Balancing equations — answer key

1. FQ11 · Practice

Model answer. There are 2 hydrogen and 2 chlorine atoms on the left, but only 1 of each in one HCl molecule. Two HCl molecules hold 2 hydrogen and 2 chlorine atoms: H₂ + Cl₂ → 2HCl.

Lowest whole-number coefficients: H₂(g) + Cl₂(g) → 2HCl(g)

A full answer includes

• H₂ + Cl₂ → 2HCl.

Also accept

• 1, 1, 2.

Watch for

• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.
• “Write H + Cl → HCl.” — Hydrogen and chlorine exist as molecules, H₂ and Cl₂. Keep the formulae and put a 2 in front of HCl.

2. FQ12 · Practice

Model answer. The atoms now balance, but H₂O₂ is hydrogen peroxide, not water. Changing a subscript changes the substance, so the equation describes a reaction that isn't happening. The formulae stay as they are, and the numbers in front change: 2H₂ + O₂ → 2H₂O.

A full answer includes

• H₂O₂ is a different substance, hydrogen peroxide; only the numbers in front of the formulae may change.

Also accept

• H₂O₂ is another substance; balance with numbers in front: 2H₂ + O₂ → 2H₂O.

Watch for

• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.
• “It is wrong because the atoms still don't balance.” — They do balance: 2 H and 2 O on each side. It is wrong because the product is no longer water.

3. FQ13 · Practice

Model answer. Zinc granules are a solid, (s). Dilute sulfuric acid is dissolved in water, (aq), and so is the zinc sulfate left in solution, (aq). The fizzing is hydrogen gas, (g): Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).

A full answer includes

• Zn(s), H₂SO₄(aq), ZnSO₄(aq), H₂(g).

Also accept

• s; aq; aq; g.

Watch for

• “An acid is (l) because it is a liquid.” — Dilute acid is dissolved in water, so it is (aq).
• “Water is (aq).” — Aqueous means dissolved in water; liquid water itself is (l).

4. FQ14 · Practice

Model answer. Chlorine: 2 atoms in each Cl₂ and 3 in each FeCl₃. Both go into 6, so take 3Cl₂ and 2FeCl₃. That makes 2 iron atoms on the right, so take 2Fe: 2Fe + 3Cl₂ → 2FeCl₃.

Lowest whole-number coefficients: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)

A full answer includes

• 2Fe + 3Cl₂ → 2FeCl₃.

Also accept

• 2, 3, 2.

Watch for

• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.
• “Any numbers that balance are fine.” — Use the smallest whole numbers: if every number can be halved, halve them.

5. FQ15 · Practice

Model answer. Hydrogen: 2 on the left (in H₂O) but 1 + 2 = 3 on the right. With 2H₂O on the left and 2NaOH on the right, hydrogen is 4 and 2 + 2 = 4, and oxygen is 2 and 2. Then 2Na balances sodium: 2Na + 2H₂O → 2NaOH + H₂.

Lowest whole-number coefficients: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

A full answer includes

• 2Na + 2H₂O → 2NaOH + H₂.

Also accept

• 2, 2, 2, 1.

Watch for

• “Some atoms are used up in the reaction, so the two sides needn't match.” — Atoms are not made or destroyed in a reaction, only rearranged, so each element has the same number of atoms on both sides.
• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.

6. FQ16 · Practice

Model answer. CaCl₂ has 2 chlorine atoms, so take 2HCl. Now hydrogen on the left is 2 (in the hydroxide) + 2 = 4, and oxygen is 2, so 2H₂O on the right: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.

Lowest whole-number coefficients: Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)

A full answer includes

• Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.

Also accept

• 1, 2, 1, 2.

Watch for

• “The number after a bracket multiplies only the last symbol.” — The number after a bracket multiplies everything inside the bracket.
• “Any numbers that balance are fine.” — Use the smallest whole numbers: if every number can be halved, halve them.

7. FQ17 · Practice

Model answer. (a) copper(II) oxide + sulfuric acid → copper(II) sulfate + water. (b) The formulae are CuO, H₂SO₄, CuSO₄ and H₂O, and every element already balances. The powder is a solid, the dilute acid and the copper(II) sulfate solution are aqueous, and water is a liquid: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).

A full answer includes

• (a) copper(II) oxide + sulfuric acid → copper(II) sulfate + water.
• (b) CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).

Also accept

• (a) Names of all four substances with + and →. (b) CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l).

Watch for

• “Water is (aq).” — Aqueous means dissolved in water; liquid water itself is (l).
• “Copper(II) oxide is CuO₂ because of the (II).” — The (II) is the charge of the copper ion, Cu²⁺. With O²⁻ the charges cancel one to one: CuO.

8. FQ18 · Practice

Model answer. Split the dissolved compounds into ions: Ba²⁺ + 2Cl⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄(s) + 2Na⁺ + 2Cl⁻. The sodium and chloride ions are dissolved on both sides, so they are spectators. What is left is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s), and the charge is 0 on both sides.

A full answer includes

• Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Also accept

• Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

Watch for

• “Keep every ion in the ionic equation.” — Ions that are the same on both sides are spectator ions; leave them out.
• “The precipitate is (aq).” — A precipitate is a solid that forms in a solution: (s).

9. FQ19 · Practice

Model answer. One Cu(OH)₂ holds one copper ion and two hydroxide ions, so take 2OH⁻. The charge on the left is +2 − 2 = 0, the same as the neutral solid on the right: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s).

Lowest whole-number coefficients: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

A full answer includes

• Cu²⁺ + 2OH⁻ → Cu(OH)₂.

Also accept

• 1, 2, 1.

Watch for

• “Only the atoms must balance in an ionic equation.” — The total charge must be the same on both sides too.
• “The number after a bracket multiplies only the last symbol.” — The number after a bracket multiplies everything inside the bracket.

10. FQ20 · Practice

Model answer. The information names one reactant, solid calcium carbonate, CaCO₃(s), and two products, solid calcium oxide, CaO(s), and carbon dioxide gas, CO₂(g). The atoms already balance: Ca 1, C 1 and O 3 on each side.

A full answer includes

• CaCO₃(s) → CaO(s) + CO₂(g).

Also accept

• CaCO₃(s) → CaO(s) + CO₂(g).

Watch for

• “Everything in a reaction is (aq).” — Use (aq) only for a substance dissolved in water; nothing here is dissolved.
• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.

11. FQ21 · Practice

Model answer. The three oxygen atoms in Fe₂O₃ make three molecules of steam, 3H₂O, which need 3H₂. The two iron atoms make 2Fe: Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g). Steam is water as a gas, so it is (g).

Lowest whole-number coefficients: Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g)

A full answer includes

• Fe₂O₃ + 3H₂ → 2Fe + 3H₂O.

Also accept

• 1, 3, 2, 3.

Watch for

• “Change a small number in a formula to make the atoms balance.” — Changing a subscript makes a different substance. Balance with numbers in front of the formulae.
• “Steam is H₂O(l).” — Steam is water as a gas, H₂O(g).

12. FT2 · Apply

Model answer. (a) iron(III) chloride + sodium hydroxide → iron(III) hydroxide + sodium chloride. (b) Fe(OH)₃ holds three hydroxide groups, so 3NaOH; then 3 sodium and 3 chlorine atoms make 3NaCl. The two solutions are (aq), the precipitate is (s), and the sodium chloride stays dissolved, (aq): FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq). (c) Split the dissolved compounds into ions: the Na⁺ and Cl⁻ ions are dissolved on both sides, so they are spectators. Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s): the charge is +3 − 3 = 0 on the left and 0 on the right.

A full answer includes

• (a) iron(III) chloride + sodium hydroxide → iron(III) hydroxide + sodium chloride.
• (b) FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq).
• (c) Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).

Also accept

• (a) Word equation. (b) FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq). (c) Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).

Watch for

• “Keep every ion in the ionic equation.” — Ions that are the same on both sides are spectator ions; leave them out.
• “The precipitate is (aq) because it is in the water.” — A precipitate is a solid that forms in a solution and does not dissolve: (s).
• “Only the atoms must balance in an ionic equation.” — The total charge must be the same on both sides too.