A2 LEVEL CHEMISTRY

TRANSITION METALS 
ASSESSED HOMEWORK
PART II

Answer all questions
Max 63 marks

	
Name		…………………………………………………………..

Mark		……../63	……....%	Grade ………

1.	In this question, one mark is available for the quality of spelling, punctuation and grammar.
	Iron and its compounds take part in several different types of reaction including ligand substitution, precipitation and redox.
	For each type of reaction
•	give an example, taken from the chemistry of iron or its compounds
•	state what you would see
•	write a balanced equation for your example.

[9]
Quality of Written Communication [1]
[Total 10 marks]
2.	Mercury thermometers are not used in some laboratories because of the danger of mercury vapour. This vapour is very easily absorbed through the lungs into the blood.
	In the blood, mercury reacts with hydrogen peroxide to form mercury(II) oxide.
		Hg + H2O2 → HgO + H2O
	The mercury(II) oxide formed accumulates within organs in the body.
	Use oxidation numbers to show that the reaction between mercury and hydrogen peroxide is an example of both oxidation and reduction.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
[Total 2 marks]
 
3.	Artists between the 13th and the 19th Centuries used a green pigment called verdigris. The artists made the pigment by hanging copper foil over boiling vinegar.
	During the preparation of verdigris, copper atoms are oxidised to copper(II) ions.
(i)	Write the oxidation half equation for the conversion of copper atoms into copper(II) ions.
........................................................................................................................
[1]
(ii)	The reduction half equation that takes place is as follows.
		O2(g) + 4H+(aq) + 4e– ® 2H2O(l)
	Construct the equation for the redox reaction between copper, oxygen and hydrogen ions.

[1]
[Total 2 marks]

4.	An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.
	When chlorine is bubbled through a suspension of iron(III) oxide in concentrated aqueous sodium hydroxide, a solution of aqueous sodium ferrate(VI) forms.
	The two relevant redox systems are shown below.
		Cl2(aq) + 2e–	→ 2C/–(aq)
		Fe2O3(s) + 10OH–(aq)	→ 2FeO42–(aq) + 5H2O(I) + 6e–
	Construct the redox equation for the reaction between chlorine, iron(III) oxide and hydroxide ions.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
[Total 2 marks]

5.	An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.
	Ferrate(VI) ions will decompose in acidic solution as shown in the equation below.
		4FeO42–(aq) + 2OH+(aq) → 4Fe3+(aq) + 3O2(g) + 10H2O(l)
	Explain, in terms of oxidation numbers, why this decomposition involves both reduction and oxidation.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
[Total 2 marks]

6.	An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.
	A student uses 1.00 g of iron(III) oxide and makes, on crystallisation, 0.450 g of sodium ferrate(VI), Na2FeO4.
	Calculate the percentage yield, by mass, of sodium ferrate(VI).
	Show your working.
	Express your answer to an appropriate number of significant figures.

percentage yield = ............................ %
[Total 4 marks]

7.	Vanadium can exist in a number of different oxidation states. One compound of vanadium is ammonium vanadate(V) and this contains the ion VO3–. This can be reduced to V2+ in several steps, using zinc metal and aqueous sulphuric acid.
	(a)	25.0 cm3 of 0.100 mol dm–3 ammonium vanadate(V) is completely reduced to V2+(aq) using zinc and aqueous sulphuric acid. The resulting solution is titrated with 0.0500 mol dm–3 MnO4–(aq) and 30.0 cm3 is required to oxidise the V2+(aq) back to VO3–(aq).
	The half equation for acidified MnO4– acting as an oxidising agent is shown below.
		MnO4– + 8H+ + 5e–  Mn2+ + 4H2O
	Show that the vanadium has changed oxidation state from +2 to +5 in this titration.

[4]
(b)	Suggest an equation for the oxidation of V2+(aq) to VO3–(aq) by MnO4–(aq) under acid conditions.
........................................................................................................................
[2]
[Total 6 marks]

 8.	Under certain conditions, VO2+ can be converted into VO2+ by reaction with KMnO4. The equation for the reaction is shown below.
		5VO2+ + H2O + MnO4– → 5VO2+ + Mn2+ + 2H+
	What volume, in cm3, of 0.0250 mol dm–3 KMnO4 would be required to convert 20.0 cm3 of 0.100 mol dm–3 VO2+ into VO2+?

volume of KMnO4 required = ...................... cm3
[Total 3 marks]
9.	The percentage purity of a sample of manganese(IV) oxide, MnO2, can be determined by its reaction with acidified iron(II) ions.
•	Stage 1 –	A sample of known mass of the impure MnO2 is added to a conical
	flask.
•	Stage 2 –	The sample is reacted with a known excess amount of Fe2+ acidified
	with dilute sulphuric acid.
•	Stage 3 –	The contents of the flask are heated gently.
•	Stage 4 –	The cooled contents of the flask are titrated with aqueous potassium
	manganate(VII) in acidic conditions to find the amount of unreacted
	Fe2+.
(i)	The reduction half-equation for manganese(IV) oxide in the presence of dilute acid is shown below.
		MnO2(s) + 4H+(aq) + 2e– → Mn2+(aq) + 2H2O(l)
	Construct the balanced equation for the redox reaction between Fe2+(aq), MnO2(s) and H+(aq).
........................................................................................................................
........................................................................................................................
........................................................................................................................
[1]
(ii)	In Stage 1 and Stage 2 a student uses a 0.504 g sample of impure MnO2 and 100 cm3 of 0.200 mol dm–3 Fe2+.
	In Stage 4 the student determines that the amount of unreacted Fe2+ is 0.0123 mol.
	1 mol of MnO2 reacts with 2 mol of Fe2+.
	Calculate the percentage purity of the impure sample of MnO2.

percentage purity = ..................................................... %
[3]
[Total 4 marks]

10.	The standard electrode potential of Cu2+(aq) + 2e–  Cu(s) is +0.34 V.
	A student measured the standard electrode potential of Cu2+(aq) + 2e–  Cu(s). She was surprised to see that the emf of the cell was less than the expected value of +0.34 V.
	She decided to measure the concentration of the Cu2+(aq) ions in the solution by titration.
	25.00 cm3 of the solution containing Cu2+ ions were pipetted into a volumetric flask and she made the volume up to 250.0 cm3 with distilled water.
	An excess of aqueous potassium iodide, KI, was added to 25.00 cm3 of the diluted solution.
The iodine formed was titrated against 0.100 mol dm–3 sodium thiosulphate, Na2S2O3.
	The volume of Na2S2O3(aq) used was 23.20 cm3.
	The equations for the formation and titration of iodine are given below.
		2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq)
		I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq)
(i)	State how the student would identify the end point of the titration.
........................................................................................................................
........................................................................................................................
[1]
(ii)	Show that the concentration of the Cu2+(aq) ions was 0.93 mol dm–3.

[4]
(iii)	Explain, in terms of chemical equilibrium, why the emf of this cell was less than the standard electrode potential.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
[2]
[Total 7 marks]
11.	Potassium dichromate(VI) can be used in a number of redox reactions. The standard electrode potentials for two half reactions are given below.
		Cr2O72– + 14H+ + 6e–		2Cr3+ + 7H2O	Eο = +1.33 V
		I2 + 2e–		2I–	Eο = +0.54 V
	Acidified potassium dichromate(VI) is added to aqueous potassium iodide to give aqueous iodine.
(i)	Construct an ionic equation to show the reaction taking place when acidified potassium dichromate(VI) is added to aqueous potassium iodide.
........................................................................................................................
........................................................................................................................
........................................................................................................................
[2]
 (ii)	An excess of aqueous sodium thiosulphate was then added. Describe and explain what you would see.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
[3]
[Total 5 marks]

12.	Chromium metal and its compounds have a number of important uses.
	Cr2O72– ions oxidise I– ions to I2 under acid conditions according to the following equation.
		Cr2O72–(aq) + 6I–(aq) + 14H+(aq)  2Cr3+(aq) + 3I2(aq) + 7H2O(l)
(i)	If you carried out this reaction, how could you see that iodine is formed?
........................................................................................................................
........................................................................................................................
[1]
(ii)	How could you use the formation of I2 in this reaction to determine the concentration of a solution of Cr2O72– ions?
	In your answer
•	state the method you would use
•	state the reagents used
•	show how you would use your results.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
[4]
[Total 5 marks]

13.	Brass is an alloy of copper and zinc.
	The percentage of copper and zinc in a sample of brass can be determined by reaction with hydrochloric acid. Only zinc reacts, as shown in the equation below.
		Zn(s) + 2H+(aq) ® Zn2+(aq) + H2(g)
•	A sample of brass powder of known mass is added to an excess of 1.00 mol dm–3 hydrochloric acid.
•	The mixture is heated gently and the hydrogen collected is measured once the reaction has finished.
	A student analyses a 1.23 g sample of brass using the method described.
The student collects 76.0 cm3 of hydrogen at room temperature and pressure.
1 mol of gas molecules occupies 24.0 dm3 at room temperature and pressure.
	Calculate the percentage by mass of copper in the sample of brass.
Give your answer to an appropriate number of significant figures.

answer .............................................. %
[Total 3 marks]

14.	The compound FeSO47H2O can be used to kill moss in grass.
	The percentage purity of an impure sample of FeSO4.7H2O can be determined by titration against potassium dichromate(VI), K2Cr2O7, under acid conditions, using a suitable indicator.
	During the titration, Fe2+(aq) ions are oxidised to Fe3+(aq) ions.
•	Stage 1 – A sample of known mass of the impure FeSO4.7H2O is added to a conical flask.
•	Stage 2 – The sample is dissolved in an excess of dilute sulphuric acid.
•	Stage 3 – The contents of the flask are titrated against K2Cr2O7(aq).
(i)	The reduction half equation for acidified dichromate(VI) ions, Cr2O72–, is as follows.
Cr2O72–(aq) + 14H+(aq) + 6e–  2Cr3+(aq) + 7H2O(l)
	Construct the balanced equation for the redox reaction between Fe2+(aq),
Cr2O72–(aq) and H+(aq).
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
[2]
(ii)	In Stage 1, a student uses a 0.655 g sample of impure FeSO4.7H2O.
	In Stage 3, the student uses 19.6 cm3 of 0.0180 mol dm–3 Cr2O72– to reach the end-point.
	One mole of Cr2O72– reacts with 6 moles of Fe2+.
	Calculate the percentage purity of the impure sample of FeSO4.7H2O.

percentage purity .........................
[4]
[Total 6 marks]

15.	An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.
	Aqueous sodium ferrate(VI) is a very powerful oxidising agent.
(i)	Predict what you would see when aqueous sodium ferrate(VI) is added to aqueous potassium iodide.
	Explain your answer.
........................................................................................................................
........................................................................................................................
[1]
(ii)	Aqueous sodium ferrate(VI) will oxidise ammonia into substance X.
	Suggest an identity for X.
........................................................................................................................
[1]
[Total 2 marks]