State two reagents required to convert vegetable oil to biodiesel.

Deduce the formula of the biodiesel formed when the vegetable oil shown is reacted with the reagents in (a).

Explain, in terms of the molecular structure, the critical difference in properties that makes biodiesel a more suitable liquid fuel than vegetable oil.

Determine the specific energy, in kJ\(\,\)g⁻¹, and energy density, in kJ\(\,\)cm⁻³, of a particular biodiesel using the following data and section 1 of the data booklet.

Density = 0.850 g\(\,\)cm⁻³; Molar mass = 299 g\(\,\)mol⁻¹;

Enthalpy of combustion = 12.0 MJ\(\,\)mol⁻¹.

methanol
OR
ethanol

strong acid
OR
strong base

Accept “alcohol”.

Accept any specific strong acid or strong base other than HNO₃/nitric acid.

[3 marks]

CH₃(CH₂)₁₆COOCH₃ / CH₃OCO(CH₂)₁₆CH₃
OR
CH₃(CH₂)₁₆COOC₂H₅ / C₂H₅OCO(CH₂)₁₆CH₃

Product must correspond to alcohol chosen in (a), but award mark for either structure if neither given for (a).

[1 mark]

lower viscosity

weaker intermolecular/dispersion/London/van der Waals’ forces
OR
smaller/shorter molecules

Accept “lower molecular mass/M_r” or “lower number of electrons”.

Accept converse arguments.

[2 marks]

Specific energy: «\( = \frac{{12\,000{\text{ kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}}}{{299{\text{ g}}\,{\text{mo}}{{\text{l}}^{ - 1}}}}\)» = 40.1 «kJ g⁻¹»

Energy density: «= 40.1 kJ\(\,\)g⁻¹ x 0.850 g\(\,\)cm⁻³» = 34.1 «kJ\(\,\)cm⁻³»

Award [1] if both are in terms of a unit other than kJ (such as J or MJ).

[2 marks]