HL Paper 3
Alloys containing at least 60 % copper reduce the presence of bacteria on their surface.The percentage of copper in brass, an alloy of copper and zinc, can be determined by UV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to 250.0 cm3 with water before analysis.
Cu (s) + 4HNO3 (aq) → Cu(NO3)2 (aq) + 2NO2 (g) + 2H2O (l)
3Zn (s) + 8HNO3 (aq) → 3Zn(NO3)2 (aq) + 2NO (g) + 4H2O (l)
The concentration of copper(II) ions in the resulting solution is then determined from a calibration curve, which is plotted by measuring the light absorbance of standard solutions.
You may find the following chart and diagram helpful.
Outline why the initial reaction should be carried out under a fume hood.
Deduce the equation for the relationship between absorbance and concentration.
Copper(II) ion solutions are blue. Suggest, giving your reason, a suitable wavelength of light for the analysis.
Outline how a solution of 0.0100 mol dm−3 is obtained from a standard 1.000 mol dm−3 copper(II) sulfate solution, including two essential pieces of glassware you would need.
The original piece of brass weighed 0.200 g. The absorbance was 0.32.
Calculate, showing your working, the percentage of copper by mass in the brass.
Deduce the appropriate number of significant figures for your answer in (e)(i).
Comment on the suitability of using brass of this composition for door handles in hospitals.
If you did not obtain an answer to (e)(i), use 70 % but this is not the correct answer.
Suggest another property of brass that makes it suitable for door handles.
Titration is another method for analysing the solution obtained from adding brass to nitric acid.
Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, Na2S2O3 (aq). Copper(I) iodide is a white solid.
4I− (aq) + 2Cu2+ (aq) → 2CuI (s) + I2 (aq)
I2 (aq) + 2S2O32− (aq) → 2I− (aq) + S4O62− (aq)
Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.
Lithium has many uses.
The emission spectra obtained by ICP-OES for a mixture containing the isotope 6Li (Li-6) and naturally occurring lithium (Li (N)) is shown.
Identify the type of bonding in lithium hydride, using sections 8 and 29 of the data booklet.
Identify the colour of the emission spectrum of lithium using section 17 of the data booklet.
Suggest why ICP-OES does not give good quantitative results for distinguishing 6Li from naturally occurring lithium.
Suggest a better method.
Lithium is obtained by electrolysis of molten lithium chloride. Calculate the time, in seconds, taken to deposit 0.694 g Li using a current of 2.00 A.
Q (charge) = I (current) × t (time)
Lithium has shown some superconductive properties when doped into graphene or when under high pressure. Under high pressure, however, the Meissner effect is absent.
Describe the Meissner effect.
At very low temperatures, lithium atoms enhance the phonon binding of electrons in graphene suggesting the formation of Cooper pairs.
Explain how Cooper pairs are formed.
Lithium forms a crystalline lattice with the unit cell structure shown below.
X-ray diffraction shows that the length of the edge of the unit cell is 3.51 × 10−8 cm.
Determine the density of lithium, in g cm−3, using sections 2 and 6 of the data booklet.