C2.3 Covalent compounds and separation techniques

Question 1

What are most non-metal compounds held together by?

Answer 1

Covalent bonds

Question 2

What are covalent bonds?

Answer 2

A bond formed when a pair of electrons is shared between two atoms

Question 3

What shell do the electrons making a covalent bond come from?

Answer 3

The outermost shell

Question 4

What is a molecule?

Answer 4

Two or more atoms joined together by covalent bonds

Question 5

What is a dot and cross diagram used for?

Answer 5

Showing electronic structures in covalent substances

Question 6

Why can covalent bonds form between non-metal elements?

Answer 6

Non-metal elements all don’t have enough electrons to have a full outermost shell

Question 7

What is the bond that allows atoms to share two pairs of electrons called?

Answer 7

A double bond

Question 8

What are simple molecular covalent substances?

Answer 8

A substance with molecules that are being held together with strong covalent bonds, but that have weak forces between themselves

Question 9

Why are simple molecular covalent substances poor conductors of electricity?

Answer 9

The atoms in simple molecular covalent substances have not lost or gained any electrons and so there aren’t any charged particles that can move around

Question 10

Why do simple molecular covalent substances have low melting and boiling points?

Answer 10

Because there are weak forces between neighbouring molecules, so it doesn’t take a lot of thermal energy to break the bonds

Question 11

What are giant molecular covalent substances?

Answer 11

Billions of atoms all joined together by covalent bonds

Question 12

What two substances can covalent compounds form?

Answer 12

• simple molecular covalent substances

- giant molecular covalent substances

Question 13

Why do giant molecular covalent substances have high melting and boiling points?

Answer 13

Because all the atoms are joined together by strong covalent bonds, and a lot of thermal energy is needed to break them

Question 14

What two atoms is sand made out of?

Answer 14

Silicon and oxygen

Question 15

What two different types of giant molecular structure can carbon form?

Answer 15

Diamond and graphite

Question 16

Why do diamond and graphite have very different properties?

Answer 16

Because of the way the carbon atoms are arranged

Question 17

What are two properties of diamond?

Answer 17

• it’s very hard

- it doesn’t conduct electricity

Question 18

What are the properties of graphite?

Answer 18

• easily rubs away in layers
• soft enough to be used as a lubricant
• conducts electricity

Question 19

Why does graphite easily rub away in layers?

Answer 19

Because there are strong covalent bonds within layers but weak forces between the layers

Question 20

What do immiscible liquids do?

Answer 20

Not mix completely with each other

Question 21

What happens when you shake immiscible liquids?

Answer 21

They seem to mix but soon separate out again

Question 22

How can you separate immiscible liquids?

Answer 22

Using a separating funnel

Question 23

What happens when two liquids dissolve in each other?

Answer 23

Their particles mix completely to form a solution

Question 24

What is a miscible solution?

Answer 24

A solution where two mixed liquids do not separate out once mixed

Question 25

How can you separate out a miscible solution?

Answer 25

Through fractional distillation

Question 26

Why does fractional distillation work on separating miscible liquids?

Answer 26

Because they have different boiling points

Question 27

What happens during fractional distillation?

Answer 27

- the mixture of liquids is heated and the liquids evaporate - the vapours condense in a fractionating column - the fraction with the highest boiling point condenses near the bottom of the column, and the fraction with the lowest boiling point condenses near the top

Question 28

What must be done before the mixture of liquids is heated for fractional distillation?

Answer 28

Nitrogen and water must be taken out of the air

Question 29

How is water taken out of air in preparation for fractional distillation?

Answer 29

The air is cooled so that the water can freeze and be removed

Question 30

What happens to the remaining air once the water has been removed in preparation for fractional distillation?

Answer 30

It is cooled to -200*C to be liquefied

Question 31

How is nitrogen taken out of the air when preparing for fractional distillation?

Answer 31

The liquefied air is warmed up to -185*C to cause the nitrogen to evaporate and go to the top of the column

Question 32

What happens to oxygen once nitrogen and water have been taken out in preparation for fractional distillation?

Answer 32

It stays as a liquid and is piped out the bottom of the column

Question 33

What part of the fractional distillation column is the coldest?

Answer 33

The top

Question 34

Why is the column warmed up to -185*C ?

Answer 34

- 185*C is above the boiling point of nitrogen so the nitrogen boils and evaporates - 185*C is below the boiling point of oxygen so the oxygen stays a liquid

Question 35

What difference between compounds does chromatography use to separate different components of a mixture?

Answer 35

Their solubilities

Question 36

How does paper chromatography work?

Answer 36

- samples at are placed near the bottom of a special sheet of paper - the bottom of the paper is placed touching the solvent but not the samples - as the solvent travels up the paper it carries the different samples at different speeds so they are separated

Question 37

What is the sheet of paper produced as a result of paper chromatography called?

Answer 37

A chromatogram

Question 38

Why is a lid used during paper chromatography?

Answer 38

To stop the solvent evaporating

Question 39

What is the R_f value?

Answer 39

The distance the compound has risen divided by the distance the solvent has risen

Question 40

What are R_f values used for?

Answer 40

Identifying different compounds

Question 41

What are three uses of chromatography?

Answer 41

- to identify food colourings and to check the safety of food - to identify DNA samples to work out if suspects have been at a crime scene - to analyse paints and dyes to restore paintings

Question 42

What structure is sand made out of?

Answer 42

Giant covalent

Question 42

Why is diamond very hard?

Answer 42

All the atoms are joined together with strong covalent bonds

Question 42

Why doesn't diamond conduct electricity?

Answer 42

There are no free electrons to move around

Question 43

Why does graphite conduct electricity?

Answer 43

There is one electron from each carbon atom that can move along the layers

Question 44

How cold is the top of a fractional distillation column?

Answer 44

-190*C