3.2.4 Properties of Period 3 elements and their oxides

• The reactions of Na and Mg with water.

  • Sodium reacts with cold water.
  • It fizzes around on surface etc.
  • 2 Na (s) + 2 H2O (l) -> 2 NaOH (aq) + H2 (g)
• The reactions of Na and Mg with water.

  Magnesium reacts very slowly with cold water to form the hydroxide but reacts more readily with steam to form the oxide

  • Mg (s) + H2O (g)  MgO (s) + H2 (g)
• Trends in the reactions of the elements Na, Mg, Al, Si, P and S with oxygen

  • Sodium burns with a yellow flame to produce a white solid.
  • Mg, Al, Si and P burn with a white flame to give white solid smoke.
  • S burns with a blue flame to form an acidic choking gas.
• • 4 Na (s) + O2 (g) -> 2 Na2O (s)
  • 2Mg (s) + O2 (g)  2MgO (s)
  • 4Al (s) + 3O2 (g)  2Al2O3 (s)
  • Si (s) + O2 (g)  SiO2 (s)
  • 4P (s) + 5O2 (g)  P4O10 (s)
  • S (s) + O2 (g)  SO2 (g)
• Explain, in terms of crystal structure and bonding, why silicon(IV) oxide has a higher melting point than phosphorus(V) oxide.

  • M1 SiO2 is macromolecular / giant covalent / giant molecule Do not allow giant, giant atomic or giant ionic 1
  • M2 Strong covalent bonds (between atoms) or covalent bonds need a lot of energy to be broken/overcome 1
  • M3 P4O10 is molecular or simple covalent molecule 1
  • M4 Weak van der Waals forces between molecules or van der Waals forces between molecules break easily
• Identify the element in Period 3, from sodium to chlorine, that has the largest atomic radius.

  • Na / sodium
• Give an equation and two observations made for the reaction that occurs when sodium is heated in oxygen.

  • 4Na + O2 → 2Na2O
  • Yellow/orange flame/light AND
  • white solid/powder/smoke/fumes/ash
• Explain the increase in melting point from sodium oxide to magnesium oxide.

  • Greater/increased charge/charge density on magnesium ion/Mg2+ (specific mention of ion(s) can be scored from M2)
  • Stronger attraction for anions/oxide ion / stronger attraction between oppositely charged ions/ stronger attraction between Mg2+ and O2– / stronger ionic bonding
• Explain why the melting point of the oxide of silicon is much higher than that of the highest oxide of phosphorus.

  • (SiO2) giant covalent / macromolecular
  • (P4O10) (simple) molecular
  • (Covalent) bonds (throughout structure) of SiO2 much stronger than the forces between molecules/intermolecular forces in P4O10
• Determining the melting point of a sample of the highest oxide of phosphorus

  1. M1: Sample in suitable melting point apparatus (e.g. capillary in oil bath/Thiele tube / melting point apparatus)

  2. M2: Heat slowly/gradually/gently to establish melting point range

  3. M3: Lower melting point / (broad) range of melting point indicates presence of impurities

  4. OR: Melting point agrees with/close to data book value / melts sharply/over narrow range / melting point exactly 573K indicates purity
• Describe a test you could carry out in a test tube to distinguish between sodium oxide and the product of the reaction in part (a)

  • React with water / add water / solution (of substances in question)
  • Add litmus paper / universal indicator / measure pH (with pH meter)
  • Universal Indicator: blue/ purple with sodium oxide (solution) and red with phosphorus oxide (solution)
• State the type of crystal structure shown in silicon dioxide and in sulfur trioxide.

  • Silicon dioxide - giant covalent (molecule)/ macromolecular
  • Sulfur trioxide - molecular / simple molecule
• Explain why silicon dioxide has a higher melting point than sulfur trioxide.

  • Covalent bonds (between atoms) in SiO2 1
  • Van der Waals between molecules / intermolecular forces in SO3 1
  • Covalent bonds are stronger than van der Waals forces 1
  • Covalent bonds take more energy to be overcome/broken or Van der Waals take less energy to be overcome/broken
• • Exam notes: Explain why silicon dioxide has a higher melting point than sulfur trioxide.
  • If covalent bonds between molecules of SiO2 lose M1 only
  • If hydrogen bonds in SO3 lose M2 only If metallic or ionic max score = 1 (either M1 or M2)
  • If IMF in SiO2 then max 1 ( M2 only)
  • Allow dipole-dipole forces between molecules
  • For M3 and M4 comparison is required/implied
• Write an equation for the reaction of sulfur trioxide with potassium hydroxide solution.

  • SO3 + 2KOH → K2SO4 + H2O
• Write an equation for the reaction of an excess of magnesium oxide with phosphoric acid.

  • 3 MgO + 2 H3PO4 → Mg3(PO4)2 + 3 H2O
• Explain why the melting point of sulfur (S8) is greater than that of phosphorus (P4).

  • S8 molecules are bigger than P4 molecules
  • (Allow sulfur molecules have bigger surface area and sulfur molecules have bigger Mr)
  • Therefore, van der Waals / dispersion / London forces between molecules are stronger in sulfur
• Explain why sodium oxide forms an alkaline solution when it reacts with water. [2 marks]

  • Sodium oxide contains O2– ions 1
  • These O2– ions react with water forming OH– ions
  • O(2–) + H2O -> 2OH– scores M1 and M2
• Element X forms an oxide that has a low melting point. This oxide dissolves in water to form an acidic solution.

  (i) Deduce the type of bonding in this oxide of X

  • Covalent
• The reactions of sodium and magnesium in water

  • Suggest why it is dangerous to react a similar piece of sodium with 10 cm3 of water in a boiling tube. [1 mark]
  • Temperature will go up more or reactants can shoot out of the tube
  • Allow the mixture could explode or glass could shatter or hydrogen could ignite/is flammable
• Give an equation for the reaction of phosphorus(V) oxide with water. Suggest a pH for the solution formed. [2 marks]

  Equation

  • P4O10 + 6 H2O → 4 H3PO4
  • Allow ‒1 to + 1
• Outline a simple experiment to demonstrate that magnesium oxide has this type of bonding (ionic)

  • Melt it
  • Molten oxide conducts electricity (or allow acts as an electrolyte)
• Give an equation and two observations made for the reaction that occurs when sodium is heated in oxygen

  • YELLOW/orange flame/light
  • white solid/powder/smoke/fumes/ash
• Write an equation for the reaction of sulfur(IV) oxide with water. Suggest the pH value of the resulting solution.

  • SO2 + H2O → H2SO3
  • pH of 1 / ONE
  • ALLOW 1 - 3
• Write an equation for the reaction of P4O10 with water to form phosphoric(V) acid.

  Give the approximate pH of the final solution.

  • P4O10 + 6H2O → 4H3PO4
  • pH must be in the range –1 to +2
• Write equations for the reactions of Na2O with water.

  Give the approximate pH for the resulting solution.

  • Na2O + H2O -> 2Na+ + 2OH– allow 2NaOH
  • 14 ; Allow 12 to 14
• Silicon(IV) oxide is insoluble in water. Explain, using an equation, why silicon(IV) oxide is classified as an acidic oxide. [2 marks]

  • Reacts with / neutralises bases / alkalis
  • SiO2 + 2NaOH → Na2SiO3 + H2O