Part 6: THERMODYNAMICS

This should have been a reasonably straight forward calculation, but care needs to be taken to get the right structures and the right molecular formula, then do the math correctly.

a. C₈H₁₄ + 11.5 O₂ --> 8 CO₂ + 7 H₂O (1 mark)

b. DHf iii = -1230.9 - (- 1214.3) = -16.6 kcal/mol (2 marks)

c. DHc i = -1230.9 - (- 35.4) = -1195.5 kcal/mol (1 mark)

DHc ii = -1230.9 - (- 26.4) = -1204.5 kcal/mol (1 mark)

d. (5 marks)

e. Isomer iii is the only chiral molecule. Neither i nor ii are chiral because both have at least one plane of symmetry OR because neither have an sp3 C atom with 4 different groups attached(2 marks)

f. A bicyclo[2.2.1] system requires a two ring system where there are 2 "bridges" with 2 C atoms and 1 bridge with 1 C atom (1 mark)

Common errors:

• incorrect coefficients in the balanced equation
• wrong sign in calculated values
• calculation error (e.g. based on number of H atoms rather than H₂ molecules i.e. using 14 H instead of 7 H₂)
• incorrect or no labelling of the most and least stable isomers
• incorrect understanding of chirality and the requirements for it
• drawing an isomer of C₈H₁₄ that did not fit the required molecular formula or was not a bicyclo[2.2.1] structure
• incorrect use of the words atom or molecule