Chemistry Paper 3 Questions and Answers - MECS Cluster Joint Mock Exam

QUESTIONS

You are provided with:

• Solid F 
• 2.0 M hydrochloric acid solution G
• 0.1 M sodium hydroxide

1. You are required to determine the;
   Enthalpy change ΔH, for the reaction between solid F and one mole of hydrochloric acid 
   Procedure:
   Using a burette place 20.0cm³ of 2.0M hydrochloric acid, solution G in a 100ml beaker. Measure the initial temperature of the solution after every ½ minute and record the values in table 1 below. At exactly 2 minutes, add all of solid F to the acid. Stir the mixture gently with the thermometer. Measure the temperature of the mixture after every half-minute and complete the table. (Retain the mixture for use in procedure II)
   Table 1
   

   Time(min)	0	½	1	1½	2	2½	3	3½	4	4½	5
   Temperature (ºC)

   1. Plot a graph of temperature (y-axis) against time.(3 marks)
   2. From the graph determine the change in temperature ΔT (1mark)
   3. Calculate the heat change for the reaction (Assume the specific heat capacity of the mixture is 4.2 J g-1 K-1 and the density of the mixture is 1g/cm³)(1mark)
      Procedure II 
      Rinse the burette thoroughly and fill it with sodium hydroxide. Transfer all the contents of the 100ml beaker from procedure I above into a 250ml volumetric flask, add distilled water to make up to the mark. Label this solution H. using a pipette and a pipette filler, place 25cm³ of solution H into a 250ml conical flask. Add two to three drops of phenolphthalein indicator and titrate against sodium hydroxide. Record your results in table 2. Repeat the titration two more times and complete the table.
      Table 2
      

      	I	II	III
      Final burette reading (cm3)
      Initial burette reading (cm3)
      Volume of solution solution D used (cm3)

      Calculate the;
      1. Average volume of sodium hydroxide used  (1mark)
      2. Number of moles of:
         1. Sodium hydroxide used (1 mark)
         2. Hydrochloric acid in 25cm³ of solution H (1 mark)
         3. Hydrochloric acid in 250cm³ of solution H (1 mark) 
         4. Hydrochloric acid in 20cm³ of solution G  (1 mark)
         5. Hydrochloric acid reacted with solid F   (1 mark)
   4. Calculate the enthalpy of reaction between solid F and one mole of hydrochloric acid. (Show the sign of ΔH) (1 mark)
2. You are provided with solid A. Carry out the tests described below and write your observations and inferences accordingly.
   1. Dissolve solid A in about 10cm³ of distilled water in a boiling tube divide the resulting solution into five portions.
   2. To the first portion and 5 drops of 2M sodium hydroxide solution 
   3. To the second portion dip a glass rod to one of the remaining portions and heat it in a non-luminous flame. 
   4. To the third portion add 2 or 3 drops of lead (II) nitrate  solution 
   5. To the forth  portion add 2 or 3 drops of barium (II) chloride followed by 2cm³ of 2M hydrochloric acid. Shake the mixture well. 
   6. To the fifth portion add 3 drops of acidified potassium manganate (VII) solution 
3. You are provided with substance B. 
   Carry out the tests described below and record your observations and inferences accordingly.
   Procedure
   1. Place a little amount of substance B in a metallic spatula and ignite it in a blue bunsen burner flame.
   2. Place a spatulaful of substance B in a boiling tube. Add about 20cm³ of distilled water and shake well. Divide the solution into three portions. Test the pH of one portion of the solution above using a full range pH chart.
   3. Add the sodium carbonate provided to the second portion.
   4. Add a few drops of potassium manganate (VII) solution to the third portion and warm mixture.

CONFIDENTIAL
In addition to the apparatus and fittings found in the chemistry laboratory, each candidate will require the following:

1. 2.1g of solid F (NaHCO₃) weighed accurately and supplied in a dry stoppered container.
2. About 60cm³ of solution G(2M HCl)
3. About 130cm³ of solution of sodium hydroxide solution.(0.1M )
4. One thermometer -10ºC – 110ºC
5. One stop watch/ clock
6. One 100ml plastic beaker
7. One burette 0 – 50ml
8. One pipette 25ml
9. One 250ml volumetric flask 
10. About 500cm³ of distilled water supplied in a wash bottle
11. One label or means of labelling
12. One pipette filler
13. Two conical flasks
14. Clamp and stand
15.  About 0.2g of solid A (Na2SO₃)supplied in a stoppered container
16. 0.2g of solid B (maleic acid)
17. Six clean dry test-tubes and One boiling tube
18. One metallic spatula
19. Glass rod
20. About 0.2g of sodium carbonate(solid)
21. 2cm piece of pH paper 

ACCESS TO

1. Means of heating.
2. 2M  aqueous sodium hydroxide supplied with a dropper
3. Acidified potassium manganate (VII) supplied with a dropper
4. Phenolphthalein indicator supplied with a dropper
5. Full range pH chart
6. Lead (II) nitrate supplied with a dropper(0.5M)
7. Barium (II) chloride supplied with a dropper(0.5M)
8. 2M hydrochloric acid supplied with a dropper

NOTES

1. Solution G is prepared by adding 172.0cm³ (1.28g/cm3) of concentrated hydrochloric acid to about 500cm³ of distilled water and diluting to one litre of solution. (2M HCI)
2. Acidified potassium manganate (VII) is made by dissolving 3.16g of the solid in about 500cm3 of 2M H₂SO₄ and diluting to one litre of solution. (0.02M KMnO₄) 
3. Sodium hydroxide is prepared by dissolving 4g of the solid in about 700cm³ of water then diluting to one litre.(0.1M NaOH) 

MARKING SCHEME 

Table 1

• Complete table ……………… 2 mks
  Penalties / conditions
  Penalize ½ mk for each space not filled in the temperature row upto a maximum 
• Use of decimals – 1mk
  Accept whole numbers or decimals up to 1 dec place (either .0 or .5) or 2nddec. (, 25, .50 or .75) used consistently, otherwise penalize fully.
• Accuracy ----------- 1 mk
  Compare the candidate’s first temperature reading to the school value (teacher’s first temperature reading)
  If within ± 2.0ºC, award 1mk, otherwise penalize fully.
• Trend ----------1mk
  Award 1mk if temperature remains constant from 0 min to 1½ min , then a drop followed by a continous rise otherwise penalise fully

1. GRAPH. 
   Labeling of axes (both) ------------- ½ mk)
   • Penalise ½ mk for wrong units used in any of the axis otherwise ignore if units not given.
   • Penalise ½ mk for inverted axis
   • Accept for ½ mk if no units shown on labeling.
     Scale --------------------½mk
   • Area covered by actual plots must be at least half of the big squares ( y – axis) and half of the big squares ( x – axis)otherwise give zero.
   • Scale used must be consistent on both axes, otherwise penalise fully.
     Plotting -------------1mk
   • Accept 8 or 9 points correctly plotted for 1mk
   • If 5 or 7 points are correctly plotted award ½ mk
   • If  less than 5  points are correctly plotted gives Zero
   • Accept correct plots even if the axes are interchanged.
     The lines------------1mk
   • Accept for ½ mark an extrapolated straight line passing through at least 2 correctly plotted points from time 0 min to 1½ min. Award another ½ mark for another extrapolated straight line passing through at least 2 correctly plotted points from time 2½ min to 5 min 
2. Showing on the graph √ ½ mk (extrapolation at time 2 minutes)
   • Stating the correct reading of ΔT√ ½ mk
     Conditions
   • penalise√ ½ mk if not shown on the graph to obtain the value.
   • Award 1mk if not shown on graph and not recorded but used correctly in the expression.(mcΔT)
   • Award 0 mk if not shown on the graph and value stated is wrong.
3. Heat change for the reactio
   ΔH = mc ΔT
   = 20×4.2×ΔT
          1000 √
   =correct answer kJ √½

Penalties / conditions

1. Accept answer given in joules /kilo joules ignore sign if given however penalize ½ for –ve 
2. Otherwise penalize ½ mk for wrong units given 

PROCEDURE II
TABLE 2 ……………..Total 5 mks distributed as below

1. Complete table ----------------------------1mk
   Complete table with 3 titrations ------1mk 
   Penalties
   • Unrealistic titre values i.e. values below 1cm3 or hundreds 
   • Burette readings beyond 50cm³  unless explained
   • Inverted table
   • Wrong arithmetic
     N/B: Penalize ½ mk each for a maximum of ½ mk
2. Decimals ----------1mk(Tied to the 1st and 2nd rows only)
   Should be 1 decimal place or 2 decimal places used consistently otherwise penalise fully.
   The 2nd decimal place should either be a 0 or 5
3. Accuracy ------------1mk
   1. If any titre value within ± 0.1 of the school value ---------------------------------award 1mk
   2. If any titre within ± 0.2 of the school value ½ mk
   3. If none of the titre values is within ± 0.2 of school value (s.v)-----------award 0mk
4. Principles of Averaging -------------1mk
   Values to be averaged MUST be shown and MUST be within ± 0.2 of each other
   Conditions 
   1. If 3 consistent values are averaged -----------1mk
   2. 3 titrations done only 2 are possible and averaged ---1mk
   3. 3 consistent values but only 2 are averaged award 0mk
   4. 3 inconsistent values are average award 0mk
      Penalties
      1. Penalise ½ mk for arithmetic error in answer outside ± 2 units in the 2nddec. Place.
      2. Penalise ½ mk for NO WORKING shown but the answer is correct.
      3. Accept rounding off to the 2nddec.places otherwise penalise ½ mk if answer is rounded off to 1stdec. place unless values divide exactly to 1 dec. place
5. Final answer --------------------1mk
   (Compared to school value (S.V) Tied to correct average titre)
   1. If within ± 0.1 of S.V ---------  award 1mk
   2. If within ± 0.2 of S.V -------- award ½mk
   3. If beyond ± 0.2 of S.V--------- award 0mk

1. Average titre x 1.5  mk =Correct answer
        1000
   Condition /penalties
   1. Penalise ½ mk for wrong Transfer (WT) of titre otherwise Penalise fully for strange value
   2. 1.5 Must be transferred INTACT otherwise penalise fully
   3. Penalises ½ mk for arithmetic error outside 2 units in the 4thdec. place
   4. Accept rounding off of the answer to the 4thdec.place
   5. Units may not be shown but it shown must be correct, otherwise penalise ½ mk for wrong units.
2.      
   1. Moles of sodium hydroxide
      = 0.1×average volume
                    1000
      = correct answer
   2. Moles of NaOH: HCI = 1:1  mk
      Moles of HCI in 25cm³ = correct answer (i) above.√½
   3. Moles of HCI in 250cm³
      = answer(ii)×250
                 25
      = correct answer √½
   4. Moles of HCI in 20cm³ of solution G. 
      =  2 × 20 
         1000√½
      = correct answer √½
   5. Moles of hydrochloric acid that reacted with solid F
      = Answer (iv) - Answer (iii) √½
      = correct answer √½
3. Molar enthalpy of reaction between solid F and one mole of hydrochloric acid 
   = 1 × answer (c)
       answer (v)
   = correct answer (+ve) kJ/mol
   (penalise ½ mk for ΔH not shown / wrong i.e given as –ve)
   Conditions /penalties
   1. Penalise ½ mk for wrong Transfer (WT) of Answer (ii), otherswisepenalise fully for strange value 
   2. Penalise ½ mk for an arithmetic error outside ± 2 units in the 4thdec. place 
   3. Same as (iv) in II above
   4. Same as (v) in II above
4. Moles in 100cm³ of solution T
   Answer 111 x 100 = Correct Answer
          25
   Conditions/ penalties
   1. penalise ½ mk for arithmetic error outside 2 units in the 4thdec. place
   2. Penalise ½ mk for wrong transfer / rounding off of answer 111
   3. Same as (IV) in 11 above
   4. Same as (V) in 11 above
      Moles ratio for reaching of Mg to H₂SO₄ is 1:1
      Moles of H₂SO₄ used in procedure 1 
      Answer 1 (d)
      Answer 1 (d) + Answer iv = Correct Answer
      Conditions / penalties
      1. Penalise ½ mk for WT of Answer 1 (d) or answer (iv) or for both otherwise penalise fully for strange value.
      2. Penalise ½ mark for arithmetic error outside ± 2 units in the 4thdec. place.
      3. Molarity of solution Q - H₂SO₄
         1000×answer (iv) = Correct answer
                  50
         Conditions /penalties
         Penalise ½ mk for wrong Transfer of answer (ii) to  (iv) otherwise penalize fully for strange value.

Question 2

Question 3