Properties of Alkanes

Question 1

Bonding in alkanes

Answer 1

• Alkanes are saturated hydrocarbons containing only carbon and hydrogen atoms joined together by single covalent bonds.
• Single covalent bonds are called sigma bonds (σ) and each carbon atom in an alkane has 4 sigma bonds.
• A sigma bond is a result of the overlap of two ‘s’ orbitals. A sigma bond has 2 electrons that are shared between the bonding atoms.

Question 2

Shape of alkanes

Answer 2

• General formula: Cn H2n+2
• Bond angle: 109.5
• Shape: Tetrahedral

Question 3

Crude oil

Answer 3

• Crude oil is a mixture of alkanes. Oil refineries separate the crude oil into fractions by fractional distillation is a distillation tower.
• Separation is possible because the boiling point of the alkanes are different and they increase as their chain length increases.
• London forces are weak intermolecular forces that can be easily broken, causing alkanes to turn to gas. The greater the intermolecular forcer, the higher the boiling point.

Question 4

Effect of chain length on B.P

Answer 4

• London forces act between molecules that are in close surface contact.
• As chain length increases, there is higher surface area, so more surface contact is possible between molecules. So the no. of London forces increase.
• London forces are also stronger as there are more electrons so a bigger partial negative charge.

Question 5

Effect of branching on B.P

Answer 5

• Branched isomers have lower boiling points compared to straight chain isomers.
• Molecules without branches have more points of contact.
• Therefore branched molecules have fewer London forces.

Question 6

Complete combustion of alkanes

Answer 6

Produces CO2 and H20.

• Number in front of CO2 is the number of Carbons on reactant side.
• Number in front of H20 is HALF the number of Hydrogens on reactants.

Question 7

Incomplete combustion of alkanes

Answer 7

Produces CO and H20.

• Number in front of CO2 is the number of Carbons on reactant side.
• Number in front of H20 is HALF the number of Hydrogens on reactants.

Question 8

Carbon Monoxide

Answer 8

• Colourless, odourless and highly toxic gas.
• Combines irreversible with haemoglobin in red blood cells to form a compound that prevents haemoglobin from transporting oxygen around the body.
• Person can be poisoned without noticing any danger.