Introduction to Hydrocarbons

Question 1

Define organic chemistry

Answer 1

a branch of chemistry which deals with carbon compounds

Question 2

Covalent bond

Answer 2

Sharing of valence electrons between non-metals to gain a stable electron arrangement.

Question 3

What is the simplest type of organic compounds

Answer 3

Hydrocarbons

Question 4

Examples of carbon compounds with localised and delocalised electrons

Answer 4

In diamond, all 4 valence electrons of C are localised between the atoms in covalent bonding. Because the movement of electrons is restricted, an electric current cannot be conducted.

In graphite, each C atom uses 3 of out 4 valence electrons to covalently bond with 3 other C in a plane. Each C atom contributes one electron to the delocalised system of electrons that is also part of the chemical bonding. The delocalised electrons are free to move throughout the plane, meaning that graphite can conduct electricity along the planes of C atoms, but not in a direction perpendicular to the plane

The electrons in CH4 are localised between the hydrogen and carbon nuclei.

Question 5

What is the simplest form of hydrocarbons?

Answer 5

Alkanes

Question 6

What are alkanes

Answer 6

A homologous series where the molecules have a single covalent bond between the carbon atoms. This means that the molecules are saturated as all the bond sites are filled with single bonds.

Question 7

How to find the name of an alkane

Answer 7

The name is based on the longest chain (so branches are disregarded). Ends in “ane”

Question 8

Prefixes of carbon compounds

Answer 8

1. Meth
2. Eth
3. Prop
4. But
5. Pent
6. Hex
7. Hept
8. Oct

Question 9

General formula of alkanes

Answer 9

C(n)H(2n+2)

eg octane = 8
so C8H18

Question 10

Ways of expressing the formula of hydrocarbons and all molecules

Answer 10

Molecular: Number of each atom
Structural: Graphical representation. Shows how atoms are arranged and bonded.
Condensed structural: eg ethane = CH3CH3
Each C atom is written separately and following each C atom are listed the other atoms that are bonded to that C atom.

Question 11

What are alkenes

Answer 11

A homologous series of hydrocarbons that contain one or more double bonds between two adjacent carbon atoms.

Question 12

Why are alkenes unsaturated

Answer 12

Because it contains at least one double bond, the two pairs of electrons are being shared between two carbon atoms. This means that these carbon atoms do not contain the maximum number of hydrogen atoms, and that the double bonds can be broken and more atoms can be added.

Question 13

What is the general formula of alkenes

Answer 13

C(2)H(2n)

Question 14

Why does methene not exist

Answer 14

Because two carbon atoms are required to form a double bond.

Question 15

How to tell the difference between alkanes and alkenes?

Answer 15

Add bromine water. The alkane should stay red-brown. The alkene should turn colourless.

This is because the double bond within an alkene is more reactive than the single bonds between the C atoms in an alkane. One of the bonds breaks and the C atoms of the double bond remain bonded with a single bond. This means that they have a spare electron to covalently bond with the bromine atoms. So, when a reactive substance, such as bromine, is added to an alkene, the double bond opens and a new type of molecule forms.

Question 16

What are alcohols

Answer 16

A homologous series of organic molecules with an alkane chain and a hydroxyl (OH) group attached to one of the carbon atoms. The hydroxyl group replaces one of the hydrogen atoms.

Question 17

Are alcohols acidic, basic or neutral

Answer 17

NEUTRAL.

Question 18

General formula for alcohols

Answer 18

C(n)H(2n+1)OH

Question 19

Eg Condensed structural formula for Octane

Answer 19

CH3(CH2)6CH3

Question 20

Physical states of alkanes at 20°C

Answer 20

Gases: Methane, Ethane, Propane, Butane
Liquids: Pentane to Octane
Solids: more than 20 C

Question 21

Explanation of the melting and boiling point trends

Answer 21

The melting and boiling points of hydrocarbons increases as the carbon chain length increases. This is because as the size of the chain of the hydrocarbon molecules increase, the stronger the force of attraction between the molecules so more energy is required to separate the molecules, hence it has a higher melting and boiling point.

Neighbouring molecules are held together by forces of attraction between the molecules. In order to push the molecules further apart, and therefore change their state, these forces of attraction must be overcome. So the longer the carbon chain, the greater the force of attraction so more E is needed to overcome these forces to change the substance’s state.

Question 22

Immiscible

Answer 22

when two insoluble liquids are formed.

Usually layers will form

Tow liquids that do mix are miscible

Question 23

Solubility of alkanes, alkenes and alcohols in water

Answer 23

There is very little attraction between alkane and alkene molecules and water molecules. This means alkanes and alkenes are all insoluble in water.

Methanol and ethanol are are soluble in water in all proportions because water molecules and alcohol molecules are strongly attracted to each other.

Question 24

State of ethene and propene at room temperature

Answer 24

Both are gases at room temp

Question 25

State of methanol and ethanol at room temp

Answer 25

colourless liquids. Because both of their boiling points are below 100°C, these liquids are described as volatile. (easily vaporised)

Question 26

Explain solubility

Answer 26

A solute will dissolve in a solvent if the forces of attraction between the solvent and the solute are great enough to overcome the forces of attraction between the solute molecules.

A solute will not dissolve if the forces of attraction between the solute and solvent are weaker than the force of attraction between the solute molecules.

Question 27

Do alcohols have higher melting and boiling points compared to corresponding alkanes?

Answer 27

Yes. The attractive forces between the alcohol molecules are stronger than those between alkane molecules, and more energy is required to break those forces.

Question 28

How to separate alcohol from water?

Answer 28

Distillation. Methanol has a lower boiling point than water. by heating the solution gently, the methanol will evaporate first, turning into a gas and leaving the liquid behind. The gaseous methanol can be collected and cooled back into a liquid.