2.3. Formulas
A subsection of Chemistry, 9701, through 2. Atoms, molecules and stoichiometry
Listing 10 of 88 questions
Hypophosphorous acid is an inorganic acid. The conjugate base of hypophosphorous acid is H2PO2 –. Give the formula of hypophosphorous acid. H2PO2 – is a strong reducing agent. It can be used to reduce metal cations without the need for electrolysis. equation 1 HPO3 2– + 2H2O + 2e– H2PO2 – + 3OH– E o = –1.57 V In an experiment, an alkaline HPO3 2–/H2PO2 – half-cell is constructed with [H2PO2 –] = 0.050 mol dm–3. All other ions are at their standard concentration. Predict how the value of E of this half-cell differs from its E o value. Explain your answer. The Cr3+/Cr half-cell has a standard electrode potential of –0.74 V. An electrochemical cell consists of an alkaline HPO3 2–/H2PO2 – half-cell and a Cr3+/Cr half-cell. Calculate the standard cell potential, Ecell. Ecell = V o o Complete the diagram in to show how the standard electrode potential of the Cr3+/Cr half-cell can be measured relative to that of the standard hydrogen electrode. Identify the chemicals, conditions and relevant pieces of apparatus. Label to show: • which is the positive electrode • the direction of electron flow in the external circuit. H2PO2 – reduces Ni2+ to Ni in alkaline conditions. Use equation 1 to construct the ionic equation for this reaction. equation 1 HPO3 2– + 2H2O + 2e– H2PO2 – + 3OH– H2PO2 –reacts with OH–. H2PO2 –+ OH–HPO3 2–+ H2Table 2.1 shows the results of a series of experiments used to investigate the rate of this reaction. Table 2.1 experiment [H2PO2 –] / mol dm–3 [OH–] / mol dm–3 volume of H2 produced in 60 s / cm3 0.40 2.00 6.4 0.80 2.00 12.8 1.20 1.00 4.8 The volume of H2 was measured under room conditions. Use the molar volume of gas, Vm, and the data from experiment 1 to calculate the rate of reaction in mol dm–3 s–1. rate of reaction = mol dm–3 s–1 The rate equation was found to be: rate = k [H2PO2 –] [OH–]2 Show that the data in Table 2.1 is consistent with the rate equation. State the units of the rate constant, k, for the reaction. The experiment is repeated using a large excess of OH–. Under these conditions, the rate equation is: rate = k1 [H2PO2 –] k1 = 8.25 × 10–5 s–1 Calculate the value of the half-life, t1 2, of the reaction. t1 2 = s Describe how an increase in temperature affects the value of the rate constant, k1. A student suggests that the reaction between H2PO2 –and OH–might happen more quickly in the presence of a heterogeneous catalyst. Describe the mode of action of a heterogeneous catalyst.
9701_m23_qp_42
THEORY
2023
Paper 4, Variant 2
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Mohr’s salt is a pale green crystalline solid which is soluble in water. Mohr’s salt is a ‘double salt’ which contains two cations, one of which is Fe2+, one anion which is SO4 2–, and water of crystallisation. The identity of the second cation was determined by the following test. Solid Mohr’s salt was heated with solid sodium hydroxide and a colourless gas was evolved. The gas readily dissolved in water giving an alkaline solution. What is the gas? What is the formula of the second cation identified by this test? In this test, a grey/green solid residue was also formed. Suggest a name or formula for this solid. The identity of the anion present in Mohr’s salt was confirmed by adding dilute hydrochloric acid followed by aqueous barium chloride to an aqueous solution of Mohr’s salt. A white precipitate was formed. Suggest the identity of the white precipitate. When a double salt such as Mohr’s salt is made, the two individual salts are mixed together in a 1:1 molar ratio, dissolved in water and the solution crystallised. Give the formula of each of the two salts that would be mixed to make the double salt, Mohr’s salt. salt 1 salt 2 Calculate the relative formula mass of each of the salts present in Mohr’s salt. salt 1 relative formula mass of salt 1 salt 2 relative formula mass of salt 2 The crystals of the double salt contain water of crystallisation. The relative formula mass of Mohr’s salt is 392. Use your answers to to calculate the number of moles of water of crystallisation present in one mole of Mohr’s salt.
9701_s06_qp_2
THEORY
2006
Paper 2, Variant 0
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Zinc chloride is one of the most important compounds of zinc. It is used in dry cell batteries, as a flux for soldering and tinning, as a corrosion inhibitor in cooling towers and in the manufacture of rayon. Draw a fully labelled diagram to show how you could use a standard hydrogen electrode to measure the standard electrode potential, E o, of zinc. The electrolysis of zinc chloride can give different electrode products, depending on the conditions used. Suggest the products formed at each electrode in the following cases. One space has been filled in for you. conditions product at anode product at cathode ZnCl2chlorine ZnCl2(concentrated aqueous) ZnCl2(dilute aqueous) Use the following data, together with relevant data from the Data Booklet, to construct a Born-Haber cycle and calculate a value for the lattice energy of zinc chloride. standard enthalpy change of formation of ZnCl2 –415 kJ mol–1 standard enthalpy change of atomisation of Zn+131 kJ mol–1 electron affinity per mole of chlorine atoms –349 kJ mol–1 lattice energy = kJ mol–1 Examiner’s Use Zinc is an essential element for plant and animal life. It is often administered in the form of a chelate, which is a complex between a metal ion and a polydentate ligand. The rate of the reaction between zinc ions and the ligand 4-(2-pyridylazo)resorcinol, PAR, has been studied. Zn2+ + 2 HO HO PAR N N Zn2+ HO –O Zn-PAR N N N N + 2H+ Both PAR and its zinc complex absorb radiation in the UV-visible region. The figure below shows their absorption spectra. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 absorbance wavelength / nm PAR Zn-PAR Devise a suitable experimental technique for studying how the rate of this reaction varies with [Zn2+]. Describe a reaction you could carry out to show that PAR is a phenol.
9701_s07_qp_4
THEORY
2007
Paper 4, Variant 0
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Explain what is meant by the term transition element. Complete the electronic configuration of the vanadium atom, 1s22s22p6 the Cu2+ ion. 1s22s22p6 List the four most likely oxidation states of vanadium. Describe what you would see, and explain what happens, when dilute aqueous ammonia is added to a solution containing Cu2+ ions, until the ammonia is in an excess. Copper powder dissolves in an acidified solution of sodium vanadate(, NaVO3, to produce a blue solution containing VO2+ and Cu2+ ions. By using suitable half-equations from the Data Booklet, construct a balanced equation for this reaction.
9701_s09_qp_4
THEORY
2009
Paper 4, Variant 0
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Complete the electronic structures of the Cr3+ and Mn2+ ions. Cr3+ 1s22s22p6 Mn2+ 1s22s22p6 Describe what observations you would make when dilute KMnO4is added slowly and with shaking to an acidified solution of FeSO4until the KMnO4 is in a large excess. Construct an ionic equation for the reaction that occurs. By selecting relevant E o data from the Data Booklet explain why acidified solutions of Fe2+are relatively stable to oxidation by air, whereas a freshly prepared precipitate of Fe(OH)2 is readily oxidised to Fe(OH)3 under alkaline conditions. relevant E o values and half equations explanation Predict the organic products of the following reactions and draw their structures in the boxes below. You may use structural or skeletal formulae as you wish. hot conc. MnO4 – + H+ hot conc. MnO4 – + H+ hot Cr2O7 2– + H+ OH OH KMnO4 and K2Cr2O7 are the reagents that can be used to carry out the following transformation. E I II CHO OH Draw the structure of intermediate E in the box above. Suggest reagents and conditions for the following. reaction I reaction II
9701_s10_qp_42
THEORY
2010
Paper 4, Variant 2
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Complete the electronic structures of the Cr3+ and Mn2+ ions. Cr3+ 1s22s22p6 Mn2+ 1s22s22p6 Describe what observations you would make when dilute KMnO4is added slowly and with shaking to an acidified solution of FeSO4until the KMnO4 is in a large excess. Construct an ionic equation for the reaction that occurs. By selecting relevant E o data from the Data Booklet explain why acidified solutions of Fe2+are relatively stable to oxidation by air, whereas a freshly prepared precipitate of Fe(OH)2 is readily oxidised to Fe(OH)3 under alkaline conditions. relevant E o values and half equations explanation Predict the organic products of the following reactions and draw their structures in the boxes below. You may use structural or skeletal formulae as you wish. hot conc. MnO4 – + H+ hot conc. MnO4 – + H+ hot Cr2O7 2– + H+ OH OH KMnO4 and K2Cr2O7 are the reagents that can be used to carry out the following transformation. E I II CHO OH Draw the structure of intermediate E in the box above. Suggest reagents and conditions for the following. reaction I reaction II
9701_s10_qp_43
THEORY
2010
Paper 4, Variant 3
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The commonest form of iron(sulfate is the heptahydrate, FeSO4.7H2O. On heating at 90 C this loses some of its water of crystallisation to form a different hydrated form of iron(sulfate, FeSO4.xH2O. 3.40 g of FeSO4.xH2O was dissolved in water to form 250 cm3 of solution. A 25.0 cm3 sample of this solution was acidifi ed and titrated with 0.0200 mol dm–3 potassium manganate(. In this titration 20.0 cm3 of this potassium manganate(solution was required to react fully with the Fe2+ ions present in the sample. The MnO4 – ions in the potassium manganate(oxidise the Fe2+ ions in the acidifi ed solution. Explain, in terms of electron transfer, the meaning of the term oxidise in the sentence above. Complete and balance the ionic equation for the reaction between the manganate(ions and the iron(ions. MnO4 –+ 5Fe2++ H+ + 5Fe3++ H2O Calculate the number of moles of manganate(used in the titration. Use the equation in and your answer to to calculate the number of moles of Fe2+ present in the 25.0 cm3 sample of solution used. Calculate the number of moles of FeSO4.xH2O in 3.40 g of the compound. Calculate the relative formula mass of FeSO4.xH2O. The relative formula mass of anhydrous iron(sulfate, FeSO4, is 151.8. Calculate the value of x in FeSO4.xH2O.
9701_s14_qp_21
THEORY
2014
Paper 2, Variant 1
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Questions Discovered
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