Revision notes

Question 1

Types of Fuels

Write the word formula for the transesterification reaction

Answer 1

Triglyceride + methanol –> mixture of fatty acid methyl esters + glycerol

Question 2

Types of Fuels

State two examples of catalysts for a tranesterification reaction

Answer 2

KOH (potassium hydroxide) & NaOH (sodium hydroxide)

Question 3

Types of Fuels

What is the flashpoint of a fuel?

Answer 3

the lowest temperature at which the vapours of the fuel will ignite

The intramolecular (covalent) bonds are not related to flashpoint; only intermolecular force are relevant.

Question 4

Types of Fuels

Name two possible sources of methane for fuel.

Answer 4

• natural gas (fossil fuel deposits)
• biogas (animal agriculture)

Question 5

Types of Fuels

Name one possible use for methanol

Answer 5

Uses for methanol include a precursor for other chemicals and an energy source for fuel cells.

Question 6

Types of fuels

What is atom economy?

Answer 6

Atom economy (or atom efficiency) is a measure of the desired useful products formed from reactants in a chemical reaction. Atom economy is commonly stated as a percent and is sometimes referred to as percentage atom economy.

Formula:
mass of atoms in desired product/mass of atoms in product x100

Question 7

Types of Fuels

Biodiesels obtained from saturated fats from animals usually has a higher melting point than biodiesel produced from plant oils. Suggest an explanation to this.

Answer 7

• Biodiesel produced from plant oils often contain carbon-carbon double bonds
• This gives a kink in the molecules, making it harder for them to pack closely together. Hence, the dispersion forces are weaker and more easily overcome
• Therefore, biodiesel molecules produced from saturated fats (with no carbon-carbon double bonds) obtained from animals has a higher melting point

Often 1 mark per point

Question 8

Types of Fuels

Explain why biodiesel is more viscous and has a higher freezing point than petrsodiesel.

Answer 8

• Biodiesel molecules are esters, whereas petrodiesel molecules are hydrocarbons
• The ester group is polar and this increases the viscosity and the freezing point of biodiesel due to the interactions between molecules

Question 9

Types of Fuels

Explain how ethanol can be considered a fossil fuel.

Answer 9

• Ethanol can be classified as a fossil fuel, as it can be obtained from crude oil
• Ethene, used to produce ethanol (by reaction with steam in the presence of an acid catalyst), is obtained from crude oil

2020 past VCAA exam q11

Question 10

Types of Fuels

Explain why biodiesel has a greater viscosity than petrodiesel

Note: this does not address the polar ester group (at least not directly)

Answer 10

The intermolecular bonding in biodiesel is stronger than that in petrodiesel because the hydrocarbon chain in biodiesel is longer and contains electronegative oxygen atoms that form dipole-dipole bonds between neighbouring molecules. Therefore, biodiesel has a higher viscosity than petrodiesel