Developing Metals 5: Rusting Flashcards
What compound is rust and give its formula
Hydrated iron ( III ) oxide
= Fe2 O3 .xH2O
When does rusting occur
When both water and oxygen are present
What kind of process is rusting
An electrochemical process involving 2 redox half equations
Give half equation for the formation iron (II) ions , Fe 2+
Fe( s) —> Fe 2+(aq) +2e-
Give a half equation for the formation of hydroxide ions, OH-
O2 (aq) + 2H2O(l) +4e- —> 4OH-(aq)
In rusting out of the 2 half equation involving iron and oxygen explain what is reduced and what oxidised and why
- 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
What happens after Fe2+ and OH- ions are produced
- 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
Name 3 methods of preventing rust
- Coating the iron
- Sacrificial Protection ( Galvanising )
- Alloying
Describe how coating iron prevents rust
- add layer of paint/oil/grease
- prevent oxygen and water coming into contact with the iron
Describe how sacrificial protection ( galvanising ) prevents rust
- galvanising = coat iron with zinc
- zinc more reactive than iron = more likely to form ions
- = any Fe2+ present is reduced to Fe
Describe how alloying prevents rust
- iron can be alloyed with nickel / carbon / chromium
- presence of other elements helps prevent Fe2+ forming and electrons being released
Give ionic equation for the formation of green rust
Fe2+( aq ) + 2OH-(aq) —> Fe(OH)2 ( s )
Give the oxidation state of Fe in red- brown rust
+3
In some conditions why is green rust formed rather than red-brown rust
Low oxygen
