What type of chemical reaction is represented by the equation Pb(NO₃)2 (aq) + 2 KI (aq) → 2 KNO₃ (aq) + PbI₂ (s)?
1) synthesis reaction
2) decomposition reaction
3) single replacement reaction
4) double replacement reaction
The correct Answer and Explanation is:
The correct answer is:
4) Double replacement reaction.
The type of chemical reaction represented by the equation:
Pb(NO₃)₂ (aq) + 2 KI (aq) → 2 KNO₃ (aq) + PbI₂ (s)
is a double replacement reaction.
Explanation:
In a double replacement (or double displacement) reaction, two compounds exchange ions to form two new compounds. This type of reaction typically occurs in aqueous solutions, where the ions are free to move and interact with each other. The general form of a double replacement reaction is:
AB + CD → AD + CB
In the given reaction:
- Lead(II) nitrate (Pb(NO₃)₂) is an ionic compound, dissociating into lead (Pb²⁺) ions and nitrate (NO₃⁻) ions in an aqueous solution.
- Potassium iodide (KI) also dissociates in solution into potassium (K⁺) ions and iodide (I⁻) ions.
When these compounds react, the positively charged lead (Pb²⁺) ions switch places with the positively charged potassium (K⁺) ions. This leads to the formation of:
- Potassium nitrate (KNO₃) as a new ionic compound, which remains dissolved in the aqueous solution.
- Lead(II) iodide (PbI₂), a solid precipitate, because it is not soluble in water.
This reaction is a precipitation reaction as well, a subtype of double replacement reactions, where one of the products (PbI₂) forms a solid that settles out of the solution.
Breakdown of the Reaction:
- Reactants:
- Pb(NO₃)₂ (Lead(II) nitrate)
- KI (Potassium iodide)
- Products:
- KNO₃ (Potassium nitrate)
- PbI₂ (Lead(II) iodide) – solid precipitate
Why it’s a double replacement reaction:
In this reaction, the cations (Pb²⁺ and K⁺) and the anions (NO₃⁻ and I⁻) from the two compounds are exchanged, forming two new compounds. This exchange of ions, and the subsequent formation of a solid precipitate (PbI₂), classifies this reaction as a double replacement reaction.