SC5 - Chemical Changes (excluding calculations)

Question 1

What is electrolysis?

Answer 1

Electrolysis is the use of electricity to split a compound up into the elements it is made from.

Question 2

Which electrode is positive and negative?

Answer 2

Positive
Anode
Negative
Is
Cathode

Question 3

How do you do molten electrolysis?

Answer 3

Heat up an ionic compound until it has fully melted (the ions can now move so it can conduct electricity). Place it in a beaker with two electrodes connected to a direct current (d.c.) supply. Turn on the electricity. The metal from the compound forms at the cathode, the non-metal forms at the anode (usually as a gas).

Question 4

What is solution electrolysis?

Answer 4

When a compound is dissolved in solution and electrolysed.

Question 5

What forms at the anode for solution electrolysis?

Answer 5

If the non-metal compound contains halide ions, the halogen will form at the anode, if there are no halide ions, oxygen from the water forms at the anode.

Question 6

What forms at the cathode for solution electrolysis?

Answer 6

If the metal from the compound is less reactive than hydrogen (usually copper or silver), the metal will form, if it is more reactive than hydrogen, hydrogen will form.

Question 7

How can copper be purified using electrolysis?

Answer 7

The anode is made of the impure copper you want to purify while the copper cathode is very pure. During electrolysis, the copper atoms in the anode lose two electrons each to become copper ions, they dissolve in the solution and migrate to the cathode, where they are deposited as pure copper. The impure copper anode loses mass while the pure copper cathode gains mass. Impurities from the anode don’t form ions, they collect at the bottom and form a ‘sludge’.

Question 8

Core practical - electrolysis of copper sulphate solution

Answer 8

Using copper electrodes:

• select two clean pieces of copper foil, label one anode and the other cathode, measure and record the mass of each
• set up an electrolysis circuit (with an ammeter and a variable resistor)
• turn in the power and adjust the variable resistor to 0.2A, leave the power on for 20 mins
• turn off the power and remove the electrodes from the beaker, wash the electrode with water and dip them in propanone
• measure and record the masses of the dry electrodes
• repeat for currents of 0.3A, 0.4A and 0.5A

Using graphite electrodes:

• set up an electrolysis circuit
• turn on the power and observe what happens at each electrode
• test for gases

Question 9

What are displacement reactions?

Answer 9

When a more reactive metal displaces a less reactive metal in a compound.

E.g. magnesium + copper sulphate —> magnesium sulphate + copper

Question 10

What is an ore?

Answer 10

A rock which contains enough metal that it is economic to mine the rock and extract the metal.

Question 11

How are gold and platinum extracted?

Answer 11

They aren’t - they are found as pure metals naturally as they are inert.

Question 12

How are copper, lead, iron and zinc extracted?

Answer 12

Their ores are heated with carbon as the metals are less reactive than carbon, so carbon displaces them to form the metal and carbon dioxide.

Question 13

Why can’t metals more reactive than zinc be extracted with carbon?

Answer 13

They are more reactive than carbon, so carbon can’t displace them.

Question 14

How are aluminium, magnesium, calcium, sodium and potassium extracted?

Answer 14

Their ores are melted and electrolysed (metal forms at cathode, oxygen forms at anode (but reacts with hot carbon electrodes to form carbon dioxide)).

Question 15

Why is molten aluminium oxide mixed with molten cryolite when aluminium is extracted?

Answer 15

Aluminium oxide has a very high melting point, whereas cryolite’s melting point is much lower, so the addition of molten cryolite lowers the melting point, meaning less energy is required, this saves money and helps the environment.

Question 16

What is bioleaching?

Answer 16

A biological method of copper extraction which can extract copper from lower grade ores. Bacteria concentrate the copper compounds in a solution (leachate). Copper can then be extracted from the leachate by displacement with scrap iron.

Question 17

What is phytoextraction?

Answer 17

A biological method of copper extraction which can extract minerals from contaminated soil. Plants are grown in contaminated soil, so copper compounds are concentrated in the plants, which are burnt to get the copper compounds (in the ash).

Question 18

What are the advantages and disadvantages of both bioleaching and phytoextraction?

Answer 18

Advantages:

• no harmful gases produced
• causes less damage to the landscape than mining
• preserves supplies of higher grade ores

Disadvantages:
- very slow

Question 19

What are the advantages and disadvantages of bioleaching?

Answer 19

Advantages:
-does not require high temperatures

Disadvantages:
- toxic substances and sulphuric acid can be produced and damage the environment

Question 20

What are the advantages and disadvantages of phytoextraction?

Answer 20

Advantages:
- can extract minerals form contaminated soils

Disadvantages:

• more expensive than mining some ores
• growing plants is dependent on weather

Question 21

What are transition metals?

Answer 21

The transition metals come between groups 2 and 3 on the periodic table. They have many uses, including aeroplane and car manufacture, civil engineering, electronics and plumbing.

Question 22

What are the properties of transition metals?

Answer 22

• high melting points
• high density
• good conductors of heat and electricity
• malleable and ductile
• less reactive than group 1 or group 2 metals
• often act as catalysts
• form coloured compounds
• elements can form different ions (iron can form Fe2+ or Fe3+)

Question 23

What is corrosion?

Answer 23

Corrosion is when a metal reacts with oxygen, making the metal weaker over time. The corrosion of iron requires water as well as air and is called rusting. Corrosion reactions are oxidation.

Question 24

Why doesn’t aluminium corrode?

Answer 24

Aluminium is quite reactive, so you’d expect it to corrode, but it doesn’t. This is because the surface atoms react quickly to form aluminium oxide. This oxide layer protects the rest of the metal and stops it from corroding (a tarnish).

Question 25

How can rusting be prevented?

Answer 25

Painting/ oiling
Using a desiccant to remove water
Coating with plastic

Question 26

How can corrosion be prevented?

Answer 26

Painting/oiling/greasing
Sacrificial protection (attach a more reactive metal to the metal you want to protect - the oxygen will react with the more reactive metal first, protecting the less reactive one)
- galvanising (coating the metal you want to protect in a more reactive metal - this acts as a barrier to air and if it gets scratched, the metal underneath will still be protected as it’s less reactive)

Question 27

What is electroplating?

Answer 27

Electroplating is when electrolysis is used to coat the surface of one metal with a thin layer of another (e.g. silver plated jewellery). The anode is the substance you want to coat, the cathode is the substance you want to coat it in (e.g. silver) and the solution must contain the metal at the cathode (e.g. silver nitrate solution).

Question 28

Why is electroplating used?

Answer 28

To make products look more attractive by plating a cheaper metal in a more expensive one.
To prevent corrosion (e.g. chromium plated taps)
To make product better at conducting electricity (e.g. gold plated cables)

Question 29

What is an alloy?

Answer 29

An alloy is a mixture of metals with one or more elements, usually other metals.

Question 30

Why are alloys created?

Answer 30

Alloys are created as they are much stronger than pure metals due to distorted layers being harder to slide. They can also be used to create substances like stainless steel.