2023 Paper 1

• Suggest why the oxidation state of chromium is zero in Cr(PF3)6

  • PF3 is neutral
  • The complex is neutral / no charge
  • THEREFORE the oxidation state of chromium is zero
• Many students did not realise that the salt bridge

  • Ionic compound should be soluble (KNO3)
  • Must not react with the acid, magnesium, chloride ions
• Drawing the displayed formula of anion when sulfur trioxide reacts with water:

  • O-H bonds must be displayed
  • Anion - MUST show charges after H+ dissociated
• Q4.4 Kp 2023 Paper 1

  • ONLY change in temperature affects the value of Kp
  • Increases pressure - the equilibrium shifts to the side with LESS moles of gas
• Explain why second ionisation energy of calcium is greater than first ionisation of calcium.

  • Answers were often vague
  • MUST ADDRESS the greater attraction between the electron removed and the more positive, smaller ion
  • MARKSCHEME =
  • Stronger attraction between same number of protons and fewer electrons
  • Electron removed from smaller ion/electron removed closer to nucleus
  • Electron removed from a positive charge/ion
• Making a volumetric solution - RP1, Topic 2 Amounts of substance

  • Many did not transfer the solution from the beaker to a volumetric flask and make the solution up to 250 cm3
  • Many did not state beaker and stirring rod should be washed with distilled water
  • AND the washings transferred to the flask
  • Several students forgot to INVERT THE FLASK to ensure thorough mixing
• State why it is good practice to remove the funnel from the burette

  • ACID COULD CONTINUE TO DRIP INTO THE BURETTE
  • THEREFORE decreases the titre (Less acid needed as there is more acid additional to burette)
• Explain why the solution containing [Al(H2O)6]3+ is acidic.

  • Al3+ has a HIGH CHARGE and SMALL SIZE
  • OR a high CHARGE DENSITY
  • Al3+ weakens the O-H bond( in water ligands and donates H+ to water)
  • MUST REFER TO ALUMINIUM IONS (or any ion question)
• State why concentration of (aluminium sulfate solution) can NOT be determined by colorimetry

  • Colourless solution
  • Does not absorb visible light
  • Allow no d-d transitions (as there are no d
  • electrons)
• State why a fuel cell does NOT need to be electrically recharged

  • Reactants supplied continuously
• Suggest one advantage of using methanol, rather than hydrogen in a fuel cell for use in cars.

  • Methanol is liquid so can be stored easily or transported easily
  • More energy can be produced from 1cm3 methanol (liquid) than 1cm3 of H2 gas
  • NOT flammability - because H2 gas and methanol are both flammable
• Time of flight exam question - when calculating mass number of an ion

  • MUST give mass number as an INTEGER / WHOLE NUMBER
• Explain how the ions are detected in the time of flight mass spectrometer

  State how the relative abundance of the ions is determined

  • (Ions hit a detector/electron multiplier and) each ion gains an electron (generating a current)
  • current is proportional to abundance
• Electrospray ionisation is used instead of electron impact for the ionisation of a protein in a mass spectrometer experiment

  Suggest why.

  • Protein cannot fragment / break up
• A student plans to titrate butanoic acid solution with a solution of ethylamine - Explain why this titration cannot be done using an indicator [2 marks]

  • STATE - this is a weak acid - weak base titration
  • MUST REFER TO GRADUAL pH change ==== pH change is too gradual/not sharp (at the equivalence point so colour change of indicator is difficult to judge)
  • OR no vertical / steep section on the pH curve