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REVISION QUESTIONS TOPIC 17

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REVISION QUESTIONS TOPIC 17 Powered By Docstoc
					                                  Revision Questions Topic 17

June 2002
3. for this question you will need, in addition to information in the book of data, the
following electrode potentials:

                              Eθ/V
   3+       +
Au (aq), Au     (aq)   / Pt +1.29
Au3+(aq)               / Au(s) +1.41
Au+(aq)                / Au(s) +1.69

(a) an electrochemical cell is set up by connecting two half cells with electrodes of
gold and silver, dipping into solutions that contain gold(III) and silver(I) ions
respectively in molar concentrations.
(i)Draw a cell diagram for this cell and calculate the Eθ cell.
(ii) Write a balanced equation for the cell reaction. State with a reason where the
position of the equilibrium lies.
(iii) Aqueous solution containing gold(I) ions are unstable. Use Eθ data to explain this
fact, giving the equation for the reaction which occurs.
(b) Solutions containing gold(I) ions are stabilised by adding cyanide ions. The
following reaction occurs, which goes to completion:

                                Au+(aq) + 2CN-(aq)  [Au(CN)2]-(aq)

(i) Predict the shape of the [Au(CN)2]- ion
(ii) Deduced, giving your reason, the signs of ∆Stotal, ∆Ssystem, ∆Ssurroundings, and hence
the sign of the enthalpy change, ∆H, for the reaction above.
(iii) In one step of the production of gold from its ore, zinc is added to a solution
containing [Au(CN)2]- ions. The following reaction occurs

                     [Au(CN)2]-(aq) + Zn(s)  2Au(s) + [Zn(CN)4]2-(aq)
(iv) Identify the reducing agent in this reaction. Explain how you reached your
decision and illustrate your answer with a half equation.

Jan 2002
2. (a) Two students were asked to plan and carry out an experiment to measure the
standard electrode potential for a nickel half-cell. The students’ plan included the cell
diagram:

                                Pt[H2(g)] │2H+(aq)    Ni2+(aq)│Ni(s)

And the block diagram of their proposed experimental set-up.
                           High resistance voltmeter




       Standard Hydrogen electrode               Standard Nickel half-cell



(i) Name and describe the essential component missing both from the cell diagram
and the block diagram.
(ii) Give the names and concentrations of the solutions required in the: (A) standard
hydrogen electrode and (B) nickel half cell.
(b)(i) The students found that the nickel half cell was the negative pole of the cell.
Which of the two elements, hydrogen of nickel, has the stronger tendency to lose
electrons, forming positive ions? Explain your answer.
(ii) Write a balanced equation, including state symbols, for the cell reaction
(iii) Which species is being reduced in this reaction? Justify your answer.
(iv) Predict how the entropy of the system will change, if at all, in each half-cell as the
reaction proceeds. Justify your answer.
(A) Standard hydrogen electrode        (B) Nickel half-cell

Jan 2002
5. This question concerns metals which form two or more differently charged ions. A
familiar example is iron, forming ions Fe2+ and Fe 3+.
(a) In this part of the question you are asked to collect electrode potential data and
assemble them into an appropriate order. Before beginning to write your answer, read
the instructions for (i) all the way through.
(i) from the book of data, table 6.1, list in order all the electrode systems and
potentials of the type M3+(aq), M2+(aq).
Add to the list, in appropriate places, the system numbered 42 and 53.
(ii) which of your listed ions is the strongest oxidising agent? Justify your answer.
(iii) Which of the M3+(aq) ions would you expect to be oxidising by oxygen under
standard conditions? Justify your answer.
(b)(i) A student predicted the under standard conditions, Sn2+(aq) ions will reduce
Fe3+(aq) to Fe2+(aq) and that the reaction will go to completion. Write a cell diagram
corresponding to this predicted reaction. Calculate the sign of Eθ and use it to
comment on the students prediction.
(ii) Write a balanced equation for the predicted reaction.
(c)(i) Item number 108 in table 6.1 includes iron with an unusual oxidation number.
What is this oxidation number?
(ii) Suggest why compounds of iron with this oxidation number are seldom
encountered.
                                                                         Total: 12 marks
June 1998
6. (a) From the book of data, tabulate electrode systems and the associated Eθ values
involving the following elements. list only those where the elements are acting as
oxidizing agents:
            I2 (aq)         Br2 (aq)       Cl2 (aq)        F2 (g)          O2 (g)
(b) Use the data, and any other data to which you might wish to refer in answering of
the following:
(i) Arrange the five substances in order of strength as oxidising agents, putting the
strongest first. Justify your answer.
(ii) Some iron (II) salts in aqueous solutions are partially hydrolysed to iron (II)
hydroxide, Fe(OH)2. It is observed that this iron (II) hydroxide becomes oxidized on
storage to iron (III) hydroxide, probably by oxygen in the air.
Write a cell diagram corresponding to a possible reaction by which this might happen,
using the appropriate electrode system from the Book of data.
Calculate the value and sign of Eθ and used the result to comment on the feasibility of
the suggested reaction.                                                               (4)
                                                                                  θ
(iii) When chlorine reacts directly with iron, iron (III) chloride results. Use E data to
compare the extent to which iron reacts similarly with iodine.                        (3)
                                                                       Total: 11 marks

                                                             TOTAL: 34 MARKS

				
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