Problem 53 - Chemistry: An Atoms-Focused Approach

Question

Write a balanced molecular equation and a net ionic equation for the following reactions: a. Solid magnesium hydroxide reacts with a solution of sulfuric acid. b. Solid magnesium carbonate reacts with a solution of hydrochloric acid. c. Ammonia gas reacts with hydrogen chloride gas.

Step-by-Step Solution

Answer

Question: Write the net ionic equation for the following reactions: (a) solid magnesium hydroxide reacts with a solution of sulfuric acid, (b) solid magnesium carbonate reacts with a solution of hydrochloric acid, (c) ammonia gas reacts with hydrogen chloride gas. Answer: (a) Mg(OH)2 (s) + 2H+ (aq) -> Mg^2+ (aq) + 2H2O (l) (b) MgCO3 (s) + 2H+ (aq) -> Mg^2+ (aq) + H2O (l) + CO2 (g) (c) NH3 (g) + HCl (g) -> NH4Cl (s)

Step 1: Write the balanced molecular equation

Magnesium hydroxide (Mg(OH)2) reacts with sulfuric acid (H2SO4) to form magnesium sulfate (MgSO4) and water (H2O). The balanced molecular equation will be: Mg(OH)2 (s) + H2SO4 (aq) -> MgSO4 (aq) + 2H2O (l)

Step 2: Write the total ionic equation

Break the soluble compounds into their respective ions: Mg(OH)2 (s) + 2H+ (aq) + SO4^2- (aq) -> Mg^2+ (aq) + SO4^2- (aq) + 2H2O (l)

Step 3: Identify and remove spectator ions

The spectator ion is SO4^2-. So, the net ionic equation is: Mg(OH)2 (s) + 2H+ (aq) -> Mg^2+ (aq) + 2H2O (l) ##Reaction b##

Step 1: Write the balanced molecular equation

Magnesium carbonate (MgCO3) reacts with hydrochloric acid (HCl) to form magnesium chloride (MgCl2), water (H2O), and carbon dioxide (CO2). The balanced molecular equation will be: MgCO3 (s) + 2HCl (aq) -> MgCl2 (aq) + H2O (l) + CO2 (g)

Step 2: Write the total ionic equation

Break the soluble compounds into their respective ions: MgCO3 (s) + 2H+ (aq) + 2Cl- (aq) -> Mg^2+ (aq) + 2Cl- (aq) + H2O (l) + CO2 (g)

Step 3: Identify and remove spectator ions

The spectator ion is Cl-. So, the net ionic equation is: MgCO3 (s) + 2H+ (aq) -> Mg^2+ (aq) + H2O (l) + CO2 (g) ##Reaction c##

Step 1: Write the balanced molecular equation

Ammonia gas (NH3) reacts with hydrogen chloride gas (HCl) to form solid ammonium chloride (NH4Cl). The balanced molecular equation will be: NH3 (g) + HCl (g) -> NH4Cl (s)

Step 2: Write the total ionic equation

Since the reactants are in the gas phase and the product is a solid compound, there is no need to break them into ions. The total ionic equation is the same as the balanced molecular equation: NH3 (g) + HCl (g) -> NH4Cl (s)

Step 3: Identify and remove spectator ions

In this reaction, there are no spectator ions. So, the net ionic equation is: NH3 (g) + HCl (g) -> NH4Cl (s)

Key Concepts

Balanced Molecular Equation

When we talk about a balanced molecular equation, we focus on the full chemical equation that represents a chemical reaction. This equation ensures that the number of each type of atom is equal on both sides of the equation. It's a reflection of the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.
The goal of balancing a molecular equation is to accurately represent the reaction while satisfying this law. For instance, in the reaction between magnesium hydroxide and sulfuric acid:

• The balanced molecular equation is: \( \text{Mg(OH)}_2 (s) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{MgSO}_4 (aq) + 2\text{H}_2\text{O} (l) \)
• Note that there are equal numbers of atoms of magnesium, oxygen, hydrogen, and sulfur on both sides.

Balancing an equation involves adding coefficients in front of compounds to ensure both sides have an equal number of each type of atom. This is crucial because it represents the stoichiometry of the reaction and helps predict the amount of products formed from given reactants.

Net Ionic Equation

The net ionic equation zooms in on the actual chemical change happening in a reaction. It reflects only those species that undergo a change in their state of oxidation or reduction, omitting the ones that remain unchanged called spectator ions. This simplifies the reaction to its core essence.
In the study of the reaction between magnesium carbonate and hydrochloric acid:

• The balanced net ionic equation is: \( \text{MgCO}_3 (s) + 2\text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \)
• Here, we only see the ions and molecules that directly participate in the reaction.

To derive the net ionic equation:1. Write the balanced molecular equation.2. Split all soluble ionic compounds into their ions.3. Cancel out the spectator ions to highlight the species that change during the reaction.This provides a clearer perspective of the chemical change at the ionic level.

Spectator Ions

Spectator ions are those ions present in a chemical reaction that do not partake in the actual chemical change. They "spectate" the reaction without partaking in the agitated dance of chemical transformation. Think of them as the audience watching a play, aware and present, but not involved in the action on stage.
For instance, during the reaction between magnesium hydroxide and sulfuric acid, the sulfate ion \( \text{SO}_4^{2-} \) acts as a spectator:

• In the ionic equation: \( \text{Mg(OH)}_2 (s) + 2\text{H}^+ (aq) + \text{SO}_4^{2-} (aq) \rightarrow \text{Mg}^{2+} (aq) + \text{SO}_4^{2-} (aq) + 2\text{H}_2\text{O} (l) \)
• The sulfate \( \text{SO}_4^{2-} \) does not change or react. It is found on both sides without alteration.

By identifying which ions do and do not form part of the net change, students can better understand the underlying chemical processes at work, focusing on the primary ionic species that drive the reaction.