Developing Metals 5: Rusting

Question 1

What compound is rust and give its formula

Answer 1

Hydrated iron ( III ) oxide

= Fe2 O3 .xH2O

Question 2

When does rusting occur

Answer 2

When both water and oxygen are present

Question 3

What kind of process is rusting

Answer 3

An electrochemical process involving 2 redox half equations

Question 4

Give half equation for the formation iron (II) ions , Fe 2+

Answer 4

Fe( s) —> Fe 2+(aq) +2e-

Question 5

Give a half equation for the formation of hydroxide ions, OH-

Answer 5

O2 (aq) + 2H2O(l) +4e- —> 4OH-(aq)

Question 6

In rusting out of the 2 half equation involving iron and oxygen explain what is reduced and what oxidised and why

Answer 6

• Fe(s) is oxidised ( OIL ) to Fe2+ because E value is more -ve
• O2(aq) is reduced ( RIG ) to 4OH-(aq)
  because E value is more positive

Question 7

What happens after Fe2+ and OH- ions are produced

Answer 7

• Fe2+ and OH- ions combine to form Fe(OH)2 (s)
• Fe(OH)2 is oxidised to form iron (III) hydroxide
• iron ( III ) hydroxide partially dehydrates to give rust, Fe2 O3 .xH2O

Question 8

Name 3 methods of preventing rust

Answer 8

1. Coating the iron
2. Sacrificial Protection ( Galvanising )
3. Alloying

Question 9

Describe how coating iron prevents rust

Answer 9

• add layer of paint/oil/grease

- prevent oxygen and water coming into contact with the iron

Question 10

Describe how sacrificial protection ( galvanising ) prevents rust

Answer 10

• galvanising = coat iron with zinc
• zinc more reactive than iron = more likely to form ions
• = any Fe2+ present is reduced to Fe

Question 11

Describe how alloying prevents rust

Answer 11

• iron can be alloyed with nickel / carbon / chromium

- presence of other elements helps prevent Fe2+ forming and electrons being released

Question 12

Give ionic equation for the formation of green rust

Answer 12

Fe2+( aq ) + 2OH-(aq) —> Fe(OH)2 ( s )

Question 13

Give the oxidation state of Fe in red- brown rust

Answer 13

+3

Question 14

In some conditions why is green rust formed rather than red-brown rust

Answer 14

Low oxygen