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