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Questions on the Period 3 elements: reactions with oxygen and water

The reactions of sodium and magnesium with water and of magnesium with steam, with equations, observations and the pH of the resulting solution; the reactions of sodium, magnesium, aluminium, silicon, phosphorus and sulfur with oxygen and what is seen; making sulfur trioxide; the oxidation states and formulae of Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂ and SO₃; and, for Cambridge, the reactions of sodium to phosphorus with chlorine and the formulae, oxidation states and bonding of NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅.

19 original questions · 65 marks · the Period 3 elements: reactions with oxygen and water notes · Periodicity and Period 3 · Flashcards for this unit

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• 6 marks · Describe · PER-Q22 AQA · CAIE

  Describe the reactions of sodium, magnesium, aluminium, silicon, phosphorus and sulfur with oxygen. Explain why they differ in how readily they start, and how the formula of each oxide follows from the element's outer electrons.

  Mark scheme

  Indicative content

  1. sodium and magnesium burn when heated, sodium with a yellow flame and magnesium with a bright white flame, each giving a white solid oxide
  2. aluminium sheet does not burn because its oxide layer keeps oxygen off the metal, but aluminium powder burns with a bright white flame to give white Al₂O₃
  3. silicon reacts only on strong heating, and slowly, because its giant covalent lattice must be broken into, giving white solid SiO₂
  4. white phosphorus catches fire in air at about 30 °C because the 60° bond angles in P₄ leave its bonds strained, and it gives white smoke of P₄O₁₀
  5. sulfur burns with a blue flame to give sulfur dioxide, a colourless gas with a choking smell, and gives no SO₃ without a catalyst
  6. the oxidation state in Na₂O, MgO, Al₂O₃, SiO₂ and P₄O₁₀ rises from +1 to +5, equal to the number of outer electrons, because all of them are used in bonding

  Levels

  • Level 3 (5–6 marks) all six elements are covered with their observations and products, the differences in how readily they start are explained, aluminium through its oxide layer, silicon through its lattice and phosphorus through bond strain, and the oxidation states of the oxides are linked to the number of outer electrons
  • Level 2 (3–4 marks) four or five elements are described with correct observations and products and the oxidation-state pattern stated; one reason for reactivity, or the link to outer electrons, is not given
  • Level 1 (1–2 marks) two or three reactions are described; burning sulfur is often said to give SO₃, and the formulae are not tied to the outer electrons

• 5 marks · Describe · PER-Q1 AQA · CAIE

  Describe what is seen when a small piece of sodium is added to water containing universal indicator. Give an equation with state symbols for the reaction, and the pH of the solution left.

  Mark scheme

  1. B1 the sodium floats, melts into a ball and moves around on the surface
  2. B1 there is fizzing, or effervescence; ignore 'hydrogen is given off', which is not an observation
  3. B1 the sodium gets smaller and disappears
  4. B1 the indicator turns purple, and the solution has a pH of 13 to 14; reject 'alkaline' with no value
  5. B1 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g), with every state symbol correct

• 4 marks · Describe · PER-Q6 AQA · CAIE

  Describe what is seen when sulfur is burned in oxygen and when white phosphorus is exposed to air at about 30 °C, naming the product each time.

  Mark scheme

  1. B1 sulfur melts and burns with a blue flame
  2. B1 a colourless gas with a choking smell forms, which is sulfur dioxide, SO₂
  3. B1 white phosphorus catches fire without being lit and burns with a white flame
  4. B1 dense white smoke forms, which is phosphorus(V) oxide, P₄O₁₀; allow P₂O₅

• 4 marks · Deduce · PER-Q12 AQA · CAIE

  Deduce the oxidation state of the Period 3 element in Na₂O, P₄O₁₀, SO₂ and SO₃. Explain why SO₂ can be oxidised further but P₄O₁₀ cannot.

  Mark scheme

  1. B1 sodium is +1 in Na₂O and phosphorus is +5 in P₄O₁₀
  2. B1 sulfur is +4 in SO₂ and +6 in SO₃
  3. B1 phosphorus has five outer electrons and uses all five in P₄O₁₀, so +5 is its highest oxidation state
  4. B1 sulfur has six outer electrons but uses only four in SO₂, so it can be oxidised further, to +6 in SO₃

• 4 marks · Outline · PER-Q18 AQA · CAIE

  Outline how magnesium can be reacted with steam in a school laboratory, including one safety precaution.

  Mark scheme

  1. B1 mineral wool soaked in water is placed at the closed end of a hard-glass boiling tube, with magnesium ribbon in the middle
  2. B1 the magnesium is heated strongly, with the flame moved to the wool at intervals to drive steam over it
  3. B1 the hydrogen is burned at a jet, or collected over water
  4. B1 if the gas is collected over water, the delivery tube is removed from the water before heating stops, to prevent suck-back cracking the hot tube

• 3 marks · Explain · PER-Q3 AQA · CAIE

  Explain why the solution formed when magnesium reacts with cold water has a pH of about 10, whereas the solution formed from sodium has a pH of 13 to 14.

  Mark scheme

  1. B1 magnesium forms magnesium hydroxide and sodium forms sodium hydroxide
  2. B1 magnesium hydroxide is only sparingly soluble, so only a small concentration of hydroxide ions reaches the solution
  3. B1 sodium hydroxide is very soluble, so the concentration of hydroxide ions is much higher and the pH is higher

• 3 marks · Explain · PER-Q4 AQA · CAIE

  Explain why magnesium reacts much faster with steam than with cold water, and why the magnesium-containing product is different.

  Mark scheme

  1. B1 with steam the reactants are at a much higher temperature, so a greater proportion of collisions have energy greater than the activation energy
  2. B1 with cold water the product is magnesium hydroxide, Mg(OH)₂, whereas with steam it is magnesium oxide, MgO
  3. B1 magnesium hydroxide is not stable at the temperature of the steam reaction and decomposes to magnesium oxide and water

• 3 marks · Describe · PER-Q5 AQA · CAIE

  Describe what is seen when steam is passed over heated magnesium, and give an equation, with state symbols, for the reaction.

  Mark scheme

  1. B1 the magnesium glows or burns with a bright white light
  2. B1 a white solid is left in the tube and a gas is given off that burns at the jet
  3. B1 Mg(s) + H₂O(g) → MgO(s) + H₂(g); reject H₂O(l)

• 3 marks · Give · PER-Q7 AQA · CAIE

  Give equations, with state symbols, for the reactions of aluminium, silicon and phosphorus with excess oxygen.

  Mark scheme

  1. B1 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
  2. B1 Si(s) + O₂(g) → SiO₂(s)
  3. B1 P₄(s) + 5O₂(g) → P₄O₁₀(s); allow 4P(s) + 5O₂(g) → P₄O₁₀(s) and multiples

• 3 marks · Calculate · PER-Q9 AQA · CAIE

  Calculate the volume of hydrogen, at room temperature and pressure, produced when 0.345 g of sodium reacts completely with water. Molar gas volume is 24.0 dm³ mol⁻¹.

  Mark scheme

  1. M1 amount of Na = 0.345 ÷ 23.0 = 0.0150 mol
  2. M1 amount of H₂ = 0.0150 ÷ 2 = 0.00750 mol, from the 2 : 1 ratio in the equation
  3. A1 volume of H₂ = 0.00750 × 24.0 = 0.180 dm³, to three significant figures; allow 180 cm³

• 3 marks · Calculate · PER-Q10 AQA · CAIE

  Calculate the mass of phosphorus(V) oxide, P₄O₁₀, formed when 1.24 g of white phosphorus burns in excess oxygen.

  Mark scheme

  1. M1 amount of P = 1.24 ÷ 31.0 = 0.0400 mol, so amount of P₄O₁₀ = 0.0400 ÷ 4 = 0.0100 mol
  2. M1 Mr of P₄O₁₀ = 4(31.0) + 10(16.0) = 284.0
  3. A1 mass of P₄O₁₀ = 0.0100 × 284.0 = 2.84 g, to three significant figures

• 3 marks · Deduce · PER-Q11 AQA · CAIE

  Deduce the empirical formula of the oxide formed when 0.540 g of aluminium powder burns in excess oxygen to give 1.020 g of oxide.

  Mark scheme

  1. M1 mass of oxygen combined = 1.020 − 0.540 = 0.480 g
  2. M1 amount of Al = 0.540 ÷ 27.0 = 0.0200 mol and amount of O = 0.480 ÷ 16.0 = 0.0300 mol
  3. A1 the ratio Al : O is 2 : 3, so the empirical formula is Al₂O₃; error carried forward from an incorrect mass of oxygen

• 3 marks · Suggest · PER-Q14 AQA · CAIE

  Suggest why a sheet of aluminium does not burn when held in a Bunsen flame, but aluminium powder sprinkled into the flame burns brightly. Give the equation, with state symbols, for the reaction of the powder.

  Mark scheme

  1. B1 the sheet is covered by a thin layer of aluminium oxide, which keeps oxygen away from the metal beneath
  2. B1 the powder has a much larger surface area, so far more aluminium is in contact with oxygen and it heats up quickly enough to ignite
  3. B1 4Al(s) + 3O₂(g) → 2Al₂O₃(s)

• 3 marks · Suggest · PER-Q15 AQA · CAIE

  Suggest why sodium is stored under oil but white phosphorus is stored under water.

  Mark scheme

  1. B1 sodium reacts with both water and oxygen, so it is kept under oil, which excludes both
  2. B1 white phosphorus catches fire in air at about 30 °C, so it must be kept away from oxygen
  3. B1 phosphorus does not react with water, so water excludes air and keeps it cool without reacting with it

• 3 marks · Compare · PER-Q16 AQA · CAIE

  Compare the reactions of sodium and magnesium with cold water.

  Mark scheme

  1. B1 sodium reacts vigorously, whereas magnesium reacts very slowly, giving only a few bubbles over several days
  2. B1 both produce hydrogen; sodium gives sodium hydroxide in solution, magnesium gives sparingly soluble magnesium hydroxide
  3. B1 the sodium solution has a pH of 13 to 14, the magnesium solution a pH of about 10

• 3 marks · Explain · PER-Q17 AQA · CAIE

  Explain why white phosphorus catches fire in air at about 30 °C, while red phosphorus can be kept in air on the side of a matchbox.

  Mark scheme

  1. B1 white phosphorus is P₄, four atoms at the corners of a tetrahedron, so each P–P–P bond angle is 60°
  2. B1 the bonds are strained and weak, so little energy is needed to start breaking them and the activation energy for burning is low
  3. B1 in red phosphorus the tetrahedra are opened out and joined into chains, so the bonds are not strained and a flame is needed to ignite it

• 3 marks · Explain · PER-Q19 CAIE

  Explain the change in the oxidation state of the Period 3 element across the chlorides NaCl, MgCl₂, AlCl₃, SiCl₄ and PCl₅, formed when sodium to phosphorus react with excess chlorine.

  Mark scheme

  1. B1 chlorine is −1 in each chloride, so the oxidation state of the Period 3 element rises from +1 in NaCl to +5 in PCl₅
  2. B1 the oxidation state rises by one at each step, matching the number of outer electrons, from one in sodium to five in phosphorus
  3. B1 each element uses all of its outer electrons in bonding to chlorine, so each chloride shows the element's highest oxidation state

• 3 marks · Calculate · PER-Q20 CAIE

  Calculate the volume of chlorine, at room temperature and pressure, that reacts with 0.496 g of white phosphorus to form PCl₅. The equation is P₄(s) + 10Cl₂(g) → 4PCl₅(s). Molar gas volume is 24.0 dm³ mol⁻¹.

  Mark scheme

  1. M1 amount of P₄ = 0.496 ÷ (4 × 31.0) = 0.00400 mol
  2. M1 amount of Cl₂ = 10 × 0.00400 = 0.0400 mol
  3. A1 volume of Cl₂ = 0.0400 × 24.0 = 0.960 dm³, to three significant figures

• 3 marks · Suggest · PER-Q21 CAIE

  Suggest why, when aluminium chloride is made by passing dry chlorine over heated aluminium, the product collects in the cooler part of the tube, away from the metal, and why the chlorine must be dry. Give the formula of the species present in the vapour.

  Mark scheme

  1. B1 aluminium chloride sublimes at about 178 °C, so it leaves the hot metal as a vapour and turns back to solid where the tube is cooler
  2. B1 aluminium chloride reacts with water, so wet chlorine would hydrolyse the product, giving hydrogen chloride
  3. B1 the vapour contains Al₂Cl₆ dimers, held by dative bonds from chlorine lone pairs to aluminium

Practise the Period 3 elements: reactions with oxygen and water one question at a time