Question

Nitrous acid, HNO, is a weak electrolyte. Nitric acid, \(\mathrm{HNO}_{3},\) is a strong electrolyte. Write equations to represent the ionization of each in water. Include the hydronium ion, and show the appropriate kind of arrow in each equation.

Short answer

HNO2: \( \text{HNO}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{NO}_2^-(aq)\). HNO3: \( \text{HNO}_3(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{NO}_3^-(aq) \).

Step-by-step solution

Understanding the Definitions

First, recognize that nitrous acid (HNO2) is a weak electrolyte, meaning it partially ionizes in water. Nitric acid (HNO3) is a strong electrolyte, meaning it completely ionizes in water.

Write the Ionization Equation for Nitrous Acid

Since nitrous acid is a weak electrolyte, it partially ionizes in water. The equation for its ionization is \( \text{HNO}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{NO}_2^-(aq) \)

Write the Ionization Equation for Nitric Acid

Since nitric acid is a strong electrolyte, it completely ionizes in water. The equation for its ionization is \( \text{HNO}_3(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{NO}_3^-(aq) \)

Key concepts

Weak Electrolytes

In chemistry, a weak electrolyte is a substance that only partially ionizes in water. This means not all of the compound's molecules break apart. Examples include nitrous acid (\text{HNO}_2) and acetic acid (\text{CH}_3\text{COOH}). When nitrous acid dissolves in water, it forms a balance between the ionized and unionized form:

• The partial ionization is represented by a double arrow ( ⇌ ) showing that the reaction is reversible.
  
• Only a fraction of \text{HNO}_2 molecules convert into \text{NO}_2^- and \text{H}_3\text{O}^+.
  

The ionization equation for nitrous acid is:\[ \text{HNO}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{NO}_2^-(aq) \]This shows that weak electrolytes don’t fully dissociate in solution, making them different from strong electrolytes.

Strong Electrolytes

Strong electrolytes are substances that completely ionize in water, breaking into ions and leaving no neutral molecules. Nitric acid (\text{HNO}_3) is an example:

• Once \text{HNO}_3 dissolves in water, it entirely splits into \text{H}_3\text{O}^+ and \text{NO}_3^- ions.
  
• This complete ionization is represented by a single arrow ( → ).
  

The ionization equation for nitric acid is:\[ \text{HNO}_3(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{NO}_3^-(aq) \]Thus, strong electrolytes, unlike weak ones, dissociate entirely in water.
If you want to test if a substance is a strong electrolyte, check if it conducts electricity well, and see if it completely dissolves into ions.

Ionization Equations

Ionization equations illustrate how acids or bases dissociate into ions in aqueous solutions. These equations are crucial in understanding acid-base reactions:

• For weak electrolytes, partial ionization is shown with a double arrow ( ⇌ ) indicating equilibrium.
  
• For strong electrolytes, complete ionization is denoted with a single arrow ( → ).
  

For example, the ionization of nitrous acid, which is weak, is shown as:\[ \text{HNO}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{NO}_2^-(aq) \]and, for strong nitric acid:\[ \text{HNO}_3(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{NO}_3^-(aq) \]Understanding these equations helps predict the behavior of different acids in aqueous solutions.

Hydronium Ion

The hydronium ion (\text{H}_3\text{O}^+) is a critically important ion in acid-base chemistry. It forms when a proton (\text{H}^+) from an acid bonds with a water molecule (\text{H}_2\text{O}):
\[ \text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ \]

• This ion is responsible for the acidic properties in solutions.
  
• It is always present in the ionization equations of acids.
  

For instance:

• In weak acid ionization, such as for nitrous acid:
  \[ \text{HNO}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{NO}_2^-(aq) \]
• In strong acid ionization like nitric acid:
  \[ \text{HNO}_3(aq) + \text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{NO}_3^-(aq) \]

Grasping the role of the hydronium ion in these reactions is key to mastering acid-base chemistry.