Topic 3 - Covalent Compounds and Separation Techniques

Question 1

What is a covalent bond?

Answer 1

A shared pair of electrons between atoms

Question 2

Why do atoms bond covalently?

Answer 2

To get a full outer shell

Question 3

What is formed when atoms covalently bond with one or more other atoms?

Answer 3

A molecule

Question 4

How is hydrogen, H2, bonded together?

Answer 4

-Covalently bond to share a pair of electrons so both have 2 in outer shell
-Form single covalent bonds
H–H

Question 5

How is hydrogen chloride, HCl, bonded together?

Answer 5

-Atoms need one more electron for full outer shell
-Single covalent bond formed
H–Cl

Question 6

How is methane, CH4, bonded together?

Answer 6

-Four covalent bonds
-Carbon needs 4 for full outer shell (half full)
-Hydrogen needs 1
H
I
H–C–H
I
H

Question 7

How is oxygen, O2, bonded together?

Answer 7

• Oxygen has 6 outer electrons
• Sometimes form ionic bonds by taking 2 electrons
• Share 2 pairs of electrons
• Covalent bondO=O

Question 8

How is water, H2O, bonded together?

Answer 8

-Covalent bonds
-Oxygen shares electrons w/ hydrogen
O
/ \
H H

Question 9

How is carbon dioxide, CO2, bonded together?

Answer 9

-Covalent bond
-Double covalent bond
-2 pairs of electrons shared
O=C=O

Question 10

What are the bonds like in a simple molecular covalent substance?

Answer 10

Very weak intermolecular forces

Question 11

How strong is a covalent bond?

Answer 11

Very strong within the atoms of the molecule

Question 12

What are the properties of a simple molecular covalent substance?

Answer 12

• Low melting + boiling points (molecules easily parted from each other as weak intermolecular forces)
• Most are gas/liquid at room temp.
• Don’t conduct electricity (no ions)

Question 13

What is the difference between and ionic lattice and a giant molecular covalent substance?

Answer 13

No charged ions in giant molecular covalent substance

Question 14

What type of bond is within a giant molecular covalent substance?

Answer 14

Strong covalent bonds

Question 15

What are the properties of giant molecular covalent bonds?

Answer 15

• Very high melting + boiling points
• Don’t conduct electricity, even when molten (except graphite)
• Usually insoluble in water

Question 16

What is the structure of a diamond atom?

Answer 16

• Made of carbon atoms
• Forms 4 covalent bonds
• Rigid giant covalent structure

Question 17

What can diamond be used for?

Answer 17

Cutting tools (very hard)

Question 18

What are the properties of diamond?

Answer 18

• Really hard (giant covalent structure)

- Doesn’t conduct electricity (no delocalised electrons)

Question 19

What is the structure of graphite?

Answer 19

• Made of carbon atoms
• Has layers that can slide over each other
• Layers are held together loosely so can be rubbed off easily
• Each carbon atom forms 3 bonds (out of 4) so lots of delocalised electrons

Question 20

What are the properties of graphite?

Answer 20

• Layers can easily slide over each other
• Layers can be rubbed off from one another
• Can conduct electricity (lots of delocalised electrons)

Question 21

What can graphite be used for?

Answer 21

• Lubricant (layers slide over each other)
• Pencil (layers easily rubbed off from one another)
• Electrodes (lots of delocalised electrons so can conduct electricity)

Question 22

What does miscible mean?

Answer 22

When 2 substances (liquids) mix together and don’t separate

Question 23

What does immiscible mean?

Answer 23

When 2 substances (liquids) mix together but separate from one another back to original substance

Question 24

How can you separate immiscible liquids from each other?

Answer 24

• Separating funnel
• 2 immiscible liquids shaken together + allowed to stand they separate into layers
• Denser liquid sinks to bottom + less dense on top
• Layers separated by separating funnel w/ tap
• Tap opened to drain denser liquid into beaker
• e.g. mixture of oil + water

Question 25

How can you separate miscible liquids from each other?

Answer 25

• Fraction distillation
• Liquids must have different boiling points
• If mixture heated, different liquid condense at different temps. so can be collected separately

Question 26

How could you fractionally distil liquid air for use in industry?

Answer 26

• Air filtered to remove dust
• Cooled to around -200⁰C + become miscible mixture of liquids
• During cooling water vapour condenses + is removed
• Carbon dioxide freezes + is removed
• Liquefied air enters fractionating column + heated slowly
• Gases separated by fractional distillation; oxygen + argon come out together so another column used to separate them

Question 27

What can chromatography be used for?

Answer 27

To identify different substances in a mixture

Question 28

Why does chromatography work?

Answer 28

Uses fact different substances wash through wet filter paper at different rates

Question 29

How would you do a chromatography experiment?

Answer 29

• Put spots of each mixture tested on pencil baseline on filter paper
• Put into beaker containing a solvent e.g. ethanol/water. Baseline must be above level of solvent
• Solvent seeps up paper taking samples with it
• Different chemicals in sample form different spots on paper
• Chromatograph forms

Question 30

What is the result of a chromatography called?

Answer 30

• Chromatograph

- e.g. if it had 4 different spots at least 4 different substances in sample mixture

Question 31

How do you interpret chromatograph?

Answer 31

• Chromatography for unknown mixture
• Chromatography for known mixtures
• See which known chromatograph the unknown chromatograph matches

Question 32

What is an R~f value?

Answer 32

Ratio between distance travelled by dissolved substance + distance travelled by solvent (in chromatography)

Question 33

What is the equation to work out the R~f from chromatography?

Answer 33

R~f = distance travelled by substance / distance travelled by substance

Question 34

What could the R~f (from chromatography) be used for?

Answer 34

• Used in forensic science + food industry

- Scientists keep tables of values + refer to them to identify substances e.g. food additives, drugs