2. Atoms, molecules and stoichiometry
A section of Chemistry, 9701
Listing 10 of 208 questions
Bubbling air through different aqueous mixtures of CoCl 2, NH4Cl and NH3 produces various complex ions with the general formula [Co(NH3)6–nCl n]3–n. Determine the oxidation state of the cobalt in these complex ions. Name the two types of reaction undergone by the cobalt ions during the formation of these complex ions. The complex [Co(NH3)4Cl 2]+ shows isomerism. Draw three-dimensional structures of the two isomers, and suggest the type of isomerism shown here. isomer 1 isomer 2 type of isomerism What is meant by the term co-ordination number? Complete the table by predicting appropriate co-ordination numbers, formulae and charges for the complexes C, D, E and F. complex metal ion ligand co-ordination number formula of complex charge on complex C Cr3+ CN– 3 – D Ni2+ H2NCH2CH2NH2 E Pt2+ Cl – 2– F Fe3+ –O2C–CO2 – [Fe(O2CCO2)3] Iron(forms complexes in separate reactions with both SCN– ions and Cl – ions. Fe3++ SCN–2+equilibrium 1 Fe3++ 4Cl –[FeCl 4]–equilibrium 2 Write the expressions for the stability constants, Kstab, for these two equilibria. Include units in your answers. Kstab1 = units = Kstab2 = units = An equilibrium can be set up between these two complexes as shown in equilibrium 3. [FeCl 4]–+ SCN–2++ 4Cl –equilibrium 3 Write an expression for Keq3 in terms of Kstab1 and Kstab2. Keq3 = The numerical values for these stability constants are shown. Kstab1 = 1.4 × 102 Kstab2 = 8.0 × 10–2 Calculate the value of Keq3 stating its units. Keq3 = units =
9701_s17_qp_43
THEORY
2017
Paper 4, Variant 3
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Cobalt is a transition element with atomic number 27. Cobalt forms ions with charges of +2 and +3. Both these ions form complexes. Complete the electronic configurations of a Co atom and a Co2+ ion. Co atom 1s22s22p6 Co2+ ion 1s22s22p6 One Co2+ ion can form a tetrahedral complex ion with four Cl – ions. This complex is blue. What is meant by the term complex ion? Draw the tetrahedral complex ion formed by one Co2+ ion with four Cl – ions. Your drawing should clearly show three-dimensional shape, and should include the overall charge on the ion. Co Explain why many transition metal complexes are coloured. Using ideas from your answer to suggest why the colour of the complex formed by one Co2+ ion with four Cl – ions is blue. Describe the colour change that occurs when water is added to blue crystals of the cobalt(‑chloride complex. Name the type of reaction responsible for this colour change. colour change type of reaction Co2+can be oxidised to Co3+. Use the Data Booklet to suggest a suitable oxidising agent for this reaction. Calculate the of this reaction. = V Write an equation for the reaction between Co2+ and the oxidising agent you chose in . Cobalt(forms two isomeric octahedral complexes with the formula [Co(NH3)3(NO2)3]. The NO2 – ion is monodentate. Complete the diagrams to show the three-dimensional structures of the two isomers and suggest the type of isomerism shown here. isomer 1 Co isomer 2 Co type of isomerism
9701_s18_qp_42
THEORY
2018
Paper 4, Variant 2
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Ethanedioicacid, HO2CCO2H, has a relative molecular mass of 90.0. Explain what is meant by the term relative molecular mass. State the empirical formula of ethanedioicacid. Calculate how many atoms of carbon are present in 0.18 g of ethanedioicacid, HO2CCO2H. Show your working.  atoms of carbon present = Solid ethanedioic acid reacts with aqueous calcium ions to make a precipitate of calciumethanedioate, CaC2O4. CaC2O4 breaks down when heated to form calciumoxide, carbondioxide and carbonmonoxide. Construct an equation to represent the reaction of CaC2O4 when heated. Include state symbols. Identify the type of reaction which occurs when CaC2O4 is heated. Identify another compound containing calcium ions which will also produce carbondioxide and calciumoxide when it is heated. 
9701_s21_qp_21
THEORY
2021
Paper 2, Variant 1
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Questions Discovered
208