Exam Questions

• Give TWO reasons why C₂H₆ has a higher boiling point than methane CH₄.

  • Ethane is a larger molecule than methane so it has more electrons (1)
  • There are stronger van der Waals’ forces between the ethane molecules (1)
• Chlorine exists as two isotopes, ³⁵Cl and ³⁷Cl.State the number of peaks in the mass spectrum of Cl₂ and state their m/z values.

  • Number of peaks: 3 peaks (1)
  • m/z values: at m/z 70, 72 and 74 (1)
• State if there is any difference between the chemical properties of the isotopes ⁶³Cu and ⁶⁵Cu and explain why.

  • Difference: no difference (1)
  • Explanation: they both have the same electron configuration (1)
• State the number of neutrons and electrons in the ⁵⁷Fe³⁺ ion.

  • Number of neutrons: 31 (1)
  • Number of electrons: 23 (1)
• Explain why the first ionisation energy of krypton is greater than that of bromine but the first ionisation energy of aluminium is less than the first ionisation energy of magnesium.

  Krypton: Krypton has more protons than bromine (1) , and its outer electrons are in the same shell (1)Aluminium: Aluminium has its outer electron in a 3p orbital, magnesium has its outer electron in a 3s orbital (1), and the energy of a 3p is greater than 3s (1)
• Explain how ions are detected and relative abundance is measured in a time of flight (TOF) mass spectrometer.

  Ions: The ion hits the detector and gains an electron. (1)Relative abundance: The abundance is proportional to the size of the current. (1)