7. Equilibria
A section of Chemistry, 9701
Listing 10 of 344 questions
Complete the following electronic confi guration of the Cu2+ ion. 1s2 2s2 2p6 In a free, gas-phase transition metal ion, the d-orbitals all have the same energy, but when the ion is in a complex the orbitals are split into two energy levels. Explain why this happens. How does this splitting help to explain why transition metal complexes are often coloured? Why does the colour of a transition metal complex depend on the nature of the ligands surrounding the transition metal ion? Draw a fully-labelled diagram of the apparatus you could use to measure the E o of a cell composed of the Fe3+/Fe2+ electrode and the Cu2+/Cu electrode. The E o for Cu2+/Cu is +0.34 V. When NH3is added to the electrode solution, the Eelectrode changes. Describe the type of reaction taking place between Cu2+and NH3. Write an equation for the reaction. Describe the change in the colour of the solution. Predict and explain how the Eelectrode might change on the addition of NH3. Fehling’s reagent is an alkaline solution of Cu2+ ions complexed with tartrate ions. It is used in organic chemistry to test for a particular functional group. Name the functional group involved. Describe the appearance of a positive result in this test. Write an equation for the reaction between Cu2+ and OH– ions and a two-carbon compound containing the functional group you named in . A solution containing a mixture of tartaric acid and its sodium salt is used as a buffer in some pre-prepared food dishes. Calculate the pH of a solution containing 0.50 mol dm–3 of tartaric acid and 0.80 mol dm–3 sodium tartrate. [Ka(tartaric acid) = 9.3 × 10–4 mol dm–3] pH =
9701_w12_qp_43
THEORY
2012
Paper 4, Variant 3
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Use the Data Booklet to draw the structure of the dipeptide val-lys. The peptide bond should be shown displayed.  The isoelectric point is the pH at which an amino acid exists as a zwitterion. The isoelectric point of valine is 6.0 and of lysine is 9.8. A mixture of the dipeptide, val-lys, and its two constituent amino acids, valine and lysine, was analysed by electrophoresis using a buffer at pH 6.0. Draw and label three spots on the diagram of the electrophoresis paper to indicate the likely position of each of these three species after electrophoresis. Explain your answer. + – mixture applied here explanation  
9701_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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Use the Data Booklet to draw the structure of the dipeptide val-lys. The peptide bond should be shown displayed.  The isoelectric point is the pH at which an amino acid exists as a zwitterion. The isoelectric point of valine is 6.0 and of lysine is 9.8. A mixture of the dipeptide, val-lys, and its two constituent amino acids, valine and lysine, was analysed by electrophoresis using a buffer at pH 6.0. Draw and label three spots on the diagram of the electrophoresis paper to indicate the likely position of each of these three species after electrophoresis. Explain your answer. + – mixture applied here explanation  
9701_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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