Exam Qs

• Suggest why the enthalpy of hydration of Ca2+ is less exothermic than Mg2+

  Ca2+ is a bigger ion than Mg2+, and so has a weaker attraction to oxygen in water
• Explain, in terms of crystal structure and bonding, why silicon(IV) oxide has a higher melting point than phosphorus(V) oxide

  SiO2 is a macromolecular structure -> has many strong covalent bonds to overcome - requires a lot of energy. P4O10 is a simple covalent molecule - has weak vdw forces between molecules to overcome easily
• Discuss the role of iron as a heterogeneous catalyst in the Haber process. (6)

  heterogenous catalyst is in a diff state / phase to reactants. a catalyst increases rate of reaction without being used up by providing an alternative pathway with lower Ea energy. H2 & N2 adsorb to irons surface, their bonds weaken and reaction takes place -> products desorb. Powdered ion or small pellets give an increased surface area. Catalyst poisoning can happen which binds to and blocks active site
• Fe2+ ions catalyse the reaction between peroxodisulfate(VI) ions and iodide ions in aqueous solution. S2O8 2–(aq) + 2 I –(aq) → 2SO4 2–(aq) + I2(aq) Explain why this reaction is slow before the catalyst is added

  activation energy is high as 2 negative ions repel each other
• Give a reason why Zn2+ ions do not catalyse reactions

  Zn2+ don't have various oxidation states
• Explain why an aqueous solution containing [Fe(H2O)6] 3+ ions has a lower pH than an aqueous solution containing [Fe(H2O)6] 2+ ions. (3)

  Fe3+ has a greater charge than Fe2+ and is smaller - greater charge to size ratio. Fe3+ are more polarising - more OH bonds in water break -- more H+ released
• Explain why the value of Kw increases as the temperature increases (2)

  increase in temp favours endo reaction - forward reaction, so equilibrium shifts right to counteract the temp increase
• Suggest why the pH probe is washed with distilled water between each of the calibration measurements.

  to wash of any residual solution - doesn't affect reading
• Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.

  to avoid missing the end point
• State two conditions needed for a half-cell to have Eo = 0.00 V

  100kPa, 298K
• Suggest why Emf values may differ

  happens under non standard conditions
• Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode

  Li+ + CoO2 + e- -> LiCoO2-
• Give an equation for the reaction that occurs at the negative lithium electrode

  Li -> Li+ + e-
• explain why complexes formed from transition metals are coloured

  absorb some wavelengths of visible light to excite electrons in d orbital, remaining wavelengths of light are reflected to give colour
• describe how a calibration graph is produced and used to find conc of iron(III) complex

  measure absorbance for known concentrations, plot graph absorbance v conc, read values of conc for measured absorbance
• State why the isotopes of rhenium have the same chemical properties

  same number of electrons - same electron configuration
• State how the relative abundance of 185Re+ is determined in a TOF mass spectrometer

  at the detector Re+ ion gains an electron, abundance depends on the size of the current created
• define mean bond enthalpy

  enthalpy change to break 1 mole of bonds, averaged over a range of compounds
• State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid

  add excess ethanoic acid to KOH
• Explain why aqueous solutions containing [CuCl4]2– ions are yellow.

  visible light absorbed, d electrons excited - only yellow light is reflected
• State three ways in which a transition metal complex can be changed to alter its colour


  change in oxidation state

  change in ligand

  change in co ordination number