Problem 38 - Chemistry and Chemical Reactivity

Question

Balance each of the following equations, and then write the net ionic equation. Show states for all reactants and products (s, \(\ell, \mathrm{g}, \text { aq })\). (a) the reaction of sodium hydroxide and iron(II) chloride to give iron(II) hydroxide and sodium chloride (b) the reaction of barium chloride with sodium carbonate to give barium carbonate and sodium chloride (c) the reaction of ammonia with phosphoric acid

Step-by-Step Solution

Answer

(a) Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s); (b) Ba²⁺(aq) + CO₃²⁻(aq) → BaCO₃(s); (c) NH₃ and H₃PO₄ form (NH₄)₃PO₄ in molecular form.

Step 1: Balance Equation (a)

The reaction between sodium hydroxide (NaOH) and iron(II) chloride (FeCl₂) forms iron(II) hydroxide (Fe(OH)₂) and sodium chloride (NaCl). Begin by writing the unbalanced equation with states:\[\text{NaOH} (\text{aq}) + \text{FeCl}_2 (\text{aq}) \rightarrow \text{Fe(OH)}_2 (\text{s}) + \text{NaCl} (\text{aq})\]To balance the equation, ensure each type of atom appears the same number of times on both sides:\[2\text{NaOH} (\text{aq}) + \text{FeCl}_2 (\text{aq}) \rightarrow \text{Fe(OH)}_2 (\text{s}) + 2\text{NaCl} (\text{aq})\]This balances sodium (Na), chlorine (Cl), and hydroxide ions (OH).

Step 2: Net Ionic Equation (a)

Identify the strong electrolytes and break them into ions. Cancel out the spectator ions to find the net ionic equation:\[\text{2Na}^+ (\text{aq}) + \text{2OH}^- (\text{aq}) + \text{Fe}^{2+} (\text{aq}) + \text{2Cl}^- (\text{aq}) \rightarrow \text{Fe(OH)}_2 (\text{s}) + \text{2Na}^+ (\text{aq}) + \text{2Cl}^- (\text{aq})\]Remove the spectator ions \(\text{2Na}^+\) and \(\text{2Cl}^-\):\[\text{Fe}^{2+} (\text{aq}) + \text{2OH}^- (\text{aq}) \rightarrow \text{Fe(OH)}_2 (\text{s})\]

Step 3: Balance Equation (b)

Barium chloride (BaCl₂) reacts with sodium carbonate (Na₂CO₃) to form barium carbonate (BaCO₃) and sodium chloride (NaCl). Write the unbalanced equation:\[\text{BaCl}_2 (\text{aq}) + \text{Na}_2\text{CO}_3 (\text{aq}) \rightarrow \text{BaCO}_3 (\text{s}) + \text{2NaCl} (\text{aq})\]Balance the equation:\[\text{BaCl}_2 (\text{aq}) + \text{Na}_2\text{CO}_3 (\text{aq}) \rightarrow \text{BaCO}_3 (\text{s}) + 2\text{NaCl} (\text{aq})\]

Step 4: Net Ionic Equation (b)

Separate the strong electrolytes into ions and eliminate spectator ions:\[\text{Ba}^{2+} (\text{aq}) + \text{2Cl}^- (\text{aq}) + \text{2Na}^+ (\text{aq}) + \text{CO}_3^{2-} (\text{aq}) \rightarrow \text{BaCO}_3 (\text{s}) + \text{2Na}^+ (\text{aq}) + \text{2Cl}^- (\text{aq})\]Remove spectator ions \(\text{2Na}^+\) and \(\text{2Cl}^-\):\[\text{Ba}^{2+} (\text{aq}) + \text{CO}_3^{2-} (\text{aq}) \rightarrow \text{BaCO}_3 (\text{s})\]

Step 5: Balance Equation (c)

The reaction between ammonia (NH₃) and phosphoric acid (H₃PO₄) forms ammonium phosphate. Write the unbalanced equation:\[\text{3NH}_3 (\text{aq}) + \text{H}_3\text{PO}_4 (\text{aq}) \rightarrow \text{(NH}_4)_3\text{PO}_4 (\text{aq})\]The equation is written as balanced.

Step 6: Net Ionic Equation (c)

Since ammonia reacts with phosphoric acid to form ammonium phosphate, the net ionic equation shows complete molecular forms as ammonia and phosphoric acid act as weak electrolytes.\[\text{3NH}_3 (\text{aq}) + \text{H}_3\text{PO}_4 (\text{aq}) \rightarrow \text{(NH}_4)_3\text{PO}_4 (\text{aq})\]

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is crucial for understanding chemical reactions. It ensures that all atoms present in the reactants are accounted for in the products. This concept is based on the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. To balance a chemical equation:

• Write the unbalanced equation with proper chemical formulas and states of matter.
• Identify the number of each type of atom present in both reactants and products.
• Adjust coefficients (the numbers before the chemical formulas) to ensure the same number of each atom appears on both sides of the equation.
• Check your work to confirm that the same number of each type of atom exists on both reactant and product sides.

A balanced equation not only complies with the conservation of mass but also provides a clearer understanding of the reaction ratios needed for quantitative chemistry calculations. Remember not to alter subscripts within chemical formulas, as this changes the substances involved.

Net Ionic Equations

Net ionic equations simplify chemical reactions by removing spectator ions - ions that do not participate directly in the reaction. This process emphasizes the actual chemical change occurring. Here is how you can write a net ionic equation:

• Write the balanced molecular equation, indicating the phase of each component.
• Separate the aqueous strong electrolytes into their constituent ions.
• Identify and eliminate spectator ions, which are ions that appear unchanged on both sides of the equation.
• Write the net ionic equation with only the species that alter during the reaction.

Net ionic equations are particularly useful for reactions in aqueous solutions, as they clarify the specific changes from reactants to products. They help focus on the important chemical transformations without the distraction of non-participating ions, making the study of reactions more straightforward.

Strong and Weak Electrolytes

Electrolytes are substances that dissolve in water to produce ions, conducting electricity. They differ based on the degree of ionization in solution into strong and weak electrolytes. **Strong Electrolytes**

• Completely dissociate into ions in solution.
• Include most salts, strong acids (like HCl and HNO₃), and strong bases (such as NaOH and KOH).
• Conduct electricity well due to the high concentration of ions.

**Weak Electrolytes**

• Partially dissociate in solution, producing fewer ions.
• Include weak acids (like acetic acid) and weak bases (such as ammonia).
• Conduct electricity poorly in comparison to strong electrolytes.

Understanding whether a substance is a strong or weak electrolyte helps predict its behavior in solutions, including its ability to conduct electricity and its role in net ionic equations. Strong electrolytes readily participate in chemical reactions, while weak electrolytes often remain mostly in their molecular form.