2011 Chains, Energy and Resources Jan

Question 1

Explain in terms of intermolecular forces, how fractional distillation works

Answer 1

• (The hydrocarbons have) different boiling points

* The larger the molecules the stronger the van der Waals’ forces

Question 2

What is meant by the term enthalpy change of reaction

Answer 2

• (Enthalpy change when) the number of moles of reactants

* As specified in the (balanced) equation react together

Question 3

What is meant by the term average bond enthalpy

Answer 3

• (Enthalpy change) when one mole of bonds

* Of (gaseous covalent) bonds is broken

Question 4

Explain the bonding in a C=C double bond

Answer 4

• (Sideways) overlap of p orbitals

* Forming a π/pi bond

Question 5

Suggest why the average bond enthalpy of a C=C bond is not twice the bond enthalpy of a C-C bond

Answer 5

• σ bond is stronger (than the π bond)

Question 6

Suggest a reason why the actual value for the enthalpy change of this reaction is different from the calculated value

Answer 6

• Bond enthalpies may not be the same as the average bond enthalpy

Question 7

Explain why many industrial manufacturing processes use catalyst

Answer 7

• Enable reactions to occur with less waste
OR enable reactions to take place with higher atom economy
• Enable reactions to produce less toxic waste/side products
• Reactions can happen at a lower pressure/temperature
• Saves energy (costs)
• Enable reactions to occur with more specificity

Question 8

Suggest advantages/disadvantages of using bio-fuels

Answer 8

• Fossil fuels are finite resources
OR biofuels are renewable
• No risk of large scale pollution from exploitation of
fossil fuels
• Land not used to grow food crops
• (Rain) forests have to be cut down to provide land
• Shortage of fertile soils