Magnesium reacts with the oxygen of the air to form a thin layer of magnesium oxide (MgO) on its surface. This oxide layer is unreactive and prevents the ribbon from burning properly. Therefore the ribbon is cleaned with sandpaper to remove the oxide layer, so that the clean magnesium can burn easily in air with a bright white flame.
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂
(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Iron is more reactive than copper, so it displaces copper from copper sulphate solution: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). The blue colour of the copper sulphate solution fades and turns pale green because the blue Cu²⁺ ions are replaced by light-green Fe²⁺ ions, and a brownish layer of copper is deposited on the iron nail. This is a displacement reaction.
Sodium sulphate reacts with barium chloride to give a white precipitate of barium sulphate: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Here the two compounds exchange their ions, so it is a double displacement reaction.
(i) Sodium (Na) is oxidised, because it gains oxygen to form sodium oxide (Na₂O). Oxygen is reduced.
(ii) Copper oxide (CuO) is reduced, because it loses oxygen to form copper (Cu). Hydrogen (H₂) is oxidised, because it gains oxygen to form water (H₂O).
- i. (a) and (b)
- ii. (a) and (c)
- iii. (a), (b) and (c)
- iv. all
In this reaction lead oxide (PbO) loses oxygen, so it is reduced to lead; carbon gains oxygen, so it is oxidised to carbon dioxide. Hence statements (c) and (d) are correct. Statement (a) is incorrect because it is lead oxide — not lead — that is getting reduced. Statement (b) is incorrect because carbon dioxide is the product of oxidation; it is carbon that is getting oxidised. So the incorrect statements are (a) and (b).
(i) (a) and (b)
- a. combination reaction.
- b. double displacement reaction.
- c. decomposition reaction.
- d. displacement reaction.
Aluminium is more reactive than iron, so it displaces iron from iron(III) oxide. This is the thermite reaction — a displacement (and redox) reaction.
(d) displacement reaction.
- a. Hydrogen gas and iron chloride are produced.
- b. Chlorine gas and iron hydroxide are produced.
- c. No reaction takes place.
- d. Iron salt and water are produced.
Dilute hydrochloric acid reacts with iron to form iron(II) chloride and hydrogen gas: Fe + 2HCl → FeCl₂ + H₂.
(a) Hydrogen gas and iron chloride are produced.
A balanced chemical equation is one in which the number of atoms of each element is equal on both the reactant side and the product side. Chemical equations should be balanced to obey the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. Since atoms are neither created nor destroyed, the total mass of the reactants must equal the total mass of the products, which means the number of atoms of each element must be the same on both sides.
(a) 3H₂ + N₂ → 2NH₃
(b) 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄
(d) 2K + 2H₂O → 2KOH + H₂
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl
(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double displacement reaction.
(b) ZnCO₃(s) → ZnO(s) + CO₂(g) — Decomposition reaction.
(c) H₂(g) + Cl₂(g) → 2HCl(g) — Combination reaction.
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement reaction.
Exothermic reactions are reactions in which heat (energy) is released along with the products. Examples: burning of natural gas, CH₄ + 2O₂ → CO₂ + 2H₂O + heat; and respiration. Endothermic reactions are reactions in which heat (energy) is absorbed from the surroundings. Example: the decomposition of calcium carbonate on heating, CaCO₃ → CaO + CO₂, which requires a continuous supply of heat.
During respiration, glucose combines with oxygen in our cells and is broken down to form carbon dioxide and water, releasing energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Because energy (heat) is released during this process, respiration is considered an exothermic reaction. This released energy is used by the body to carry out its life processes.
In a combination reaction, two or more substances combine to form a single new substance, whereas in a decomposition reaction a single substance breaks down into two or more simpler substances. Since decomposition is the exact reverse of combination, decomposition reactions are called the opposite of combination reactions.
Combination: CaO + H₂O → Ca(OH)₂
Decomposition: CaCO₃ → CaO + CO₂ (on heating)
Heat (thermal decomposition): CaCO₃(s) → CaO(s) + CO₂(g)
Light (photolytic decomposition): 2AgCl(s) → 2Ag(s) + Cl₂(g)
Electricity (electrolytic decomposition): 2H₂O(l) → 2H₂(g) + O₂(g)
In a displacement reaction, a more reactive element displaces a less reactive element from its compound (an element and a compound react), e.g. Fe + CuSO₄ → FeSO₄ + Cu. In a double displacement reaction, two compounds react and exchange their ions (radicals) to form two new compounds, e.g. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. So displacement involves one element replacing another, while double displacement involves an exchange of ions between two compounds.
Copper is more reactive than silver, so it displaces silver from silver nitrate solution: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s). This is a displacement reaction.
A precipitation reaction is a reaction in which two solutions react to form an insoluble solid called a precipitate. For example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq), in which insoluble white barium sulphate is formed as a precipitate. Another example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), where white silver chloride is precipitated.
(a) Oxidation is the gain of oxygen (or loss of hydrogen) by a substance. Examples: 2Cu + O₂ → 2CuO (copper gains oxygen); 2Mg + O₂ → 2MgO (magnesium gains oxygen).
(b) Reduction is the loss of oxygen (or gain of hydrogen) by a substance. Examples: CuO + H₂ → Cu + H₂O (copper oxide loses oxygen); ZnO + C → Zn + CO (zinc oxide loses oxygen).
The shiny brown coloured element 'X' is copper (Cu). On heating in air, copper reacts with oxygen to form a black coloured compound, copper(II) oxide (CuO): 2Cu + O₂ → 2CuO.
Iron articles are painted to prevent them from rusting (corrosion). Rusting takes place when iron is exposed to moisture (water) and oxygen of the air. A coat of paint forms a protective layer on the surface that prevents air and moisture from coming into contact with the iron, and so prevents it from rusting.
Oil and fat containing food items are flushed with nitrogen, which is an unreactive (inert) gas, to prevent them from being oxidised. Oxidation of oils and fats makes the food rancid, giving it a bad smell and taste. Flushing the packet with nitrogen removes oxygen and provides an inert atmosphere, which prevents oxidation and keeps the food fresh for a longer time.
(a) Corrosion is the slow eating up of the surface of a metal by the action of air, moisture or chemicals around it. Example: the rusting of iron, in which iron forms a brown flaky layer of hydrated iron(III) oxide. (Other examples: the black coating on silver and the green coating on copper.)
(b) Rancidity is the oxidation of oils and fats present in food, which produces an unpleasant smell and taste. Example: the change in smell and taste of fried or oily food (such as chips) when it is left exposed to air for a long time.