Titration can be used to determine the concentration of dissolved oxygen in samples of river water. The procedure for the experiment is given. step 1 Use five $$\(50 \mathrm{~cm}^{3}\)$$ graduated syringes, A, B, C, D and E, to collect five separate $$\(30.0 \mathrm{~cm}^{3}\)$$ samples of river water. step 2 In the laboratory, carefully add $$\(5.0 \mathrm{~cm}^{3}\)$$ of $$\(0.220 \mathrm{moldm}^{-3}\)$$ manganese(II) sulfate, $$\(\mathrm{MnSO}_{4}(\mathrm{aq})\)$$, into syringe $$\(\mathbf{A}\)$$ and mix well. step 3 Add $$\(5.0 \mathrm{~cm}^{3}\)$$ of alkaline aqueous potassium iodide into syringe $$\(\mathbf{A}\)$$ and mix well. step 4 Add $$\(10.0 \mathrm{~cm}^{3}\)$$ of dilute sulfuric acid into syringe $$\(\mathbf{A}\)$$ and mix well. step 5 Transfer the contents of syringe $$\(\mathbf{A}\)$$ into a $$\(150 \mathrm{~cm}^{3}\)$$ conical flask. Rinse syringe $$\(\mathbf{A}\)$$ using $$\(10 \mathrm{~cm}^{3}\)$$ of distilled water and add washings to the conical flask. step 6 Carry out one accurate titration of all the contents in the conical flask with $$\(0.00200 \mathrm{moldm}^{-3}\)$$ aqueous sodium thiosulfate, $$\(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})\)$$, using starch indicator. Repeat steps 2-6 for the samples in syringes B-E. The sample in the conical flask and the prepared solution of sodium thiosulfate are provided. Describe the following procedures for the experiment using syringe $$\(\mathbf{A}\)$$. (i) Preparing the clean burette before taking any readings. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ................................................................................................................................. (ii) Carrying out the one accurate titration in step 6. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . .................................................................................................................................
Exam No：9701_s24_qp_51 Year：2024 Question No：1(c)
Answer:
Knowledge points:
5.1.1 understand that chemical reactions are accompanied by enthalpy changes and these changes can be exothermic (ΔH is negative) or endothermic (ΔH is positive)
5.1.2 construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy
5.1.3.1 standard conditions (this syllabus assumes that these are 298 K and 101 kPa) shown by
5.1.3.2 enthalpy change with particular reference to: reaction, , formation, , combustion, , neutralisation,
5.1.4 understand that energy transfers occur during chemical reactions because of the breaking and making of chemical bonds
5.1.5 use bond energies (ΔH positive, i.e. bond breaking) to calculate enthalpy change of reaction,
5.1.6 understand that some bond energies are exact and some bond energies are averages
5.1.7 calculate enthalpy changes from appropriate experimental results, including the use of the relationships q = mcΔT and ΔH = –mcΔT/n
5.2.1 apply Hess’s Law to construct simple energy cycles
5.2.2.1 determining enthalpy changes that cannot be found by direct experiment
5.2.2.2 use of bond energy data
5.3 pending
Solution:
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