Problem 39 - Chemistry and Chemical Reactivity

Question

Write balanced net ionic equations for the following reactions: (a) the reaction of nitrous acid (a weak acid) and sodium hydroxide in aqueous solution (b) the reaction of calcium hydroxide and hydrochloric acid

Step-by-Step Solution

Answer

(a) HNO₂(aq) + OH⁻(aq) → NO₂⁻(aq) + H₂O(l) (b) OH⁻(aq) + H⁺(aq) → H₂O(l)

Step 1: Understand the Reaction Components

For the reactions given: (a) Nitrous acid (HNO₂) is a weak acid and sodium hydroxide (NaOH) is a strong base. (b) Calcium hydroxide (Ca(OH)₂) is a strong base and hydrochloric acid (HCl) is a strong acid.

Step 2: Write the Balanced Molecular Equations

(a) For nitrous acid with sodium hydroxide, the complete molecular equation is: \[ \text{HNO}_2 (aq) + \text{NaOH} (aq) \rightarrow \text{NaNO}_2 (aq) + \text{H}_2\text{O} (l) \](b) For calcium hydroxide with hydrochloric acid, the complete molecular equation is: \[ \text{Ca(OH)}_2 (aq) + 2 \text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + 2 \text{H}_2\text{O} (l) \]

Step 3: Identify the Strong Electrolytes and Write Complete Ionic Equations

Complete ionic equations dissociate strong electrolytes in water:(a) In \( \text{HNO}_2 (aq) + \text{NaOH} (aq) \rightarrow \text{NaNO}_2 (aq) + \text{H}_2\text{O} (l) \), because HNO₂ is weak, only NaOH dissociates: \[ \text{HNO}_2 (aq) + \text{Na}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{Na}^+ (aq) + \text{NO}_2^- (aq) + \text{H}_2\text{O} (l) \](b) In \( \text{Ca(OH)}_2 (aq) + 2 \text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + 2 \text{H}_2\text{O} (l) \), fully dissociate strong acids and bases: \[ \text{Ca}^{2+} (aq) + 2 \text{OH}^- (aq) + 2 \text{H}^+ (aq) + 2 \text{Cl}^- (aq) \rightarrow \text{Ca}^{2+} (aq) + 2 \text{Cl}^- (aq) + 2 \text{H}_2\text{O} (l) \]

Step 4: Write the Net Ionic Equations by Removing Spectator Ions

Remove ions that appear unchanged on both sides:(a) After canceling \(\text{Na}^+\) ions: \[ \text{HNO}_2 (aq) + \text{OH}^- (aq) \rightarrow \text{NO}_2^- (aq) + \text{H}_2\text{O} (l) \](b) After canceling \(\text{Ca}^{2+}\) and \(\text{Cl}^-\) ions: \[ 2 \text{OH}^- (aq) + 2 \text{H}^+ (aq) \rightarrow 2 \text{H}_2\text{O} (l) \] Simplified, this equation becomes: \[ \text{OH}^- (aq) + \text{H}^+ (aq) \rightarrow \text{H}_2\text{O} (l) \]

Key Concepts

Weak Acids

Weak acids are fascinating in chemistry because they don't fully ionize in solution. This partial ionization differentiates them from strong acids. A common example of a weak acid is nitrous acid (HNO₂).

When dissolved in water, a weak acid like HNO₂ only releases a small fraction of its hydrogen ions (H⁺). This means that at equilibrium, both the ionized and non-ionized forms of the acid exist together.

• This property affects how reactions proceed since not all molecules are available to react.
• In net ionic equations, the weak acid generally remains in its molecular form.

Understanding weak acids helps in predicting the outcome and extent of chemical reactions involving them.

Strong Bases

Strong bases are key players in neutralization reactions because they completely dissociate in water. Sodium hydroxide (NaOH) is a prototypical strong base.

In solution, NaOH dissociates fully into Na⁺ and OH⁻ ions. This characteristic makes strong bases highly reactive, especially with acids.

• Full dissociation means that all molecules of the base contribute hydroxide ions to the reaction.
• This ensures that strong bases are very effective at raising the pH of a solution.

Recognizing a strong base in reactions helps understand its behavior and predict the products formed.

Strong Acids

Strong acids are known for their ability to completely ionize in aqueous solutions. Hydrochloric acid (HCl) is a prime example.

In water, HCl fully dissociates into H⁺ and Cl⁻ ions. This total ionization is key to their potency in reactions with bases.

• The complete release of hydrogen ions makes them effective in driving reactions to completion.
• In net ionic equations, strong acids give away their ions, contributing largely to the reactivity.

Understanding strong acids allows chemists to predict reaction behaviors and product formation accurately.

Chemical Reactions

Chemical reactions are processes where substances interact to form new products. They often involve bonds breaking and new bonds forming.

When acids react with bases, a special type of reaction occurs called neutralization. This process typically results in the formation of water and a salt.

• Reactions are represented by balanced chemical equations.
• Molecular equations, complete ionic equations, and net ionic equations describe reactions at different levels of detail.

Grasping the concept of chemical reactions is crucial in understanding how substances interact and change.

Spectator Ions

Spectator ions are ions present in the reaction mixture that do not take part in the overall chemical change. They remain unchanged in the solution.

When writing net ionic equations, spectator ions are excluded to simplify the equation to show only the participating species.

• For instance, in reactions involving strong acids and bases, ions like Na⁺ and Cl⁻ often remain unchanged.
• Identifying and removing spectator ions helps highlight the actual chemical changes occurring.

Understanding spectator ions streamlines the equation-writing process and clarifies the reaction.