Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
31 questions
Unit 15 · Chemical Reaction
2. Write the differences between
Balanced and unbalanced chemical equations
Answer
| Balanced chemical equation | Unbalanced chemical equation |
|---|---|
| has the same number of atoms of each element on both sides | the number of atoms of at least one element differs between the two sides |
| obeys the law of conservation of mass | doesn’t show that mass is conserved |
| gives the ratio of molecules and of masses that react | can’t be used to find how much reacts |
| e.g. 2H₂ + O₂ → 2H₂O | e.g. H₂ + O₂ → H₂O |
Endothermic and exothermic reactions
Answer
| Endothermic reaction | Exothermic reaction |
|---|---|
| absorbs (takes in) heat | releases (gives out) heat |
| heat must usually be supplied to keep it going | once started, it often keeps itself going |
| the surroundings may get colder | the surroundings get hotter |
| e.g. CaCO₃ → CaO + CO₂ (heated in a kiln) | e.g. C + O₂ → CO₂ + heat |
3. Answer the following questions
How is a chemical change different from a physical change?
Answer
| Chemical change | Physical change |
|---|---|
| new substances with new properties are formed | no new substance is formed; only the state, shape or size changes |
| the composition of the substance changes | the composition stays the same |
| usually can’t be reversed easily | usually can be reversed easily |
| heat or light is usually given out or taken in | little energy change, except in changes of state |
| e.g. burning wood, rusting, curd from milk | e.g. melting ice, dissolving sugar, breaking glass |
What is a chemical equation? Explain with an example.
Answer
A chemical equation is a way of writing a chemical reaction in words or in symbols and formulae, with the reactants on the left of an arrow and the products on the right. For example, when zinc is put in dilute hydrochloric acid, zinc chloride and hydrogen gas are formed:
Word equation: zinc + hydrochloric acid → zinc chloride + hydrogen
Formula equation (balanced): Zn + 2HCl → ZnCl₂ + H₂
Here zinc and hydrochloric acid are the reactants, and zinc chloride and hydrogen are the products.
“Our daily life would be nearly impossible without chemical reactions.” Explain this statement, with the importance of chemical reactions.
Answer
Chemical reactions keep us alive and run almost everything we do:
- Respiration in our cells releases energy from food (glucose + oxygen → carbon dioxide + water + energy).
- Digestion breaks food into small molecules that the body can absorb.
- Photosynthesis in plants makes all our food, and the oxygen we breathe.
- Medicines cure diseases by reacting in the body; an antacid neutralizes extra stomach acid.
- Burning fuels (wood, LPG, kerosene, biogas) gives heat for cooking and warmth, and runs vehicles.
- Food: cooking, curd from milk, bread rising.
- Industry: cement, steel, soap, fertilizers and plastics are all made by chemical reactions.
Without these reactions we would have no energy, no food, no medicines and none of the materials we use, so daily life would be nearly impossible.
Someone is cooking food in an oven fueled by firewood. Which reactions are exothermic and which are endothermic?
Answer
The burning of the firewood is exothermic: the wood reacts with oxygen in the air and gives out heat and light (for its carbon, C + O₂ → CO₂ + heat).
The cooking of the food is endothermic: the reactions that soften rice and vegetables and cook meat take in the heat from the fire, and they stop when the fire goes out.
4. Change these word equations into balanced formula equations
Nitrogen + hydrogen → ammonia
Answer
N₂ + 3H₂ → 2NH₃
Sodium + oxygen → sodium oxide
Answer
4Na + O₂ → 2Na₂O
Magnesium + nitrogen → magnesium nitride
Answer
3Mg + N₂ → Mg₃N₂ (Mg has valency 2 and N valency 3, so magnesium nitride is Mg₃N₂.)
Calcium carbonate → carbon dioxide + calcium oxide
Answer
CaCO₃ → CO₂ + CaO (already balanced; it needs heat)
Zinc + hydrochloric acid → zinc chloride + hydrogen
Answer
Zn + 2HCl → ZnCl₂ + H₂
Aluminium + nitrogen → aluminium nitride
Answer
2Al + N₂ → 2AlN
Potassium + chlorine → potassium chloride
Answer
2K + Cl₂ → 2KCl
Hydrogen + chlorine → hydrogen chloride
Answer
H₂ + Cl₂ → 2HCl
5. Balance these equations
Na + O₂ → Na₂O
Answer
4Na + O₂ → 2Na₂O (Na 4 = 4; O 2 = 2)
Hg + O₂ → HgO
Answer
2Hg + O₂ → 2HgO
Ca + N₂ → Ca₃N₂
Answer
3Ca + N₂ → Ca₃N₂
Fe + O₂ → Fe₂O₃
Answer
4Fe + 3O₂ → 2Fe₂O₃ (Fe 4 = 4; O 6 = 6)
Fe + CuSO₄ → FeSO₄ + Cu
Answer
Fe + CuSO₄ → FeSO₄ + Cu. It is already balanced: Fe 1 = 1, Cu 1 = 1, S 1 = 1, O 4 = 4.
HCl + NaOH → NaCl + H₂O
Answer
HCl + NaOH → NaCl + H₂O. Already balanced: H 2 = 2, Cl 1 = 1, Na 1 = 1, O 1 = 1.
HCl + KOH → KCl + H₂O
Answer
HCl + KOH → KCl + H₂O. Already balanced: H 2 = 2, Cl 1 = 1, K 1 = 1, O 1 = 1.
HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
Answer
2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
Check, counting NO₃ as one unit: NO₃ 2 = 2; Ca 1 = 1; H 2 + 2 = 4 on the left, 2 × 2 = 4 on the right; O (outside NO₃) 2 = 2.
6. Complete these equations and balance them
Fe + HCl → …… + H₂
Answer
Fe + 2HCl → FeCl₂ + H₂ (iron forms ferrous chloride with dilute hydrochloric acid)
HCl + …… → NaCl + H₂O
Answer
HCl + NaOH → NaCl + H₂O
H₂SO₄ + NaOH → …… + H₂O
Answer
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Na + …… → Na₂O
Answer
4Na + O₂ → 2Na₂O
Ca + O₂ → ……
Answer
2Ca + O₂ → 2CaO
…… + O₂ → HgO
Answer
2Hg + O₂ → 2HgO
Fe + …… → Fe₂O₃
Answer
4Fe + 3O₂ → 2Fe₂O₃
…… + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
Answer
2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
Al + N₂ → ……
Answer
2Al + N₂ → 2AlN