Alkanes (page 92-93)

Question 1

What are Alkanes?

Answer 1

Alkanes are Saturated Hydrocarbons.

Question 2

What is the general formula CnH2n+2 for Alkanes?

Answer 2

alkanes general formula is CnH2n+2. They’ve got only carbon and hydrogen atoms, so they’re hydrocarbons.

Question 3

How many single bonds do every carbon atom in an alkane have with other atoms?

Answer 3

they have four single bonds with other atoms.

Alkanes are saturated - all the carbon-carbon bonds are single bonds.

Look at diagram 1 on page 92 of a few examples of alkanes. (metane, ethane, propane & cyclohexane - (cycloalkanes have two fewer hydrogens than alkanes. Their general formula is CnH2n).

Question 4

Alkane Molecules are Tetrahedral around each carbon, explain what this means?

Answer 4

In an alkane molecule, each carbon atom has four pairs of bonding electrons around it. They all repel each other equally. So the molecule forms a tetrahedral shape around each carbon. Each bond angle is 109.5°

(if you draw lines joining up the Hs in CH4, the shape you get is a tetrahedron). - see diagram 2 on page 92.

Question 5

How many carbons do….

a) Methane have?
b) Ethane have?
c) Propane have?

Answer 5

a) Methane has 1 carbon (1 tetrahedral carbon)
b) Ethane has 2 carbons, (2 tetrahedral carbons)
c) Propane has 3 carbons, (3 tretrahedral carbons)

see diagram 2 on page 92 of tetrahedral around each carbon.

for more about shapes of molecules see pages 46-47.

Question 6

What is the Boiling Point of an Alkane?

Answer 6

this depends on its size and shape.

Question 7

The smallest alkanes, like methane, are gasses at room temperature and pressure, what boiling points do they have?

Answer 7

they have very low boiling points.

(larger alkanes are liquids - they have higher boiling points).

Question 8

Alkanes have what bonds inside the molecules?

Answer 8

alkanes have ‘covalent bonds’ inside the molecules. between the molecules, there are induced dipole-dipole interactions (also called london forces) which hold them all together.

Question 9

Why is it, the longer the carbon chain, the stronger the induced dipole-dipole interaction is?

Answer 9

This is because there’s more surface contact and more electrons to interact.

As the molecules get longer, it takes more energy to overcome the induced dipole-dipole interactions, and the boiling point rises.

Question 10

A branched-chain alkane has a lower boiling point thant its straight-chain isomer, why?

Answer 10

Branched-chain alkanes can’t pack closely together and they have smaller molecular surface areas - so the induced dipole-dipole interactions are reduced.
look at example on page 92 diagram 3.

Question 11

If you burn (oxidise) alkanes with oxygen, what do you get?

Answer 11

you get carbon dioxide and water - this is a combustion reaction.

Question 12

What is the equation for the combustion of propane?

Answer 12

C3H8(g) + 5O2(g)&raquo_space;» 3CO2(g) + 4H2(O)g

Question 13

Why do combustion reactions happen between gasses?

Answer 13

so liquid alkanes have to be vaporised first. Smaller alkanes turn into gases more easily (they’re more volatile), so they’ll burn more easily too.

Question 14

Why do larger alkanes release heaps more energy per mole?

Answer 14

because they have more bonds to react.

(because they release so much energy when they burn, alkanes make excellent fuels).

Question 15

Combustion reactions happen between gases. All gases at the same temperature and pressure have the same molar volume. This means you can use the ratio of the volumes of gases reacting together to calculate the molar ratios, and then work out what hydrocarbon is combusting.

Give an example how you work this out using:

30 cm³ of hydrocarbon X combusts completely with 240 cm³ oxygen.
150 cm³ carbon dioxide is produced. What is the molecular formular of hydrocarbon X?

Answer 15

1) using the volumes provided, the reaction equation can be written: 30X + 240O²&raquo_space;> 150CO² + ?H²O
2) this can be simplified by dividing everything by 30: X + 80² 》》》 5CO² + nH²O
3) 8 moles of oxygen reacts to form 5 mole of carbon dioxide and n moles of water. So any oxygen atoms that don’t end up in CO² must be in H²O. This means that n = (8 x 2) - (5 x 2) = 6.
4) This means the combustion equasion is: X + 80² 》》》 5CO² + 6H²O. you can use this to identify X.
5) All the carbon atoms from X end up in carbon dioxide molecules, and all the hydrogen atoms from X end up in water, so the number of carbon atoms in X is 5 and the number of hydrogen atoms in X is 12.
6) The molecular formular of X is C5H12.

see equation on page 93

Question 16

Burning Alkanes in limited oxygen produces what?

Answer 16

carbon monoxide.

(if there isn’t much oxygen around, the alkan will still burn, but it will produce carbon monoxide and water.

e.g. burning methane with not much O2 - 2CH4(g) + 3O2(g) &raquo_space;» 2CO(g) + 4H2O(g).

Question 17

Carbon Monoxide is poisonous. how is this dangerous in your body?

Answer 17

1) The oxygen in your bloodstream is carried around by haemoglobin.
2) Carbon monoxide is better at binding to haemoglobin than oxygen is, so it binds to the haemoglobin in your bloodstream before the oxygen can.
3) this means that less oxygen can be carried around your body, leading to oxygen deprivation. At very high concentrations, carbon monoxide can be fatal.

Question 18

What is the general formula for alkanes?

Answer 18

the general formula is CnH2n+2 (they’ve only got carbon and hydrogen atoms so they’re hydrocarbons.

(remember n is the numer of carbon atoms in the molecule).

Question 19

What is the H-C-H bond angle in a molecule of methane?

Answer 19

109.47°

bond angle: the angle formed by three contiguous bonded atoms, the H-C-H bond angles in methane are all equal at 109.47°

Question 20

What kind of intermolecular forces are there between alkane molecules?

Answer 20

Alkane molecules have weak Van der Waals forces between them. As C-C bonds are non-polar and C-H bonds are practically polar molecules which can therefore only be attracted to each other by induced dipole-dipole interactions.

Question 21

The alkane ethane is a saturated hydrocarbon. It is mostly unreactive, but will react with oxygen in the combustion reaction.

a) what is a saturated hydrocarbon? (2 marks)
b) write a balanced equation for the complete combustion of ethane (2 marks)

Answer 21

a) One with no double bonds OR all the carbon-carbon bonds are single bonds [1mark] It contains only hydrogen and carbon atoms [1mark]

b) C2H6(g) + 3½O2(g) + 3H2O(g) (2marks).

1 mark for the correct symbols, 1 mark for balancing.

Question 22

Noanne is a hydrocarbon with the formula C9H20

a) which would you expect to have a higher boiling point, nonane or 2,2,3,3,-tetramethylpentane? Explain your answer (2 marks)

b) When noname burns in a limited air supply, the products are carbon monoxide and water.

i) Write a balanced equation for this reaction (1 mark)
ii) Explain why carbon monoxide is such a dangerous gas? (2 marks)

Answer 22

a) Noanne will have a higher boiling point than 2,2,3,3, - tetramethylpentant (1 mark) beacuse the molecules of branched-chain alkanes like 2,2,3,3,- tetramethylpentane are less cloesly packed together than their straight-chain isomers, so they have fewer induced dipole-dipole interactions holding them together (1 mark).

b)
i) C9H20 + 9½O2 &raquo_space;» 9CO + 10H2O (1mark).
ii) carbon monoxide binds to haemoglobin in the blood in preference to oxygen (1 mark), so less oxygen can be carried around the body, leading to oxygen deprivation (1 mark).

Question 23

Alkanes are the simplest organic compounds you’re going to meet. They’re very stable so they don’t get u to much. Make sure you can explain whay different alkanes have different boiling points, even if their molecular formulas are the same. why?

Answer 23

It’s all about the strength of the intermolecular forces and how closely the molecules can pack together. You need to remember this x