Basic concepts and alkanes

Question 1

What is a general formula?

Answer 1

An algebraic formula that can describe any member of a family of compounds.

Question 2

What is the general formula of an alcohol?

Answer 2

CnH2n+1OH

Question 3

What is a homologous series?

Answer 3

A family of compounds that have the same functional group and general formula, but each successive member differs by CH2.

Question 4

What is the general formula of an alkane?

Answer 4

CnH2n+2

Question 5

What is molecular formula?

Answer 5

It gives the actual number of atoms of each element in a molecule.

Question 6

What is empirical formula?

Answer 6

The simplest whole number ratio of atoms of each element in a compound.
Divide the molecular formula by the smallest number of atoms for a given element in the molecule.

Question 7

What are aromatic compounds?

Answer 7

They contain a benzene ring.

Question 8

What are aliphatic compounds?

Answer 8

They contain hydrogen and carbon joined together in straight chains, branched chains or non-aromatic rings.

Question 9

What are alicyclic compounds?

Answer 9

When an aliphatic compound contains a ring that is not benzene.

Question 10

What is an alkyl group?

Answer 10

In compounds with a branched carbon skeleton, it is any hydrocarbon branched chain.
The general formula is CnH2n+1.

Question 11

What are saturated compounds?

Answer 11

They contain only carbon-carbon single bonds.

Question 12

What are unsaturated compounds?

Answer 12

They contain multiple carbon-carbon bonds.

Question 13

What are structural isomers?

Answer 13

They have the same molecular formula, but the structural formula is different.

Question 14

What are chain isomers?

Answer 14

Molecules that have different arrangements of the carbon skeleton.
They have similar chemical properties, but different physical properties, due to the change in shape of the molecule.

Question 15

What are positional isomers?

Answer 15

The skeleton and functional group can be the same, but the functional group could be attached to a different carbon atom.
They have different physical properties, and chemical can also be different.

Question 16

What are cycloalkanes?

Answer 16

They have a ring of carbon atoms with 2 hydrogens attached to each carbon.
They have 2 less hydrogens than other alkanes.

Question 17

What are IUPAC rules?

Answer 17

Count the carbon atoms in the longest continuous chain to get the stem.
Number the chain so the main functional group has the lowest number. Or pick the one with the most side chains.
Put the side chains in alphabetical order after the number it is attached to, at the start.
Use di, tri or tetra if there’s multiple identical things.

Question 18

What is the bonding in alkanes?

Answer 18

Atomic orbitals on carbon and hydrogen atoms are positioned so that they overlap to form a new, shared orbital lying directly between the bonded atoms.
This type of covalent bond is a sigma bond.

Question 19

What is the strength of sigma bonds?

Answer 19

The high electron density between the nuclei means there is strong electrostatic attraction between the nuclei and the shared pair of electrons.
So they have high bond enthalpy.

Question 20

What is the rotation of sigma bonds?

Answer 20

The bonded atoms are free to rotate around the sigma bond, because the atomic orbitals between them will always overlap.

Question 21

What is a sigma bond?

Answer 21

Overlap of orbitals directly between bonding atoms.

Question 22

What is the reactivity of alkanes?

Answer 22

Low reactivity because sigma bonds are very strong and so difficult to break.
They are also non-polar - won’t attract positively or negatively charged particles to react with them.

Question 23

What are the boiling points of alkanes?

Answer 23

It depends on the size and shape.
Small alkanes, e.g. methane, are gases at room temperature and pressure.
Larger alkanes are liquid.

Question 24

How does the intermolecular forces of alkanes affect boiling points?

Answer 24

The longer the carbon chain, the more induced dipole dipole interactions.
This is because of larger surface contact, and so more electrons to interact.
So it takes more energy to overcome the interactions and boiling point increases.

Question 25

How does branching of alkanes affect boiling point?

Answer 25

Branched alkanes have a lower boiling point because it can’t pack closely together and so has less surface contact, so induced dipole dipole interactions are reduced.

Question 26

What is the complete combustion of alkanes?

Answer 26

Burning alkanes with enough oxygen produces carbon dioxide and water.

Question 27

Which alkanes burn best?

Answer 27

Complete combustion happens to gases, so liquids must be vaporised first.
Smaller alkanes turn into gases easier (more volatile) so will burn more easily.

Question 28

Which alkanes produce more energy in combustion?

Answer 28

Some energy is used to break the covalent bond but more energy is produced in forming the new product bonds.
So larger alkanes release more energy per mole than smaller, as more CO2 and H2O are formed.
They make excellent fuels.

Question 29

What is incomplete combustion of alkanes?

Answer 29

When there is limited oxygen, so carbon monoxide and water are formed.

Question 30

Why is carbon monoxide bad?

Answer 30

It is poisonous, and better at binding to haemoglobin than oxygen is.
So less oxygen can reach the cells and they suffer oxygen deprivation - fatigue, headaches, nausea.

Question 31

What is bond fission?

Answer 31

Breaking a covalent bond, a shared pair of electrons between two atoms.

Question 32

What is heterolytic fission?

Answer 32

The bonds break unevenly with one of the atoms recieving both electrons from the bonded pair.
A cation and an anion are formed.

Question 33

What is homolytic fission?

Answer 33

The bond breaks evenly and each atom recieves one electron from the bonded pair.
Two electrically uncharged radicals are formed.
Because of the unpaired electron, radicals are very reactive.

Question 34

What do curly arrows show?

Answer 34

It shows the movement of a pair of electrons during a reaction.

Question 35

How are haloalkanes formed?

Answer 35

When halogens react with alkanes in photochemical reactions - they’re started with light.
A hydrogen atom is substituted by a halogen such as chlorine or bromine in radical substitution

Question 36

What is the synthesis of chloromethane?

Answer 36

CH4 + Cl2 -UV-> CH3Cl + HCl
Initiation: Cl2 -UV-> 2Cl*
Propagation: Cl* + CH4 –> CH3* + HCl
CH3* + Cl2 –> CH3Cl + Cl*
Termination: Cl* + Cl* –> Cl2
CH3* + Cl* –> CH3Cl
CH3* + CH3* –> C2H6

Question 37

What are the steps of radical substitution?

Answer 37

Initiation: UV light produces radicals.
Propagation: Radicals are used up and created in a chain reaction. Cl* attacks methane. The new methyl radical CH3* attacks Cl2. The new Cl* attacks CH4. This continues until all the Cl2 or CH4 are used up.
Termination: Two radicals join up to form a stable molecule and terminate the reaction.

Question 38

What are the limitations of radical substitution?

Answer 38

A mixture of products is made, so the desired product must be separated from the unwanted by-products.
The chance of this happening is reduced by using an excess of alkane.
It can also take place at any point on the carbon chain, so a mixture of isomers is formed.