exam questions mistakes

• STEAM + MAGNESIUM - state symbols

  Mg (s) + H2O (g) ---> MgO (s) + H2 (g)
• Explain why the melting point of magnesium is higher than the melting point of sodium.

  • Mg atoms smaller than Na atoms and Mg has more delocalised electrons than Na
  • So Mg has a stronger attraction to delocalised sea of electrons
• State what is observed when dilute aqueous sodium hydroxide is added to separate solutions of magnesium chloride and barium chloride.

  • Observation with MgCl2: (slight) white precipitate
  • Observation with BaCl2: no change/ no reaction
• Explain why the second ionisation energy of calcium is lower than the second ionisation energy of potassium.

  • In Ca(+) (outer) electron(s) is further from nucleus and Ca(+) loses electron from 4th energy shell
  • More shielding (in Ca+ )
  • while K(+) loses electron from 3rd energy shell
• A sample of strontium has a relative atomic mass of 87.7 and consists of three isotopes, 86Sr, 87Sr and 88Sr. Why isotopes of strontium have identical chemical properties?

  • Same electronic configuration / same number of electrons in outer shell
• Describe how a student could distinguish between aqueous solutions of magnesium chloride, MgCl2, and aluminium chloride, AlCl3, using one simple test-tube reaction.

  • sodium hydroxide (other Group 1 hydroxides)
  • white precipitate
  • white precipitate which dissolves in excess (NaOH)
• Chlorine has a low boiling point because the forces between the molecules are weak. Explain how these forces arise between molecules of chlorine.

  • Random movement of electrons in one molecule creates a dipole
  • Induces a dipole in a neighbouring molecule
  • temporary attraction between δ + and δ –
• Silver nitrate is added to the solution. Suggest why an excess is used.

  • To ensure that all the halide ions (chloride and iodide) are removed from the solution
• periodicity is the repeating of trends and patterns across a period
• identify the element in period 4 with the largest atomic radius and explain your answer.

  • Potassium
  • it has the smallest nuclear charge having the smallest number of protons
  • same amount of shielding
• Explain why the first ionisation energy of group 2 elements decrease down the group?

  • the atom gets larger down the group so there is an increase in shielding of nuclear charge
  • there is a weaker attraction between the nucleus and outer electron
• third ionisation energy of magnesium equation:

  Mg 2+ (g) ---> Mg 3+ (g) + e-
• explain why the third ionisation energy of magnesium is much higher than the second ionisation energy of magnesium

  • The electron is removed from the 2nd energy level closer to the nucleus
  • (Electron being removed is) less shielded
• explain in terms of electron transfer the meaning of the term oxidising agent?

  • species that accepts electrons/ electron acceptor
• one reason why the reaction of sodium fluoride (NaF) with concentrated sulphuric acid (H2SO4) is different from the reaction with sodium iodide (NaI)?

  • Fluoride less easily oxidised than iodide
• Give a half-equation to show the conversion of iodide ions to iodine.

  • 2I– → I2 + 2e

  Give a half-equation to show the conversion of sulfuric acid to sulfur

  • 6H+ + 6I- + H2SO4 → 3I2 + S + 4H2O
• Silver nitrate is added to the solution. Suggest why an excess is used

  • To ensure that all the halide ions (chloride and iodide) are removed from the solution

  
• what is the role of the chloride ions: NaCl + H2SO4 → NaHSO4 + HCl

  • base/ proton acceptor
• 2Br- + 2H+ + H2SO4 → SO2 + Br2 + 2H2O

  • Br changes oxidation state from −1 to 0 and is oxidised
  • S changes oxidation state from +6 to +4 and is reduced
• State the role of the sulfuric acid in this reaction.

    NaCl + H2SO4 → NaHSO4 + HCl

  • Proton donor or (Bronsted-Lowry) acid
• Explain why the melting point of magnesium is higher than the melting point of sodium.

  • Mg2+ has a higher charge than Na+ so Mg2+ has a greater charge density
  • Stronger attraction to delocalised sea of electrons
• Explain why different observations are made when aqueous barium chloride is added separately to aqueous magnesium sulfate and to aqueous magnesium nitrate.

  Write the simplest ionic equation, including state symbols, for any reaction that occurs.

  • BaSO4 is insoluble but Ba(NO3)2 is soluble
  • Ba2+(aq) + SO4 2-(aq) → BaSO4(s)
• Use your knowledge of the reactions of Group 2 metals with water to explain why water should not be used to put out a fire in which magnesium metal is burning.

  • H2 produced (Mg + H2O ---> MgO + H2)
  • which is highly flammable