Problem 12 - Introductory Chemistry Atoms First

Question

Aqueous solutions of lead nitrate and sodium sulfate combine to produce a precipitate. (a) What are the spectator ions? (b) Write the net ionic equation for this precipitation reaction.

Step-by-Step Solution

Answer

(a) The spectator ions are sodium (Na⁺) and nitrate (NO₃⁻). (b) The net ionic equation is: \(Pb^{2+}(aq) + SO_4^{2-}(aq) \rightarrow PbSO_4(s)\).

Step 1: Write the balanced chemical equation

Write the balanced chemical equation for the reaction between aqueous lead nitrate and sodium sulfate. The reactants are lead nitrate (Pb(NO₃)₂) and sodium sulfate (Na₂SO₄), and their products are lead sulfate (PbSO₄, the precipitate) and sodium nitrate (NaNO₃). The balanced chemical equation is: Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)

Step 2: Identify the spectator ions

The spectator ions are the ions that remain unchanged during the chemical reaction (i.e., the ones that do not participate in forming the product). In this reaction, sodium (Na⁺) and nitrate (NO₃⁻) ions are the spectator ions, as they remain in the solution and do not form the precipitate of lead sulfate.

Step 3: Write the complete ionic equation

Rewrite the chemical equation by listing all the ions present in the reaction (excluding the solid precipitate). The equation becomes: Pb²⁺(aq) + 2NO₃⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → PbSO₄(s) + 2Na⁺(aq) + 2NO₃⁻(aq)

Step 4: Write the net ionic equation

To write the net ionic equation, remove the spectator ions (Na⁺ and NO₃⁻) from both sides of the complete ionic equation. The net ionic equation is: Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s) This net ionic equation shows the formation of the precipitate (PbSO₄) from aqueous lead ions (Pb²⁺) and sulfate ions (SO₄²⁻).

Key Concepts

Spectator Ions

In a chemical reaction that occurs in an aqueous solution, not all ions participate in forming the end products. Some ions remain unchanged and float around in the solution. These ions are known as spectator ions.
For example, when lead nitrate and sodium sulfate solutions are mixed, you can find spectator ions. In this case, sodium ions \((Na^+)\) and nitrate ions \((NO_3^-)\) don't participate in forming the solid product, lead sulfate \((PbSO_4)\), and remain in the aqueous solution without being involved in any reaction.
To identify spectator ions, you first write down all ions involved in the solution and then spot the ones that appear identically on both sides of the chemical equation. These are the ions that don't participate in the actual chemical change, thereby being named spectator ions. Their primary role is maintaining electrical neutrality in the solution.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction in which two soluble salts in aqueous solution combine to form an insoluble product, known as a precipitate.
In these reactions, the precipitate appears as a solid that emerges from the liquid solution. This is due to the limited solubility of certain compounds in water. When solutions of lead nitrate \((Pb(NO_3)_2)\) and sodium sulfate \((Na_2SO_4)\) are mixed, lead sulfate \((PbSO_4)\) precipitates, leaving a solid that you can see and, often, remove from the solution.

• Precipitation reactions are useful in a variety of fields, including in detoxifying soluble metals or salts, and in qualitative analysis for detecting particular ions.
• The formation of a precipitate can be a clear indication that a chemical reaction has taken place.

Understanding precipitation reactions involves recognizing the solubility rules for ionic compounds and predicting which combinations will result in insoluble products.

Balanced Chemical Equations

A balanced chemical equation accurately represents the quantities of reactants and products involved in a chemical reaction.
Balancing an equation ensures that the number of atoms for each element is equal on both sides of the equation, upholding the Law of Conservation of Mass.
For the reaction between lead nitrate \((Pb(NO_3)_2)\) and sodium sulfate \((Na_2SO_4)\), the balanced equation is:
\[ Pb(NO_3)_2(aq) + Na_2SO_4(aq) \to PbSO_4(s) + 2NaNO_3(aq) \]
In this equation, each side contains the same number of each type of atom, meaning the reaction is perfectly balanced:

• 1 lead (Pb) atom
• 2 nitrate (NO₃⁻) ions
• 2 sodium (Na⁺) ions
• 1 sulfate (SO₄²⁻) ion

Balancing is a crucial component of writing chemical equations as it reflects the reality of the chemical process and is essential for accurate stoichiometric calculations.