exam questions mistakes

• Explain how your results could be used to calculate the enthalpy of a solution (determine enthalpy of a solution of 0.8g of anhydrous MgCl2)

  1. measure 50cm3 of water in measuring cylinder and pour into insulating container
  2. record initial temp using a thermometer (degrees C) for at least 3 mins
  3. add a known mass - 0.08g - of MgCl2 to container
  4. measure temp every minute for 7 mins
  5. plot temp on graph
  6. extrapolate 2 best of fit lines for initial temp and for after solid added - find max temp rise
  7. use Q = mc∆T in joules and divide by number of moles to find ΔH
• Explain why the enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller numerical value than the value calculated in Question 1.2

  • Agl contains covalent character
  • (small difference in electronegativities) so the bonding in the lattice will be stronger
• explain why the standard entropy value for carbon dioxide is greater than that of carbon

  • CO2 is a gas so it is more disordered than carbon
• state the temp at which the standard entropy of aliminium is 0Jk-1 mol-1

  • 0k
• Give an equation, including state symbols, to represent the process that occurs when the enthalpy of solution of magnesium chloride is measured.

  • MgCl2 (s) → Mg2+(aq) + 2 Cl‒ (aq)
• suggest why is magnesium 2+ more exothermic than calcium 2+

  • Ca 2+ is a bigger ion
  • so has a weaker attraction the to O- in water
• State why there is a difference between the theoretical and experimental values.

  • It has covalent character or partial covalent bonding
  • bonds holding the lattice together are stronger
• Explain why the enthalpy of hydration becomes less exothermic from Li+ to K+

  • From Li+ to K+ size of ion increases
  • Electrostatic attraction between metal ion and Oδ– of water decreases
• lattice formation from experiment = -585 Jk mol-1

  lattice formation from calculation using perfect ionic model = -582 Jk mol-1. Deduce the bonding means:

  • it is almost perfectly ionic (there is little difference)
• enthalpy of lattice formation = Enthalpy change when 1 mol of solid ionic compound is formed from its gaseous ions
• suggest how enthalpy of lattice formation of NaCl compares to that of NaF. Justify your answer.

  • formation of NaCl is less exothermic
  • Cl- ion is bigger than F- ion
  • so attraction of Na is weaker