Alkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds.

Question 2

What is the general formula of an alkane?

Answer 2

CnH2n+2

Question 3

What is the general formula of a cycloalkane?

Answer 3

CnH2n

Question 4

Explain alkanes’ structure in terms of bonding and structure

Answer 4

Each carbon atom is joined to four other atoms by single covalent bonds, which are called sigma bonds.

Question 5

Explain what a sigma bond is (2 points).

Answer 5

1. The result of the overlap of two orbitals, one from each bonding atom.
2. Each overlapping orbital contains one electron, so the sigma bond has two electrons shared between the bonding atoms.

Question 6

Explain the shape of alkanes (4 points).

Answer 6

1. Each carbon atom is surrounded by four electron pairs in four sigma bonds.
2. Repulsion between these electron pairs result in 3D tetrahedral arrangement around each carbon atom.
3. Each bond angle is approximately 109.5 degrees.
4. The sigma bonds act as axes around which the atoms can rotate freely, so these shapes are not rigid.

Question 7

Explain the effect of chain length on boiling/melting points (3 points).

Answer 7

1. As the chain length increases, the molecules have a larger surface area, so more surface contact is possible between molecules.
2. The London forces between the molecules will be greater and so more energy is required to overcome the forces.
3. As the carbon chain length increases, bpt/mpt also increases because the number of electrons gets bigger, so the strength of induced dipole-dipole interactions increases.

Question 8

Explain the effect of branching has on bpt/mpt (3 points).

Answer 8

1. As the carbon chain becomes more branched, bpt/mpt decreases.
2. Molecules can’t pack together as closely as they’d like to, so less surface contact.
3. Therefore weaker and fewer induced dipole-dipole interactions between moelcules.

Question 9

Why are alkanes very unreactive? (3 points).

Answer 9

1. C-C and C-H sigma bonds are strong- very high bond enthalpies.
2. C-C and C-H bonds are non-polar, so they don’t attract an attacking species.
3. The electronegativity of carbon and hydrogen is so similiar that the C-H bond can be considered to be non-polar.

Question 10

What is the general equation for complete combustion of an alkane

Answer 10

CxHy + (x+y/4)O2 …. xCO2 + y/2H2O

Question 11

Name a common place incomplete combustion can occur

Answer 11

Car engines

Question 12

Explain what happens when oxygen is limited during combustion (3 points).

Answer 12

1. The hydrogen atoms in the alkane are always oxidised to water.
2. Combustion of the carbon may be incomplete, forming the toxic gas of carbon monoxide or even carbon itself as soot.
3. Carbon can be produced as a black solid.

Question 13

Why is carbon monoxide bad?

Answer 13

It prevents Fe^2+ ion in haemoglobin in red blood cells from binding with oxygen and the body’s tissues become starved of oxygen.

Question 14

6 problems with using alkanes for fuels

Answer 14

1. Crude oil is non-renewable and will run out.
2. Carbon monoxide is produced (a pollutant)- toxic gas from incomplete combustion.
3. Nitrogen oxides are produced (pollutants)- adds to acid rain and destruction of forests.
4. Sulfur dioxide is produced ( a pollutant)- adds to acid rain.
5. Carbon dioxide is produced (a pollutant)
6. Other greenhouse gases released into the atmosphere which prevent heat from escaping the atmosphere and lead to increased temperatures on Earth.

Question 14

2 problems with using alkanes for fuels

Answer 14

1. Crude oil is non-renewable and will run out.
2. Pollutants produced when burning hydrocarbons.

Question 15

What 4 pollutants are produced when alkanes are used for fuels? Describe the effect of each of these pollutants.

Answer 15

1. Carbon monoxide- toxic gas from incomplete combustion.
2. Nitrogen oxides- adds to acid rain and destruction of forests.
3. Sulfur dioxide- adds to acid rain.
4. Carbon dioxide- and other greenhouse gases released into the atmosphere which prevent heat from escaping the atmosphere and lead to increased temperatures on Earth. This could be one of the causes of climate change.

Question 16

Definition of radical substitution

Answer 16

A type of substitution in which a radical substitutes a different atom or group of atoms (hydrogen).

Question 17

What causes homolytic fission?

Answer 17

UV-radiation

Question 18

What are the reagents and conditions for radical substitution of alkanes?

Answer 18

Reagents- Br2/Cl2
Conditions- UV radiation

Question 19

Describe what happens in the initiation stage of radical substitution of alkanes (4 points).

Answer 19

1. Formation of radicals.
2. The reaction is started when the covalent bond in the halogen molecules is broken by homolytic fission.
3. Each halogen atom takes one electron from the pair, forming two highly reactive halogen radicals.
4. The energy from this bond fission is provided by UV radiation.

Question 20

What happens in the propagation stage for radical substitution of alkanes? (2 points).

Answer 20

1. Radicals are used up and regenerated.
2. Propagation is terminated whenever two radicals collide (in practice).

Question 21

What happens in the termination stage of radical substitution of alkanes? (3 points).

Answer 21

1. Radicals are used up.
2. Two radicals collide, forming a molecule with all electrons paired.
3. When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction.

Question 22

What are the three limitations to radical substitution?

Answer 22

1. Uncontrollable termination steps lead to a mxi=ture of products.
2. Further substitution: products will be made that are not wanted.
3. If the carbon chain is longer, a mixture of monosubstituted isomers by substitution at different positions in the carbon chain exist.