Unit 4 practice test: Chemical Reactions

Each HCl molecule gives its H⁺ to a water molecule. The diagram shows only H₃O⁺ and Cl⁻ afterward, with no HCl molecules left.

• Correct: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq): Right: a proton moves from HCl to water.
• HCl(g) → H(g) + Cl(g): The diagram shows ions, not neutral H and Cl atoms.
• HCl(g) + H₂O(l) → H₂(g) + HOCl(aq): No hydrogen gas forms; the H ends up in hydronium.
• HCl(g) → HCl(aq), with no other change: If HCl simply dissolved, the right box would show HCl molecules, but it shows none.

Water takes the H⁺ and becomes H₃O⁺, so water is the Brønsted-Lowry base.

• Correct: H₂O: Right: water accepts the proton.
• HCl: HCl gives the proton away, so it is the acid.
• Cl⁻: Cl⁻ is a product, the conjugate base of HCl.
• H₃O⁺: H₃O⁺ is a product, the conjugate acid of water.

Pure HCl is made of neutral molecules, so nothing carries charge. In water, the proton transfer makes H₃O⁺ and Cl⁻ ions that move through the solution and conduct.

• Correct: In water, HCl gives its proton away, forming ions: Right: the reaction with water creates mobile ions.
• Water itself conducts electricity well, whatever is dissolved: Pure water conducts very poorly; the ions from HCl are what make the solution conduct.
• Liquid HCl is made of ions that are held in place: Pure HCl is molecular, not ionic, which is why it does not conduct.
• Dissolving breaks HCl into neutral atoms that carry charge: Neutral atoms carry no charge; the diagram shows ions.

K⁺ and NO₃⁻ are spectators (their compounds are soluble). The yellow solid must be lead(II) iodide. Pb²⁺ needs two I⁻ to make a neutral solid, and the charges balance: +2 + 2(−1) = 0 on the left, 0 on the right.

• Correct: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s): Right: atoms and charges both balance, and only the changing species appear.
• Pb²⁺(aq) + I⁻(aq) → PbI(s): PbI would need lead to be +1. With Pb²⁺, the neutral solid is PbI₂, and the left side carries a +1 charge that the right side lacks.
• K⁺(aq) + NO₃⁻(aq) → KNO₃(s): Potassium nitrate is soluble, so its ions stay dissolved; they are the spectators.
• Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq): This is the molecular equation, which includes the spectator ions in its formulas.

Mixing the solutions only gives a solution of four kinds of ions, K⁺, Cl⁻, Na⁺ and NO₃⁻, all spectators. Nothing changes, so there is no net ionic equation (no reaction).

• Correct: There is no reaction, since every ion stays dissolved: Right: all four ions are spectators.
• The net ionic equation is K⁺(aq) + NO₃⁻(aq) → KNO₃(aq): KNO₃(aq) is just K⁺ and NO₃⁻ dissolved, the same as before; nothing has changed.
• A reaction occurs, but its product is a colorless solid: Every possible combination is soluble according to the data given, so no solid can form.
• The solutions were too dilute for their ions to meet: At 0.1 M the ions meet constantly; wells 1, 2 and 4 react at the same concentration.

Sodium compounds are soluble, so Na⁺ and SO₄²⁻ stay in solution. The blue solid is copper(II) hydroxide; Cu²⁺ needs two OH⁻.

• Correct: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s): Right: atoms and charge balance and only the changing ions appear.
• Cu²⁺(aq) + OH⁻(aq) → CuOH(s): With Cu²⁺, one OH⁻ leaves a +1 charge; the neutral solid needs two hydroxide ions.
• Na⁺(aq) + SO₄²⁻(aq) → NaSO₄(s): Sodium sulfate is soluble (all sodium compounds are), and its formula would be Na₂SO₄ in any case.
• CuSO₄(s) + 2NaOH(s) → Cu(OH)₂(s) + Na₂SO₄(s): The reactants are dissolved, not solids; writing (s) for them misdescribes the solutions in the wells.