2017 Paper 1

• 2017 Paper 1 Q3.4 - Calculate the pH of a 0.0131 moldm^-3 solution of calcium hydroxide at 10C. Give your answers to 2dp

  • STEP 1 - RECOGNISE THAT CALCIUM HYDROXIDE IS DIBASIC
  • [OH-] = 2 * 0.0131
• Predict whether the pH of magnesium hydroxide is smaller, bigger, or equal to pH of calcium hydroxide.

  • smaller / lower pH / less alkaline / more acidic 1
  • magnesium hydroxide is less soluble / sparingly soluble/ solubility of hydroxide increases down group II
  • NOTE: Do NOT accept Mg(OH)2 is INSOLUBLE
• Explain why silicon dioxide has a higher melting point than sulfur trioxide [4 marks]

  • Covalent bonds (between atoms) in SiO2
  • Van der Waals between molecules / intermolecular forces in SO3
  • M3 IS THE MARK I LOST = Covalent bonds are stronger than van der Waals forces
  • Covalent bonds take more energy to be overcome/broken or (Van der Waals) take less energy to be overcome/broken
• Explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.

  
  

  Suggest why the value of the enthalpy change for this reaction (ethanedioate ions and aqueous iron(III) ions) is close to zero [2 marks]

  • [Fe(H2O)6] 3+ + 3C2O4 2 -> [Fe(C2O4)3] 3– + 6H2O
  • There are SIX Fe –O bonds broken and then made / same number and type of bond being broken and made.
• Explain why colorimetry cannot be used to determine the concentration of solutions containing [CuCl2]-

  In your answer refer to the electron configuration of the metal ion [2 marks]

  • (3)d10 OR has full (3)d (sub) shell/orbital
  • It is colourless/cannot absorb (frequencies of) visible light