Part 6: THERMODYNAMICS

a. But-1-yne, buta-1,2-diene and buta-1,3-diene are constitutional isomers. (1 mark)

b. H₂C=C=CHCH₃+ 5.5 O₂ -----> 4 CO₂ + 3 H₂O (2 marks)

c calculations are shown on the diagram (4 marks) and d. see below (4 marks)

e. 1,2-butadiene is H₂C=C=CHCH₃ and 1,3-butadiene is H₂C=CHCH=CH₂. Therefore 1,3-butadiene has 6 sp2C-H compared to 3 sp2C-H and 3 sp3C-H in the 1,2-isomer. There is also a pi interaction (resonance) between the C2 and C3 in 1,3-butadiene that further stablises the system. (2 marks)

Common errors:

• couldn't write a balanced equation
• could not apply Hess's law equation and made algebraic mistakes with the signs of the terms.
• used 6 moles of H2 instead of the required 3.
• couldn't relate calculated stablity to the energy diagram.
• didn't understand the terms heat of combustion or formation in terms of what they represent.
• poorly structured diagram (e.g. products or elements not aligned)
• incomplete rationale of the energy differences of the two dienes.