2.2 Redox, Rusting and Iron

Question 1

What is oxidation?

Answer 1

Oxidation is the gain in oxygen, loss of hydrogen and loss of electrons.

Question 2

What is reduction?

Answer 2

Loss of oxygen, gain of hydrogen and gain of electrons.

Question 3

What is a redox reaction?

Answer 3

A redox reaction is one in which oxidation and reduction occur at the same time (simultaneously).

Question 4

How do you answer a three mark redox question (exam technique)?

Answer 4

-Name the substance that undergoes gain/loss in oxygen/hydrogen.
-Gain/loss of oxygen/hydrogen is oxidation is reduction.
-Name substance that has been oxidised/reduced.

OIL RIG

Question 5

What is rust, what is its appearance and what is its formula?

Answer 5

Rust is hydrated iron (III) oxide, which is a red brown solid, and its formula is: Fe2O3.xH2O

Question 6

What are displacement and combustion reactions?

Answer 6

A displacement reaction is one in which a more reactive element will form ions and cause the ions of a less reactive metal to change to atoms. The process involves the transfer of electrons. The more reactive element displaces the less reactive element from a compound.

A combustion reaction involves oxygen being added to an element and so the element is oxidised (the burning of fuels).

Question 7

What conditions are required for iron to rust?

Answer 7

When iron is exposed to air (oxygen) and moisture (water in the air), the iron rusts.

Question 8

Describe the method for the experiment to investigate what factors cause rust

Answer 8

Method:
Tube A- put a clean nail into a boiling tube containing a little distilled water and place a bored bung on top of the boiling tube.
Tube B- boil some distilled water for a few minutes in a beaker and then allow it to cool. Put a nail in the boiling tube and then cover it with water. Pour about 0.5cm depth of olive oil onto the water surface. Stopper the tube.
Tube C- put about 2cm depth of anhydrous calcium chloride granules into a boiling tube, followed by a plug of cotton wool. Put a nail in the tube and stopper it.
Leave all the tubes undisturbed for two to three days before re-examining the nails.

Question 9

Describe the results for the experiment to investigate what factors cause rust

Answer 9

Tube A (oxygen and water)- red brown solid forms on the nail- rust forms.
Tube B (water: boiled water has less oxygen)- nail stays a shiny silvery grey solid- no rust forms.
Tube C (dry air: no water as CaCl2 dries moisture in the air)- nail stays a shiny silvery grey solid- no rust forms.

Question 10

Describe how a barrier or protective layer is used to prevent rust from forming

Answer 10

Preventing the surface of the iron coming into contact with water and air by using a barrier or protective layer.
• Paint is used to protect cars, bridges and railings
• Oil or grease are used to protect tools and machinery
• Plastic coating- such as that used to cover garden chairs and dish racks. Car manufacturers are increasingly using plastic in cars to reduce the problem of rust
• Plating with another metal: tin is used in making cans for food- these are made from steel and are coated on both sides with a thin layer of tin. Tin is unreactive and non-toxic. It is deposited on the steel by electrolysis. Chromium is used to coat steel giving it a shiny attractive appearance. This is used for some vehicle bumpers and bicycle handlebars. Chromium can be applied by electrolysis.

Question 11

Describe how putting a more reactive metal into contact with the iron or steel prevents rust from forming

Answer 11

Putting a more reactive metal in contact with the iron or steel. The more reactive metal reacts first, leaving the iron intact.
• Bars of magnesium are attached to the sides of ships and underwater pipes to prevent rusting. The magnesium corrodes instead of the iron or steel and must be replaced with fresh magnesium periodically. This method is called sacrificial protection.
• Iron can be coated with zinc- this method is called galvanising. Zinc is more reactive than iron and oxidises to form a layer of zinc oxide readily. Galvanising protects by sacrificial protection if the surface is scratched, and also the zinc oxide provides a barrier to air and water.

Question 12

Describe how alloying prevents iron from forming

Answer 12

Alloying- an alloy is a mixture of two or more elements, at least one of which is a metal. Alloys have metallic properties. Alloys are often stronger and more resistant to corrosion than pure metals. Stainless steel is an alloy resistant to corrosion.

Question 13

What is the ore from which iron is extracted called and what is it mostly made up of?

Answer 13

Haematite- mostly iron (III) oxide with the formula Fe2O3.

Question 14

How is iron extracted from its ore?

Answer 14

The iron oxide has to be reduced. This reduction is carried out with coke which is a form of carbon. Coke is made from coal.

Question 15

What the three substances which make up the charge for the blast furnace?

Answer 15

-Coke (Carbon, C)
-Iron ore (haematite, mainly Fe2O3 with sand impurities, SiO2.
-Limestone (calcium carbonate, CaCO3).

Question 16

How is iron extracted in the blast furnace

Answer 16

-The hot air that enters at the bottom of the furnace reacts with coke (carbon) to form CO2. This gas then reacts with more coke to produce CO.
-The CO gas then comes in contact with the iron ore (haematite) containing Fe2O3 and reduces it to iron. The molten iron trickles down to the bottom of the furnace where it is removed.
-The purpose of limestone is to remove sand impurities, SiO2, from the iron ore.
-The limestone decomposes to give calcium oxide which reacts with the main impurity, silicon dioxide, to give calcium silicate, often known as slag (CaSIO3).
-The slag formed on top of the molten iron metal and is easily removed.
-The slag can be used for road building as hardcore.
-The iron that comes out of the furnace is called cast iron and is too brittle for most uses of iron and so the iron must be converted to steel by removing some of the carbon it picked up on its way down through the furnace.

Question 17

What are the equations for the chemical reactions occurring in the blast furnace?

Answer 17

1. Coke reacts with oxygen:
   C (s) + O2 (g) —> CO2 (g) [oxidation]
2. Carbon dioxide reacts with more coke to produce the reducing agent, carbon monoxide:
   C (s) + CO2 (g) —> 2CO (g)
   [reduction]
3. The carbon monoxide reacts with the iron (III) oxide:
   Fe2O3 (s) + 3CO (g) —>
   2Fe (l) + 3CO2 (g) [reduction of the metal oxide]
   The molten iron sinks to the bottom and is tapped off
   The limestone is added to remove impurities (mainly sand) from the haematite:
4. The limestone undergoes thermal decomposition:
   CaCO3 (s) —> CaO (s) + CO2 (g)
5. The alkaline calcium oxide then combines with the acidic silicon dioxide to give calcium silicate (slag):
   CaO (s) + SiO2 (s) —> CaSiO3 (l)
   The molten slag floats on top of the iron and is easily removed (iron is denser than the slag).

Question 18

Describe the use of iron

Answer 18

Iron is a cheap metal because the ore is abundant and it is relatively easy to extract from its ore. It is used in many structures and bridges due to its strength.