LLG
Paris–Abu Dhabi
Advanced Math and Science Pilot Class
Academic year 2014-2015
Test– Chemistry
Test: Electrochemical cells
DOCUMENT 1:
Rusting of iron consists of the formation of hydrated oxide: Fe(HO)3, FeO(HO) or Fe2O3 from
iron atoms (Fe).
It is an electrochemical process that requires the presence of water, oxygen and a conductive
medium. In air, a relative humidity of 50% provides the necessary amount of water that will
dissolve efficiently some oxygen.
It always starts with the transformation of solid iron: Fe(s) into its corresponding ions: Fe2+(aq)
The processes can be as followed:
► Process 1: When a droplet of water containing a little dissolved oxygen (O 2(aq)) falls on an
iron pipe (Fe(s)), the solid iron under the droplet is corroded. The electrons made during this
process are taken by dissolved oxygen, at the edge of the droplet to produce water.
► Process 2: More acidic water (containing a larger amount of H+ ions) increases the corrosion as it is then the hydrogen ions
H+ that will consume the electrons, making hydrogen gas instead of water.
Source: www.corrosion-doctors.org
DATA: redox pairs involved Fe2+(aq) / Fe(s)
O2(aq) / H2O
H+ / H2(g)
The following questions are related to the transformations described in document 1:
1.
2.
3.
4.
5.
6.
Write the half-equation associated to the transformation of solid iron (Fe(s))
Is this transformation an oxidation or a reduction?
Write the half-equation associated to the transformation of O2(aq).
Is this transformation an oxidation or a reduction?
Deduce the equation of the reaction occurring during process 1.
Using data and information about process 2, check that the equation associated with this process is:
Fe(s) + 2 H+ = Fe2+(aq) + H2(g). (Write each half-equations and combine them)
7. Using the example of this reaction, explain :
- which substance is the oxidant, the reducer
- which substance has been oxidized, which one has been reduced
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in
electrical contact, in the presence of an electrolyte. Dissimilar metals have different reducing properties and when two come into
contact, one metal acts as anode and the other one as cathode. A spectacular example of galvanic corrosion occurs in the Statue
of Liberty when regular maintenance checks in the 1980’s revealed that corrosion had taken place between the outer copper
(Cu(s)) skin and the wrought iron (Fe(s)) structure due to an electrochemical cell between the Fe2+/Fe half-cell and the O2(aq) /H2O
half-cell ( through the inert copper electrode ); when discovered, more than 60% of the iron structure had been corroded.
A conventional drawing of this cell is given below:
Element 1
Fe(s)
Cu(s)
INERT ELECTRODE
Fe2+(aq)
O2 (aq)
LLG
Paris–Abu Dhabi
Advanced Math and Science Pilot Class
Academic year 2014-2015
Test– Chemistry
8. What is the half-equation taking place at the Fe2+/Fe half-cell?
9. Deduce the half-equation of the reaction taking place at the other half-cell.
10. Deduce the polarity of the iron electrode. Is the iron electrode the anode or the cathode?
11. What does the word “inert” means?
12. What is the polarity of the inert copper electrode. Is the inert copper electrode the anode or the
cathode?
13. Deduce that the reaction taking place when this cell is delivering electricity is:
2 Fe(s) + O2(aq) + 4 H+ = 2 Fe2+(aq) + 2 H2O
14. What is the name of element 1 in the circuit?
15. What is the role of element 1?
16. Add an external electric circuit connecting the two electrodes to the diagram and show using arrows
the direction of all the charge carriers (electrons, cations, anions).
17. What is obviously the limiting reactant in the electrochemical reaction? When does it stop?
To prevent the rust from forming, the oxidation of iron must be stopped. This can be achieved by sacrificial anode protection.
The idea is to provide an alternative oxidation reaction at the anode by introducing another metal, more reactive than iron (Fe),
iron becoming an inert electrode.
DOCUMENT 2: Reactivity series (sample)
When a redox reaction takes place between an oxidant and a
reducer; if more than one reaction is possible (more than one
reducer and/or more than one oxidant are present), the reaction
taking place will involve the strongest oxidant and the strongest
reducer among the possible ones, according to the reactivity
series:
DOCUMENT 3: (picture) the hull of a ship
Reducer strength
Mg2+ -- Mg
Al3+ -- Al
Zn2+ -- Zn
Fe2+ -- Fe
Cu2+-- Cu
Ag+ -- Ag
O2 – H2O
Oxidant strength
The white patches visible on the ship's hull are zinc block sacrificial anodes (brand new, picture on the
left hand side) . The same one after some time (picture on the right hand side).
18. Justify that it is possible to protect a piece of iron from corrosion by connecting it to a piece of zinc.
19. Check that this leads to an electrochemical cell whose negative electrode is Zinc, positive electrode
iron (being here the inert electrode) and overall reaction:
2 Zn + O2 + 4 H+ = 2 Zn2+ + 2 H2O
20. Justify that this process is named “sacrificial anode” (justify both words: sacrificial and anode). Who
is the sacrificial anode in this example?
21. Give two other examples of metals that could be used as sacrificial anode. Write the corresponding
reactions.