HKDSE 2014

17, 21 November 2014

Candidates’ Performance in Paper I (Q6‐14)

   

                                         

                        

6 (a) Fair

• about half of the candidates failed to mention the relationship between the boiling point and the molecular size/chain length of the alkene molecules.

• wrongly stated that the separation was based on the melting point of the alkanes.

   

                      

                       

       

6 (a) Fair

• definition of standard enthalpy change of combustion ‐‐ failed to give a complete answer including ‘enthalpy change involved’, ‘one mole of substance’, ‘complete combustion’ and ‘under standard conditions’.

• chemical equation – missing state symbols

   

     

                          

6 (b) Fair

• misspelt ‘catalytic converter’.

• calculation of the standard enthalpy change – incorrect ‘+ / –’ signs for the enthalpy terms

 

                                       

                              

7 (a) Poor

• unable to calculate the concentration of the acid by using the data of density and % by mass of HCl in the acid.

• converting the density 1.18 g cm–3 into g dm–3

‐ divided 1.18 by 1000 instead of multiply it by 1000

 

                           

                    

                       

7 (b) Poor

• Commonly observed mistakes include ‐ treating the whole process as a dilution of a

concentrated acid, ‐ failing to use distilled water / deionized

water for dissolving Na2CO3, ‐ wrongly dissolving the sodium carbonate

powder directly inside the volumetric flask, ‐ using inappropriate apparatus

 

                     

7 (c) Poor

• Only a few candidates realised that HCl(g) would escape from concentrated hydrochloric acid.

 

                             

               

8 Satisfactory

• Part (a) ‐ failed to realise that H+ are easier to be reduced than Mg2+

• Part (b) ‐ confused about the anodic and cathodic changes which occurred in beaker B

• Part (c) – Very good

• Part (d) ‐ Fair

 

                               

                             

 

9 Satisfactory

• Part (a) – some candidates gave wrong colour for the copper(II) hydroxide

• Part (b) – common mistakes for the chemical equations given for KMnO4 and Na2SO3: ‐ failed to give balanced equations ‐ only gave the half equation for the reduction

‐of MnO4

    

                              

              

                             

10 Fair Common mistakes • use of more concentrated HCl (e.g. 2 M or 4 M) • omitted the description of diluting the given1 M HCl to obtain solutions of different concentrations. • failed to mention how to perform a faircomparison. (e.g. kept constant for theexperimental conditions)

   

                                            

11 • Part (a) – Satisfactory

• Part (b) – Poor • failed to give the correct colour • only stated the colour of the solution without giving a reason

• failed to point out that VO2+ received 2

electrons from SO2

                    

                            

                                            

12 (a) Fair • did not give a specific example for the reagent Y

• Very few candidates were able to give an explanation for the higher solubility of sodium benzoate when compared with benzoic acid. • confused about obtaining an insoluble substance from a mixture with that of obtaining a soluble compound from a solution.

                  

                              

12 (a) Satisfactory • wrongly suggested the use of Br2 to carry out the final step

• wrongly suggested treating the intermediate C6H5CH2OH with concentrated H2SO4 to give a non‐existing alkene before adding HBr.

                              

           

              

13 (a) Satisfactory Common mistakes • Failed to correctly calculate the amounts of thevarious species at equilibrium (molecalculation) • Used incorrect container volume (50 cm3 vs. 50 dm3) • Incorrect numerical calculations / units /significant figures

                                                    

       

14 Fair • Part (a) ‐Many candidates had no idea about the structure of a triglyceride

• Part (c) – Some omitted H+ when using K2Cr2O7

or KMnO4 for the chemical test • Part (d) – Wrong answers include like addition, hydrolysis, saponification and esterification, etc.

       

 

              

                                 

General Comments Candidates are general weak in • Calculations (calculations involved in reactions, equilibrium and volumetric analysis) • Describe correct procedures, even for simple practical tasks (preparation of standard solutions, isolating a substance from a mixture, etc.)

       

 

                    

                

General Comments Candidates are general weak in • Explain physical properties of substances using chemical concepts – bonding and structure (boiling/melting points, solubilities, etc.) • Did not read the questions carefully