12.1. Nitrogen and sulfur
A subsection of Chemistry, 9701, through 12. Nitrogen and sulfur
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Explain why the thermal stability of the Group2 nitrates increases down the group. Sodiumnitrite, NaNO2, is a decomposition product from heating sodiumnitrate, NaNO3. A student analysed a sample of sodium nitrite by titration with aqueous cerium(ions, Ce4+. The equation for the titration reaction is shown. NO2 –+ 2Ce4++ H2O2Ce3++ NO3 –+ 2H+● ●0.138 g of impure sodium nitrite was dissolved in water and made up to 100 cm3 in a volumetric flask. ● ●25.0 cm3 of this solution required 21.80 cm3 of 0.0400 mol dm–3 Ce4+to reach the end‑point. You should assume the impurity does not react with Ce4+. Calculate the percentage purity of the sample of sodium nitrite.  % Acidified manganate(ions, MnO4 –, can also be used to analyse solutions containing nitrite ions, NO2 –, by titration. In acidic solution, NO2 – ions exist as HNO2. Use the DataBooklet to construct an ionic equation for this reaction. Use E o values to calculate the for this reaction.  = V Nitrousacid, HNO2, is a weak acid with a Ka of 6.9×10–4 mol dm–3 at 298 K. Explain the difference between a strong acid and a weak acid. Write the expression for the acid dissociation constant, Ka, for HNO2. Ka =  Calculate the pH of 0.15 mol dm–3 HNO2.  pH = Calculate the percentage of HNO2 molecules that are ionised in 0.15 mol dm–3 HNO2.  % ionisation = Solutions containing a mixture of HNO2 and NaNO2 are buffer solutions. Define what is meant by the term buffer solution. Write two equations to show how a solution containing a mixture of HNO2 and NaNO2 acts as a buffer. Nitrousacid is used in the preparation of diazonium salts. The –N2 + group in the diazonium ion can be replaced with Cl, Br or CN as shown. The reagent used is a copper(salt, CuX. N2 + X CuX diazonium ion where X = Cl, Br or CN This reaction can be used in the synthesis of compound Z as shown. N2 + H3C H3C CN W Y H3C H3C reaction 1 aq HCl, heat reaction 2 an excess of MnO4 – / H+ heat reaction 3 Z Suggest the reagent used in reaction 1. Suggest structures of compounds Y and Z and draw them in the boxes above. Compounds W and Z were analysed using carbon‑13 NMR spectroscopy. Predict the number of peaks in the carbon‑13 NMR spectra of W and Z. number of peaks W Z  
9701_w18_qp_42
THEORY
2018
Paper 4, Variant 2
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