concepts

Net Ionic Equation

A net ionic equation is a simplified chemical equation that shows only the ions and molecules directly involved in the chemical reaction, eliminating the spectator ions. In the context of an acid-base reaction, it highlights the essential chemical change without the clutter of unreactive ions.
For example, when hydrochloric acid (HCl) and sodium hypochlorite (NaClO) are mixed, both compounds dissociate in water. Hydrochloric acid dissociates to form hydrogen ions (\( \text{H}^+ \)) and chloride ions (\( \text{Cl}^- \)), while sodium hypochlorite dissociates into sodium ions (\( \text{Na}^+ \)) and hypochlorite ions (\( \text{ClO}^- \)).
The net ionic equation reflects the actual chemistry behind the reaction:

• Hydrogen ions from HCl react with hypochlorite ions from NaClO.
• This forms hypochlorous acid (HClO), a new compound.

The resulting net ionic equation is:\[ \text{H}^+_{(aq)} + \text{ClO}^-_{(aq)} \rightarrow \text{HClO}_{(aq)} \]This concise equation shows the transformation of active chemical species into products.

Equilibrium Position

The equilibrium position of a chemical reaction refers to the direction in which the reaction naturally tends to proceed to maintain balance among the reactants and products. It is a key aspect in understanding how and why chemical reactions occur.
In an acid-base reaction, the equilibrium position can tell us which side of the equation is favored. For our specific reaction involving hydrochloric acid and sodium hypochlorite, the position of equilibrium lies to the right.

• This is because hydrochloric acid (HCl) is a strong acid that completely dissociates in water, providing a large concentration of \( \text{H}^+ \) ions.
• On the other hand, the product, hypochlorous acid (HClO), is a weak acid.

Since reactions tend to favor the formation of weaker acids and bases (to achieve lower energy and more stability), the equilibrium shifts towards the right.
Thus, the formation of hypochlorous acid is favored, showing the dynamic behavior of the equilibrium as it adjusts to the strengths of the acids involved.

Acid Strength Comparisons

Acid strength is a measure of the tendency of an acid to donate a proton (\( \text{H}^+ \)). It is indicative of how strongly an acid behaves in releasing its hydrogen ions. Comparing acid strengths is important in predicting the outcome of acid-base reactions.
Strong acids, like hydrochloric acid (HCl), completely dissociate in solution. This means they readily release their \( \text{H}^+ \) ions, making them very effective proton donors. Conversely, weak acids like hypochlorous acid (HClO) do not fully dissociate; they release fewer \( \text{H}^+ \) ions.
In acid-base reactions:

• Strong acids will dominate in creating chemical reactions due to their higher ionization levels.
• The reaction typically proceeds in the direction that forms a weaker acid from the stronger one.

The specific reaction between HCl and NaClO results in the transformation of a strong acid (HCl) into a weak acid (HClO).
Thus, understanding acid strength helps in determining the equilibrium direction and predicting the compounds formed during reactions. This insight is crucial when writing net ionic equations and analyzing equilibrium positions.