3.2.4 Properties of Period 3 elements and their oxides

Cards (27)

  • 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 + 2KOHK2SO4 + 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 + H2OH2SO3
    • 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