Problem 47 - 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) \( \text{HNO}_2 + \text{OH}^- \rightarrow \text{NO}_2^- + \text{H}_2\text{O} \); (b) \( \text{OH}^- + \text{H}^+ \rightarrow \text{H}_2\text{O} \).

Step 1: Write the Chemical Equations

Write the full chemical equations for each reaction. (a) Nitrous acid reacts with sodium hydroxide: \[ \text{HNO}_2 (aq) + \text{NaOH} (aq) \rightarrow \text{NaNO}_2 (aq) + \text{H}_2\text{O} (l) \] (b) Calcium hydroxide reacts with hydrochloric acid: \[ \text{Ca(OH)}_2 (aq) + 2\text{HCl} (aq) \rightarrow \text{CaCl}_2 (aq) + 2\text{H}_2\text{O} (l) \]

Step 2: Split into Ionic Equations

Separate the aqueous compounds into their ions. Ignore weak acids and undissociated molecules. (a) Nitrous acid is weak and remains undissociated: \[ \text{HNO}_2 (aq) + \text{Na}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{Na}^+ (aq) + \text{NO}_2^- (aq) + \text{H}_2\text{O} (l) \] (b) Calcium hydroxide and hydrochloric acid dissociate: \[ \text{Ca}^{2+} (aq) + 2\text{OH}^- (aq) + 2\text{H}^+ (aq) + 2\text{Cl}^- (aq) \rightarrow \text{Ca}^{2+} (aq) + 2\text{Cl}^- (aq) + 2\text{H}_2\text{O} (l) \]

Step 3: Cancel Spectator Ions

Identify and cancel out the ions that appear on both sides of the ionic equation, as they do not participate in the reaction. (a) The spectator ion is \(\text{Na}^+\). Removing it gives: \[ \text{HNO}_2 (aq) + \text{OH}^- (aq) \rightarrow \text{NO}_2^- (aq) + \text{H}_2\text{O} (l) \] (b) Spectator ions are \(\text{Ca}^{2+}\) and \(\text{Cl}^-\). Removing them gives: \[ 2\text{OH}^- (aq) + 2\text{H}^+ (aq) \rightarrow 2\text{H}_2\text{O} (l) \]

Step 4: Write the Balanced Net Ionic Equations

The remaining equations after canceling spectator ions are the net ionic equations, which are already balanced.(a) The balanced net ionic equation is: \[ \text{HNO}_2 (aq) + \text{OH}^- (aq) \rightarrow \text{NO}_2^- (aq) + \text{H}_2\text{O} (l) \] (b) The balanced net ionic equation simplifies to: \[ \text{OH}^- (aq) + \text{H}^+ (aq) \rightarrow \text{H}_2\text{O} (l) \]

Key Concepts

Chemical Reactions

Chemical reactions occur when substances undergo changes to form new substances. In a reaction, the reactant molecules break bonds and form new bonds to create the products. - Chemical reactions can be represented by chemical equations, which visually show the transformation from reactants to products. - These equations provide vital information, such as the types of atoms involved and their quantities, and the phases in which they exist (solid, liquid, gas, or aqueous). - Understanding chemical reactions is key to analyzing processes like combustion, photosynthesis, and cellular respiration. In studying chemical reactions involving solutions, it's common to work with equations showing substances dissolved in water, known as aqueous solutions. This format allows us to highlight the reactants and products as well as the participating ions.

Spectator Ions

Spectator ions are ions in a chemical reaction that do not participate in the actual chemical change. Their presence is significant only in maintaining the charge balance of a solution. - During reactions in aqueous solutions, many compounds dissociate into ions. - Some of these ions participate directly in forming new substances, while others remain unchanged throughout. - Spectator ions are identified by their presence on both sides of a complete ionic equation—meaning their removal doesn't affect the actual reaction. Knowing how to spot spectator ions helps streamline chemical equations, simplifying them into net ionic equations. This leads to a clearer view of the substances undergoing change and allows for a deeper understanding of the chemistry behind the reactions.

Balanced Equations

Balanced equations are integral to chemical reactions as they obey the Law of Conservation of Mass, which dictates that matter cannot be created or destroyed. Therefore, the amount of each element must be the same on both sides of a chemical equation. - To balance an equation, adjust the coefficients before the chemical symbols to ensure the same number of atoms for each element on both the reactant and product sides. - This precise balancing is essential for accurately representing the stoichiometry of the reaction—telling us how much of each reactant is needed and how much product is formed. In the context of net ionic equations, balancing involves ensuring that the number of atoms and the charge are equal on both sides. This step is crucial for correctly representing the true nature of the chemical change occurring in the system.