Unit 7 Chemical reactions
Key unit competence: Write balanced chemical equations to explain chemical reactions in accordance with the law of conservation of matter.
Key ideas
- A chemical reaction changes reactants into new substances called products. Signs: colour change, gas given off (bubbles), precipitate formed, temperature change, light or sound.
- The law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed, so the total mass of products equals the total mass of reactants.
- A word equation names the substances: magnesium + oxygen → magnesium oxide. A chemical equation uses formulae: 2Mg + O2 → 2MgO.
- To balance an equation, change only the coefficients (numbers in front), never the subscripts in formulae, until each element has the same number of atoms on both sides.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). Example: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g).
- Useful diatomic gases to remember: H2, O2, N2, Cl2. Water is H2O, carbon dioxide CO2, ammonia NH3.
Common misconceptions
- "You can balance an equation by changing a formula, e.g. H2O to H2O2." No: Changing a subscript makes a different substance; only change coefficients.
- "Mass is lost when a gas escapes in an open beaker." No: The mass is still there, in the gas that escaped; in a closed container the mass stays the same.
- "Oxygen in an equation is written as O." No: Oxygen gas is diatomic and is written O2.
Worked examples
Example 1. Balance the equation Mg + O2 → MgO and explain each step.
Solution
Count atoms: left 1 Mg, 2 O; right 1 Mg, 1 O.
Put 2 in front of MgO: right has 2 Mg, 2 O.
Put 2 in front of Mg: left has 2 Mg, 2 O.
Balanced: 2Mg + O2 → 2MgO.
Example 2. 6.4 g of copper reacts with oxygen to form 8.0 g of copper(II) oxide. Find the mass of oxygen used and name the law applied.
Solution
Mass of oxygen = mass of product − mass of copper
= 8.0 − 6.4 = 1.6 g.
Law: conservation of mass.
Practice questions
Name two signs that a chemical reaction has happened.
Show answer
Any two: gas given off, colour change, precipitate formed, temperature change, light produced.
Common mistake: Giving a change of state such as melting.
Write the word equation for burning carbon in oxygen.
Show answer
Carbon + oxygen → carbon dioxide.
Common mistake: Writing carbon monoxide without saying oxygen is limited.
State the law of conservation of mass.
Show answer
In a chemical reaction, the total mass of products equals the total mass of reactants.
Common mistake: Saying 'mass increases when gas forms'.
Balance H2 + Cl2 → HCl.
Show answer
H2 + Cl2 → 2HCl.
Common mistake: Writing H2 + Cl2 → H2Cl2.
Balance Zn + HCl → ZnCl2 + H2.
Show answer
Zn + 2HCl → ZnCl2 + H2.
Common mistake: Forgetting to double HCl.
How many atoms of each element are in Ca(OH)2?
Show answer
Ca 1, O 2, H 2.
Common mistake: Counting only one O and one H.
Balance Na + O2 → Na2O.
Show answer
4Na + O2 → 2Na2O.
Common mistake: Writing 2Na + O → Na2O.
Write the balanced equation with state symbols for magnesium reacting with dilute hydrochloric acid.
Show answer
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
Common mistake: Giving hydrogen the state symbol (aq).
Balance CH4 + O2 → CO2 + H2O.
Show answer
CH4 + 2O2 → CO2 + 2H2O.
Common mistake: Balancing O before H, which gives wrong numbers.
20 g of calcium carbonate is heated and forms 11.2 g of calcium oxide. Calculate the mass of carbon dioxide formed.
Show answer
20 − 11.2 = 8.8 g.
Common mistake: Adding the two masses.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. What does the state symbol (aq) mean? a. Dissolved in water 2. Which equation is balanced? b. 2Ca + O2 → 2CaO 3. What is the balanced equation for calcium reacting with oxygen? c. 2H2 + O2 → 2H2O 4. Balance _C3H8 + _O2 → _CO2 + _H2O. d. 1, 5, 3, 4 Show answer
1 → a (Dissolved in water) · 2 → c (2H2 + O2 → 2H2O) · 3 → b (2Ca + O2 → 2CaO) · 4 → d (1, 5, 3, 4)
- Aqueous means in water solution.
- Left: 4 H, 2 O; right: 4 H, 2 O.
- Two Ca atoms and two O atoms on each side.
- C: 3 CO2; H: 8 → 4 H2O; O: 6 + 4 = 10 → 5O2.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
2 and 3 · 4 and 3 · 2 and 2 · zinc sulfate and hydrogen- Balance _Al + _O2 → 2Al2O3. The coefficients of Al and O2 are .
- In the reaction zinc + sulfuric acid → zinc sulfate + hydrogen, the products are .
- Balance: _Na + Cl2 → _NaCl. The coefficients are .
Show answer
1: 4 and 3 · 2: zinc sulfate and hydrogen · 3: 2 and 2
- Right: 4 Al, 6 O. Left: 4Al and 3O2.
- Products are written on the right of the arrow.
- 2Na + Cl2 → 2NaCl: 2 Na and 2 Cl on each side.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. What is the coefficient needed in front of HCl to balance Mg + _HCl → MgCl2 + H2? a. 8 g 2. Which balanced equation shows iron reacting with chlorine to form iron(III) chloride? b. 2Fe + 3Cl2 → 2FeCl3 3. Which word equation matches C + O2 → CO2? c. Carbon + oxygen → carbon dioxide 4. 12 g of magnesium burns completely in oxygen to form 20 g of magnesium oxide. What mass of oxygen reacted? d. 2 Show answer
1 → d (2) · 2 → b (2Fe + 3Cl2 → 2FeCl3) · 3 → c (Carbon + oxygen → carbon dioxide) · 4 → a (8 g)
- Two Cl atoms are on the right, so 2HCl is needed on the left.
- Left: 2 Fe, 6 Cl; right: 2 Fe, 6 Cl.
- C is carbon and CO2 is carbon dioxide.
- By conservation of mass: 20 − 12 = 8 g.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
2 · 2, 2, 3 · 9 · 3- In CH4 + 2O2 → CO2 + 2H2O, how many atoms are on the reactant side in total? →
- How many oxygen atoms are in 2H2O? →
- Balance _KClO3 → _KCl + _O2. →
Show answer
1: 9 · 2: 2 · 3: 2, 2, 3
- CH4: 5 atoms; 2O2: 4 atoms; total 9.
- Each H2O has one O atom; 2 × 1 = 2.
- Step 1. O: 3 on left, 2 on right; LCM 6.
Step 2. 2KClO3 → 2KCl + 3O2.