1.3. Electrons, energy levels and atomic orbitals
A subsection of Chemistry, 9701, through 1. Atomic structure
Listing 10 of 113 questions
Complete the electronic configuration of a copper atom. 1s22s22p6 ● Explain why most copper(salts are coloured. ● Suggest why copper(salts are usually white.  Brass is an alloy of copper and zinc. The following reaction can be used to determine the amount of copper in a sample of brass. 2Cu2++ 4I–2CuI+ I2The procedure was carried out using the following steps. ● ● A solution of Cu2+was obtained by dissolving a 1.50 g sample of brass in concentrated sulfuric acid and diluting with water. ● ● An excess of I–was added. ● ● The iodine produced was titrated against a 0.500 mol dm–3 solution of thiosulfate ions, S2O3 2–. I2+ 2S2O3 2–2I–+ S4O6 2–● ● The volume of S2O3 2– solution needed to reach the end-point was 28.35 cm3. Calculate the percentage by mass of copper in the sample of brass.  percentage by mass of copper =  Use standard electrode potential data from the Data Booklet to calculate for the reaction. 2Cu2++ 4I–2CuI+ I2 = V Explain how the value of calculated in predicts that the reaction is not likely to occur. In an experiment, a solution of I–is added to a solution of Cu2+. A reaction does occur and a precipitate of sparingly soluble CuIis formed. The concentration of Cu2+remaining in the solution is 1.00 mol dm–3. The concentration of Cu+in a saturated solution of CuI is 1.3×10–6 mol dm–3. Use the Nernst equation to calculate the electrode potential, E, for the Cu2+ / Cu+ half cell in this experiment.  E(Cu2+ / Cu+) = V Copper(chloride is also sparingly soluble in water. Suggest why the following reaction does not occur. 2Cu2++ 4Cl –2CuCl + Cl 2X When chloride ions are added to a solution containing Cu2+, the complex ion [CuCl 4]2–is formed. State the colours of Cu2+and [CuCl 4]2–. Cu2+[CuCl 4]2– Name the type of reaction that occurs when [CuCl 4]2–is formed from Cu2+. Write an expression for the stability constant, Kstab, for [CuCl 4]2–. Include the units in your answer. Kstab =  units =  
9701_m19_qp_42
THEORY
2019
Paper 4, Variant 2
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Iron is a transition element in the fourth period. Iron forms compounds containing the ions Fe2+ and Fe3+. Define the term transition element. Compare the melting point and density of iron with those of calcium, an s-block element in the fourth period. melting point density  Complete the electronic configuration of an isolated gaseous Fe2+ ion. 1s2 Aqueous Fe3+ ions form coloured complexes. Explain the origin of the colour in transition element complexes. When an excess of CN–ions is added to green [Fe(H2O)6]2+ions, yellow [Fe(CN)6]4– complex ions are formed. Heating [Fe(CN)6]4– with dilute nitricacid and then neutralising the product with Na2CO3produces red crystals, containing the [Fe(CN)5NO]2– complex ion. NO is a neutral, monodentate ligand. State the shape of the [Fe(H2O)6]2+complex ion. Write the equation for the reaction between [Fe(H2O)6]2+ions and an excess of CN–ions. Deduce the oxidation states of iron in: [Fe(CN)6]4– [Fe(CN)5NO]2–.  Define the term monodentate ligand. Complete the diagram to show the three-dimensional structure of the [Fe(CN)5NO]2– complex ion. Fe  The two complex ions [Fe(CN)6]4– and [Fe(CN)5NO]2– are different colours. Explain why the colours of the two complex ions are different. E is a complex ion, [Fe(C2O4)2Cl 2]4–, containing Fe2+ with a coordination number of 6. Define the term coordination number. E shows both optical isomerism and cis-trans isomerism. One isomer of E is shown. The C2O4 2– ion is represented as ox . In the boxes, draw three-dimensional diagrams to show: ● ● the trans isomer of E ● ● the optical isomer of E. Fe trans isomer 4– Fe optical isomer 4– Fe cis isomer E Cl Cl 4– ox ox  [Fe(C2O4)2Cl 2]4– contains ligands which are anions of ethanedioic acid, HO2CCO2H. Complete the table to show any observations for the reactions of HO2CCO2H with the named reagents. Where no change is observed, write ‘none’. reagent observations with HO2CCO2H warm acidified manganate(2,4-dinitrophenylhydrazine warm Tollens’ reagent  
9701_m20_qp_42
THEORY
2020
Paper 4, Variant 2
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Iron is a transition metal in Group 8 of the Periodic Table. Explain why iron has variable oxidation states. Complete the shorthand electronic configurations of Fe and Fe3+. Fe Fe3+ An aqueous solution of Fe(NO3)3 contains the complex [Fe(H2O)6]3+. When solutions of KSCNand [Fe(H2O)6]3+are mixed, a colour change is observed. The red complex [Fe(H2O)5SCN]2+ forms. Define complex. State the coordination number of Fe in [Fe(H2O)6]3+. The H—O—H bond angle in water is 104.5°. Suggest the H—O—H bond angle in [Fe(H2O)6]3+. Explain your answer. Explain why iron complexes are coloured. Aqueous solutions of complexes [Fe(H2O)6]3+ and [Fe(H2O)5SCN]2+ are different colours. Explain why these complexes are different colours. Table 3.1 gives values for the stability constants, Kstab, of different complexes of iron. Table 3.1 complex stability constant, Kstab [Fe(H2O)5(H2PO4)]2+ 5.90 × 101 [Fe(H2O)5SCN]2+ 1.30 × 102 [Fe(H2O)5(H2PO4)]2+ can form when H3PO4 reacts with [Fe(H2O)6]3+. Write an equation for this reaction. Write an expression for Kstab of [Fe(H2O)5SCN]2+ and give its units. Kstab = units = Use the stability constant data in Table 3.1 to calculate the value of the equilibrium constant, Kc, for the following equilibrium. [Fe(H2O)5(H2PO4)]2+ + SCN– [Fe(H2O)5SCN]2+ + H2PO4 – value of Kc =
9701_m24_qp_42
THEORY
2024
Paper 4, Variant 2
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Define standard cell potential, E o cell. An electrochemical cell is set up to measure E o cell of a cell consisting of an Fe3+ / Fe2+ half-cell and a Cl 2 / Cl – half-cell. Draw a labelled diagram of this electrochemical cell. Include all necessary substances. It is not necessary to state conditions used. The cell reaction for the electrochemical cell in is shown. Cl 2 + 2Fe2+ 2Fe3+ + 2Cl – E o cell = +0.59 V Calculate ΔG o , in kJ mol–1, for this cell reaction. ΔG o = kJ mol–1
9701_s23_qp_42
THEORY
2023
Paper 4, Variant 2
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On the following diagram draw a clear labelled sketch to describe the shape and symmetry of a typical d-orbital. Although the five d-orbitals are at the same energy in an isolated atom, when a transition element ion is in an octahedral complex the orbitals are split into two groups. Draw an orbital energy diagram to show this, indicating the number of orbitals in each group. energy Use your diagram as an aid in explaining the following. • Transition element complexes are often coloured. • The colour of a complex of a given transition element often changes when the ligands around it are changed. Heating a solution containing potassium ethanedioate, iron(ethanedioate and hydrogen peroxide produces the light green complex K3Fe(C2O4)3, which contains the ion [Fe(C2O4)3]3–. The structure of the ethanedioate ion is as follows. –O O– C O O C Calculate the oxidation number of carbon in this ion. Calculate the oxidation number of iron in [Fe(C2O4)3]3–. The iron atom in the [Fe(C2O4)3]3– ion is surrounded octahedrally by six oxygen atoms. Complete the following displayed formula of this ion. Fe O 3– O O In sunlight the complex decomposes into potassium ethanedioate, iron(ethanedioate and carbon dioxide. Use oxidation numbers to help you balance the following equation for this decomposition. K3Fe(C2O4)3 K2C2O4 + FeC2O4 + CO2
9701_w11_qp_41
THEORY
2011
Paper 4, Variant 1
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On the following diagram draw a clear labelled sketch to describe the shape and symmetry of a typical d-orbital. Although the five d-orbitals are at the same energy in an isolated atom, when a transition element ion is in an octahedral complex the orbitals are split into two groups. Draw an orbital energy diagram to show this, indicating the number of orbitals in each group. energy Use your diagram as an aid in explaining the following. • Transition element complexes are often coloured. • The colour of a complex of a given transition element often changes when the ligands around it are changed. Heating a solution containing potassium ethanedioate, iron(ethanedioate and hydrogen peroxide produces the light green complex K3Fe(C2O4)3, which contains the ion [Fe(C2O4)3]3–. The structure of the ethanedioate ion is as follows. –O O– C O O C Calculate the oxidation number of carbon in this ion. Calculate the oxidation number of iron in [Fe(C2O4)3]3–. The iron atom in the [Fe(C2O4)3]3– ion is surrounded octahedrally by six oxygen atoms. Complete the following displayed formula of this ion. Fe O 3– O O In sunlight the complex decomposes into potassium ethanedioate, iron(ethanedioate and carbon dioxide. Use oxidation numbers to help you balance the following equation for this decomposition. K3Fe(C2O4)3 K2C2O4 + FeC2O4 + CO2
9701_w11_qp_42
THEORY
2011
Paper 4, Variant 2
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Questions Discovered
113