C15 : 15.1 - Rusting

Question 1

what is corrosion

Answer 1

the destruction of materials by chemical reactions with
substances in the environment

Question 2

where does corrosion occur in a metal

Answer 2

at the surface of the metal only, where it is exposed to air

Question 3

what is rusting

Answer 3

corrosion of Iron

Question 4

what is the corrosion of iron called

Answer 4

RUSTING

Question 5

What is necessary for iron to rust

Answer 5

BOTH oxygen (air) and water

Question 6

what is the word equation for rusting

Answer 6

Question 7

what compound is rust

Answer 7

hydrated iron (III) oxide -> water loosely bonded in structure of iron oxide

Question 8

what kind of reaction is Rusting and give chemical formula (not needed)

Answer 8

REDOX reaction

Question 9

how does iron rust

Answer 9

• Rust is a soft, crumbly substance that flakes off the surface of iron easily - exposing fresh iron below which then undergoes rusting
• this means over time, all the iron rusts and its structure becomes weaker

Question 10

what affect does rusting have to iron

Answer 10

• weakens strength iron (and affects appearance)
• major concern as iron extensively used in industries like transport and construction

Question 11

how is the corrosion of aluminium different to rusting of iron

Answer 11

• aluminium also corrodes in air - but aren’t completely destroyed like iron
• Aluminium reacts with oxygen to produce aluminium oxide, Al2O3
• The aluminium oxide forms a tough protective layer that covers the aluminium, preventing further corrosion

Question 12

how to investigate the conditions needed for rusting

Answer 12

• Set up 3 test tubes
• 1st test tube : iron is dipped in water and also exposed to air
• 2nd test tube : iron is dipped in boiled water covered with a layer of oil
• 3rd test tube : iron is put in a test tube with calcium chloride (drying agent - absorbs any water)
• results :
• nail in test tube 1 rusts (in contact w/ both air and water)
• nail in test tube 2 doesn’t rust (not in contact w/ air)
• nail in test tube 3 doesn’t rust (not in contact w/ water)
• results show that both air and water must be present for rusting to occur

Question 13

what does boiling the water and layer of oil in the test tube do

Answer 13

• water boiled to remove any dissolved oxygen
• oil provides barrier to stop O2 diffusing into water

Question 14

why does the mass of the rusty nail increase

Answer 14

• because the iron atoms in the nail have now bonded to oxygen and water molecules
• resulting in a compound heavier than iron alone

Question 15

rusting speeds up in the presence of what ?

Answer 15

• electrolytes (water containing ions)
• such as salt water - presence of sodium chloride catalyses the process

Question 16

formula for rust

Answer 16

Question 17

what are the 2 main ways of preventing rust

Answer 17

• Barrier protection
• Sacrificial protection / Galvanisation

Question 18

what are egs of barrier protection and when are they used

Answer 18

• paint - can be used on big+small structures . Decorative
• oil or grease - has to be used when moving parts are involved (eg bike chains)
• electroplating
• coating with plastic - can be used on big+small structures . Decorative

Question 19

how does barrier protection work and cons of it

Answer 19

• coating w/ a barrier will prevent iron from coming in contact w/ air and water - preventing rust
• but, if coating is washed away or scratched , iron will rust
• once iron begins to rust -> continue to corrode internally as rust is porous and allows both air and water to come into contact with fresh metal underneath any barrier surfaces that have been broken or scratched

Question 20

how does electroplating work

Answer 20

• Electroplating : using electrolysis to put a thin layer of a metal on the object:
• at cathode is the iron/steel object to be plated
• at anode is the plating metal
• electrolyte contains ions of plating materials (eg metal nitrate)
• plating metal ions lose e- at anode and gain e- at cathode - where they get deposited

Question 21

how does sacrificial protection work

Answer 21

• iron coated w/ more reactive metal (eg zinc, aluminium, Mg)
• the metal firstly acts as a barrier, but if scratched - zinc around the scratch will react w/ air/water instead of iron
• since, zinc is more reactive, it will more readily lose e- to form Zn2+ ions
• the Zn atoms become oxidised readily
• iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation
• so any water/air reacts w/ zinc rather than iron - preventing it from oxidation and corrosion

Question 22

what happens if the sacrificial metal is scratched or damaged

Answer 22

the iron is still protected from rusting because zinc preferentially corrodes as it is higher up the reactivity series than iron

Question 23

what is galvanising

Answer 23

iron to be protected is coated with a layer of zinc

Question 24

when is sacrificial protection used and give eg

Answer 24

• used under harsh conditions
• eg : bars of magnesium/ magnesium alloys are attached to steel ships’ hulls
• blocks slowly corrode and protect the hull - more reactive - loses e- more easily - pushed onto the iron which prevents it from being reduced to iron(III) ions