Problem 59 - Chemistry The Science in Context

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. d. Gaseous sulfur trioxide is dissolved in water and reacts with a solution of sodium hydroxide.

Step-by-Step Solution

Answer

Question: Provide the net ionic equation for the reaction between solid magnesium hydroxide and a solution of sulfuric acid. Answer: Mg(OH)2 (s) + 2H+ (aq) → Mg+2 (aq) + 2H2O (l)

Step 1: Balanced molecular equation

Mg(OH)2 (s) + H2SO4 (aq) → MgSO4 (aq) + 2H2O (l)

Step 2: Rewrite to show ions

Mg(OH)2 (s) + 2H+ (aq) + SO4^2- (aq) → Mg+2 (aq) + SO4^2- (aq) + 2H2O (l)

Step 3: Cancel out spectator ions

SO4^2- (aq) is the spectator ion.

Step 4: Net ionic equation

Mg(OH)2 (s) + 2H+ (aq) → Mg+2 (aq) + 2H2O (l) b. Solid magnesium carbonate reacts with a solution of hydrochloric acid.

Step 1: Balanced molecular equation

MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g)

Step 2: Rewrite to show ions

MgCO3 (s) + 2H+ (aq) + 2Cl- (aq) → Mg+2 (aq) + 2Cl- (aq) + H2O (l) + CO2 (g)

Step 3: Cancel out spectator ions

2Cl- (aq) is the spectator ion.

Step 4: Net ionic equation

MgCO3 (s) + 2H+ (aq) → Mg+2 (aq) + H2O (l) + CO2 (g) c. Ammonia gas reacts with hydrogen chloride gas.

Step 1: Balanced molecular equation

NH3 (g) + HCl (g) → NH4Cl (s)

Step 2: Rewrite to show ions

This reaction does not involve ions, as both reactants are in the gas phase.

Step 3: Cancel out spectator ions

Not applicable to this reaction.

Step 4: Net ionic equation

NH3 (g) + HCl (g) → NH4Cl (s) d. Gaseous sulfur trioxide is dissolved in water and reacts with a solution of sodium hydroxide.

Step 1: Balanced molecular equation

SO3 (g) + H2O (l) → H2SO4 (aq) --> H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + 2H2O (l)

Step 2: Rewrite to show ions

2H+ (aq) + SO4^2- (aq) + 2Na+ (aq) + 2OH- (aq) → 2Na+ (aq) + SO4^2- (aq) + 2H2O (l)

Step 3: Cancel out spectator ions

2Na+ (aq) and SO4^2- (aq) are the spectator ions.

Step 4: Net ionic equation

2H+ (aq) + 2OH- (aq) → 2H2O (l)

Key Concepts

Balanced Molecular Equation

A balanced molecular equation is the starting point for analyzing a chemical reaction. It shows all reactants and products in their molecular form and ensures that the number of atoms of each element is conserved. This means that both sides of the equation have the same number of each type of atom, reflecting the law of conservation of mass. For example, when solid magnesium hydroxide reacts with sulfuric acid, the equation is:

• Mg(OH) _{2} (s) + H _{2}SO _{4} (aq) → MgSO _{4} (aq) + 2H _{2}O (l)

This equation is balanced because there are equal numbers of each type of atom on both sides. Every balanced equation must meet this requirement to accurately depict the physical reality of the reaction.

Net Ionic Equation

A net ionic equation simplifies a chemical reaction by showing only the species that actually change during the process. This form highlights the essential chemical change, stripping away spectator ions that do not participate. For the reaction between magnesium hydroxide and sulfuric acid, the net ionic equation is:

• Mg(OH) _{2} (s) + 2H ^{+} (aq) → Mg ^{2+} (aq) + 2H _{2}O (l)

This equation is derived by removing ions that appear on both sides of the equation, simplifying the reaction to just the components that undergo change. It is particularly useful for understanding precipitation, acid-base, and redox reactions, as it clarifies which elements and compounds are truly involved.

Spectator Ions

Spectator ions are ions in a solution that do not participate in a chemical reaction. They remain unchanged and are not included in the net ionic equation. In the case of magnesium carbonate reacting with hydrochloric acid, chloride ions ( Cl^{-} ) are spectator ions since they appear on both sides of the reaction without taking part in the transformation:

• MgCO _{3} (s) + 2H ^{+} (aq) → Mg ^{2+} (aq) + H _{2}O (l) + CO _{2} (g)

By identifying and removing these ions, students can focus on the essential reaction components, aiding in the comprehension of how ions move and interact in aqueous solutions.

Acid-Base Reaction

An acid-base reaction involves the transfer of hydrogen ions ( H^{+} ) between reactants. This type of reaction can form a salt and water, as seen with sulfur trioxide and sodium hydroxide:

• SO _{3} (g) + H _{2}O (l) → H _{2}SO _{4} (aq)
• H _{2}SO _{4} (aq) + 2NaOH (aq) → Na _{2}SO _{4} (aq) + 2H _{2}O (l)

The resulting net ionic equation for the interaction in the solution highlights the formation of water from hydrogen and hydroxide ions:

• 2H ^{+} (aq) + 2OH ^{-} (aq) → 2H _{2}O (l)

This neutralization reaction is a classic example of an acid reacting with a base, showcasing the fundamental process of exchanging ions to balance charges and form new compounds. Understanding these reactions is vital for grasping the basics of chemistry and its applications in real-world chemical processes.