2021 RVHS Prelim P2 Ans

River Valley High School 8873/02 [Turn over 2021 JC 2 Preliminary Examination Paper 2 RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME Suggested Answer CLASS 2 0 J INDEX NUMBER H1 CHEMISTRY 8873/02 Paper 2 Structured Questions 15 Sep 2021 2 hours Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST. DO NOT OPEN THIS BOOKLET UNTIL YOU ARE TOLD TO DO SO. Write your name, class and index number in the spaces at the top of this page. Write in dark blue or black pen. You may use an pencil for any diagrams or graphs. Do not use staples, paper clips, highlighters, glue or correction fluid. Section A Answer all questions in the space provided. Section B Answer one question in the space provided. A Data Booklet is provided. Do NOT write anything on it. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Paper 1 30 Paper 2 1 9 2 7 3 6 4 5 5 18 6 15 7 20 8 20 s.f. units Paper 2 80 Total 110 This paper consists of 20 printed pages.

2 River Valley High School 8873/02 2021 JC 2 Preliminary Examination Paper 2 Section A (60 marks) Answer all the questions in this section. 1 (a) (i) Describe the reactions, if any, of the chlorides MgC l2 and PCl5 with water. Write equations for all reactions that occur, and suggest the pH of the resulting solutions. Relate the reactivity of the se chlorides to their structure and bonding. ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… [3] MgCl2 has a giant ionic lattice structure and strong electrostatic attraction between Mg 2+ and C l ions. MgC l2 dissolves in water readily to form an acidic solution of pH 6.5. Both hydration of ions and partial hydrolysis of Mg 2+(aq) occurs. Hydrolysis occurs due to the polarisation of water molecules by the Mg2+ ion MgCl2(s) + 6H2O(l)  [Mg(H2O)6]2+(aq) + 2Cl(aq) [Mg(H2O)6]2+(aq) + H2O(l)  [Mg(H2O)5OH]2+(aq) + H3O+(aq) PCl5 has a simple covalent structure. PCl5 dissolves and undergoes hydrolysis in water to form an acidic solution of pH 2. P atom in PCl5 has energetically accessible vacant 3d orbitals to form dative bonds with water molecules. PCl5(s) + 4H2O(l)  H3PO4(aq) + 5HCl(aq) (ii) Carbon is in the same group as silicon. Suggest and explain if CC l4 will react with water.

3 River Valley High School 8873/02 [Turn over 2021 JC 2 Preliminary Examination Paper 2 ……………………………………………………………………………… ……………………………………………………………………………… [1] CCl4 does not react with water as carbon does not have energetically accessible empty 3d orbitals to accept the lone pair of electrons from water molecules. (b) Phosphorus exist as 31P, 32P and 33P isotopes. 31P is the most stable isotopes. (i) Define the term isotopes. ……………………………………………………………………………… ……………………………………………………………………………… [1] Isotopes are atoms of the same element with the same number of protons but different number of neutrons. (ii) 31P can react with chlorine to form phosphorus chloride. Phosphorus in phosphorus chloride can exhibit variable oxidation states. Based on your knowledge in periodicity, state the two common oxidation states of phosphorus in phosphorus chloride ……………………………………………………………………………… [1] +5, +3 (iii) Phosphorus can exist as cations or anions. The angle of deflection for a bea m of 31P+ ions in an electric field is 6. Calculate the angle of deflection for a beam of 33P3 ions in the same electric field and show in the diagram below, Fig 1, how a beam of 33P3 ions will deflect in the electric field. Fig. 1 [2]  + Beam of 33P3 ions

4 River Valley High School 8873/02 2021 JC 2 Preliminary Examination Paper 2 charge mass for 31P+ =  +1 31  = 0.03225 charge mass for 33P3 =  -3 33  = 0.09091 Since chargeangle of deflection mass  , Angle of deflection= 0.09091 6 16.90.03225  (iv) 32P is unstable and decays rapidly to another element. During the decay, a neutron is converted to a proton with the release of an electron, forming a charged ion. Write an equation to show the reaction. ……………………………………………………………………………… [1] 32P  32S+ + e− [Total : 9]  + Beam of 33P3 ions 16.9

5 River Valley High School 8873/02 [Turn over 2021 JC 2 Preliminary Examination Paper 2 2 (a) Propanoic acid is a weak acid and can be used to make a buffer solution. (i) State the difference between a weak acid and a strong acid. ………………………………………………………………………....... ………………………………………………………………………....... ………………………………………………………………………....... [1] A strong acid is one that ionises/dissociates completely in solution to give a large amount of H+. A weak acid is one that ionises/dissociates partially in solution to give a small amount of H+. (ii) Explain what is meant by the term buffer solution. ………………………………………………………………………....... ………………………………………………………………………....... ………………………………………………………………………....... [1] A buffer solution is one that can resist a change in pH when a small amount of acid or base is added to it. (iii) The hydrogencarbonate ion, HCO 3, is part of the buffer in the blood. Write an equation showing how HCO 3 reacts with acid in the blood. ………………………………………………………………………....... [1] HCO3– + H+ → H2CO3 (iv) After consuming food or drink containing sugar, the pH in the mouth can drop from pH 6.8 to a pH of approximately 4.8 as the sugar is broken down into lactic acid. In time, hydrogencarbonate ions in saliva restore the pH to its original value. Define pH a nd calculate the H + ion concentration in the mouth at pH 4.8. ………………………………………………………………………....... ………………………………………………………………………....... ………………………………………………………………………....... ………………………………………………………………………....... [2]

6 River Valley High School 8873/02 2021 JC 2 Preliminary Examination Paper 2 ………………………………………………………………………....... ………………………………………………………………………....... pH is the negative logarithm to the base ten of the concentration of H+(aq) in mol dm3. or pH = lg [H+(aq)] [H+] = 104.8 = 1.58  105 mol dm3 (b) (i) Write an equation for the reaction between propanoic acid and sodium hydroxide. Label the conjugate acid and the conjugate base in your equation. ………………………………………………………………………....... ………………………………………………………………………....... [2] CH3CH2COOH + NaOH → CH3CH2COONa+ + H2O Conjugate base Conjugate acid [Total: 7] 3 When 40 cm 3 of a gaseous organic compound, C xHyO, underwent complete combustion with an excess of oxygen, the total volume reduced by 130 cm3. A further contraction of 200 cm 3 took place when the residual gas was passed through aqueous sodium hydroxide. All volumes were measured at 25 C and 1 atm. (a) Write a balanced equation for the complete combustion of CxHyO. ……………………………………………………………………….................. [1] CxHyO(g) +(𝑥 + 𝑦 4 − 1 2)O2(g)  xCO2(g) + 2 y H2O(l) (b) State the function of NaOH(aq). ……………………………………………………………………….................. [1] To react with CO2(g). (c) (i) State the volume of CO2 produced in this reaction ………………………………………………………………………....... [1] 200 cm3

7 River Valley High School 8873/02 [Turn over 2021 JC 2 Preliminary Examination Paper 2 (ii) Hence, determine the values of x and y. [3] x = 200  40 = 5 volume r