SAJC H2 CHEM P2 QNS Prelim

1       [Turn over  1 Answer all the questions Planning (P) Iron(II) sulfate is one of the most readily available sources of iron( II). Hence it is used to make iron supplement tablets to treat anaemia and other iron deficiencies. Women are at a higher risk for iron-deficiency anaemia than men, especially pregnant women. However, excess iron in the body can lead to health problems, and in severe cases, death. The percentage by mass of iron in an iron supplement tablet can be determined by titrating it with acidified potassium manganate( VII). (a) Using the Data Booklet, write an ionic equation for the reaction between iron( II) sulfate and acidified potassium manganate(VII). _________________________________________________________________ [1] (b) If the concentration of a standard solution of potassium manganate( VII) is 0.500 mol dm ‒3, calculate the range of mass of powdered iron supplement tablets needed for a suitable and reliable titration. You may assume that the percentage mass of iron in iron supplement tablets is between 80% to 90%. [2]

2       [Turn over  1 (c) Write a plan for the titration that you would carry out to calculate the mass of iron present in the tablet. You are given a 0.500 mol dm ‒3 standard solution of potassium manganate( VII) and excess powdered iron tablets. You may use the reagents and apparatus normally found in a school or college laboratory. In your plan, you should give: Details, including quantities of all reagents and capacities of apparatus used; The essential details of the titration procedure. _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  _________________________________________________________________ _________________________________________________________________  _________________________________________________________________  [6]

3       [Turn over  1 d) Some women take multi-vitamin pills to supplement their diet which often also contain mineral ions such as Cu 2+. However, too much Cu 2+ in our body causes various health problems. In a manufacturing error, a batch of multi-vitamin pills contained too much Cu 2+ and was also contaminated accidentally with a small amount of Ba2+ ions which are poisonous. A sample from this batch of multi-vitamin pills is dissolved to form a solution. You are to devise a series of chemical tests which can be conducted under laboratory conditions using common laboratory reagents and apparatus to separate the two cations above, such that each cation is present in a separate precipitate. Record your plan in the table below. Test Expected observation Identity of cation in precipitate [3] [Total: 12 marks]

4       [Turn over  2 Phosphorus tribromide is a colourless liquid that is commonly used in laboratories for the manufacture of alkyl bromide from alcohols. (a) (i) Draw a dot and cross diagram of phosphorus tribromide. (ii) Draw the structure for the compound formed when phosphorus tribromide reacts with boron tribromide (iii) Suggest whether phosphorus tribromide or boron tribromide has a higher boiling point. Explain. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ [5] (b) Phosphorus tribromide reacts in a similar way as phosphorus trichloride. Write an equation, with state symbols, to show the reaction of phosphorus tribromide with water. _________________________________________________________________ [1]

5       [Turn over  2 (c) Using the information from the table below and any relevant data from the Data Booklet, construct an energy level diagram to calculate the bond energy of the P-Br bond. ∆H/ kJmol-1 Atomisation energy of phosphorus +315 Enthalpy change of formation of phosphorus tribromide -185 Enthalpy change of vapourisation of phosphorus tribromide +39 [4]

6       [Turn over  2 (d) The general equation for the formation of alkyl bromide from alcohols using phosphorus tribromide is: PBr 3 + 3ROH  3RBr +H3PO3 The reaction mechanism for the above reaction using ethanol and phosphorus tribromide is shown below. (i) State the type of reaction for the overall reaction _____________________________________________________________ (ii) Complete the mechanism above by using partial charges, lone pairs and curly arrows to represent the electron flow. (iii) Suggest a reason why this reaction method cannot be used to prepare C(CH3)3Br. _____________________________________________________________ _____________________________________________________________  (iv) Using the above mechanism in 2(d), suggest the organic compound formed when ethanoic acid is reacted with phosphorus tribromide. _____________________________________________________________ [7]

7       [Turn over  2 (e) PBr3 is used in the following reaction scheme to produce a sweet-smelling compound D. (i) Draw the structures of A to D in the spaces provided above.

8       [Turn over  2 (e) (ii) Give the reagents and conditions for Steps II, III, IV and VI. Step II: ____________________________________________________ Step III:____________________________________________________ Step IV: ____________________________________________________ Step VI: _________________________________________________________ [8] [Total: 25 marks] 3 Hydrogen peroxide exhibits oxidising and reducing properties, depending on pH. In acidic solutions, H2O2 is one of the most powerful oxidising agents known and is used commonly as a bleaching agent and disinfectant. In one of its reactions, it oxidises potassium iodide to form iodine and water. (a) With reference to the Data Booklet, write the balanced equation for the reaction between hydrogen peroxide and potassium iodide. _____________________________________________________________  [1]

9       [Turn over  3 (b) To study the kinetics of the above reaction, two sets of separate experiments were performed, in which the initial concentrations of each of the reactants were varied in turn, the other two being kept constant. The results are shown below. Graph 1 Graph 2 s 2s t 2t W: [