Chemistry - Saturday 8th April

Question 1

Explain the process of fractional distillation of crude oil.

Answer 1

Fractional distillation is used to separate the alkanes according to their size.

The smaller molecules have a lower boiling point so they travel further up the chamber and are distilled higher up. The larger molecules have a lower boiling point so they condense closer to the bottom of the chamber.

Question 2

Name and describe he use of each of the fractions of crude oil’s fractional distillation products.

Answer 2

Question 3

Describe the differences between the alkanes distilled at the top versus at the bottom of the chamber.

Answer 3

As you go up the chamber, the alkanes are…

• Easier to ignite
• Flow more easily
• Are more volatile (this means they evaporate more easily)
• Have a lower boiling point

Question 4

What are the two types of cracking and how do they differ?

Answer 4

Catalytic cracking

• Uses a catalyst called silica or alumina
• The temperature is 600 to 700 degrees Celsius

Steam cracking

• Mix with steam
• The temperature is 800 degrees Celsius

Remember: Cracking breaks larger alkenes into smaller, more useful alkanes and alkenes

Question 5

What is the difference between an alkane and an alkene?

Answer 5

Alkanes and alkenes are both hydrocarbons.

Alkanes have no double bonds, whereas alkenes have at least one double C=C bond.

This means alkanes are saturated whereas alkenes are unsaturated.

Question 6

What is the chemical test used to distinguish between alkanes and alkenes?

Answer 6

Add bromine water, which is orange brown, to the test liquid.

If the colour of the mixture stays orange brown, it was an alkane - it’s not reacting.

If the colour of the mixture becomes colourless, it was an alkene.

Question 7

How do alkenes react with halogens?

Answer 7

It is an addition reaction.

Both atoms of the halogen attach - so there’s only one product, the halogenoalkane

Ethene + Bromine → Dibromoethane

Question 8

How do alkanes react with halogens?

Answer 8

Requires the presence of UV light

Substitution reaction: Replacing a H with the Halogen

Product includes Hydrogen-Halogen

Question 9

How does the BP and MP of Noble Gases change as you go down the group?

Answer 9

BP and MP increase as you go down the group as the RAM (relative atomic mass) gets bigger so more energy is needed to overcome the forces of attraction between the atoms.

Question 10

What do alkali metals have in common?

Answer 10

Soft metals

First 3 are less dense than water - this means they float on water

They all have one electron on the outer shell

Alkali metals = Group 1, lose one electron when they react, called alkali metals as they form hydroxides when react with water.

Question 11

Describe the word equation for the reaction between an alkali metal and water.

Answer 11

G1 Metal + Water —> Metal Hydroxide + Hydrogen

Question 12

Describe the difference between Lithium, Sodium and Potassium in their reactions with water.

Answer 12

Question 13

Write the word equation for the reaction between an alkali metal and oxygen.

Answer 13

G1 Metal + Oxygen –> G1 Metal Oxide

Question 14

What colour are group1 metal oxides?

Answer 14

White solids

Question 15

Describe the reaction between an alkali metal and chlorine.

Answer 15

Question 16

What happens when you dissolve chlorides in water?

Answer 16

They form colourless solutions.

Question 17

Describe the trends that occur as you go down group 1.

Answer 17

As you go down group 1…

• Density increases
• MP and BP decreases - as you go down, the ions get larger, which means the attraction between the nucleus and outer electron is weaker - the electrostatic forces get weaker - so less energy is needed to overcome the attraction so MP/BP gets lower
• Reactivity increases as you go down the group because it is easier to lose the outer electron. This is because the attraction between the nucleus and outer electron is weaker. The attraction is weaker because as you go down the group the distance from the nucleus to the outer electron increases and there are more shells in-between the nucleus and outer electron, so the shielding effect increases.

Question 18

What do chlorides look like as solids?

Answer 18

White solids.

Question 19

What do all halogens have in common?

Answer 19

7 electrons in the outer shell

Halogens form diatomic molecules e.g. Cl2, I2, Br2

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. These are called displacement reactions.

Question 20

How does boiling and melting point change as you go down Group 7?

Answer 20

As you go down the group, the size of the atoms and therefore the size of the molecules get bigger.

The bigger the molecules, the stronger the intermolecular forces.

This means more energy is needed to overcome them, hence BP and MP increases.

Question 21

How does reactivity change as you go down Group 7?

Answer 21

Reactivity decreases as you go down the group because…

1. As you go down the group, the atoms have more shells
2. This means the distance between the nucleus and the outer electron increases
3. This also means that there are more shells in-between the nucleus and outer electron, so the shielding effect increases
4. Therefore the electrostatic attraction between the outer electron and the nucleus is weaker
5. This means it is more difficult to gain the outer electron, making it less reactive.

Question 22

What does chlorine, bromine and iodine look like?

Answer 22

Chlorine = green gas

Bromine = red-brown liquid

Iodine = grey-black solid

As you go down the group, the colour gets darker and the MP increases so it goes gas, liquid, solid.

Question 23

What do chlorides look like?

Answer 23

Chlorine is a green gas.

Question 24

What happens when I react Bromine with Potassium Iodide?

Answer 24

The bromine displaces the iodine, to get Iodine and Potassium Bromide

The brown bromine liquid would disappear. A black solid would appear.

Question 25

What are the differences between transition metals and alkali metals?

Answer 25

• All group 1 metals form ions with a +1 charge but the transition metals can form ions with variable charges
  
  • E.g. Iron can form Fe(II) or Fe(III)
• Transition metals are much harder, stronger and denser than the group 1 metals, which are very soft and light
• Transition have much higher melting points e.g. titanium melts at 1,688 ºC whereas potassium melts at only 63.5 ºC, not far off the average cup of tea!
• Transition metals are much less reactive than the group 1 metals
• Transition metals react very slowly or do not react with water, oxygen and halogens whereas the alkali metals do.
  
  • A group 1 metal will tarnish in the presence of oxygen as a metal oxide is formed.
  • Iron on the other hand can take several weeks to react with oxygen to form iron oxide (rust) and requires the presence of water.

Question 26

What is one of the main uses of transition metals?

Answer 26

Transition metals are very useful as catalysts.