SIR MICHELANGELO REFALO

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1 SIR MICELANGELO REFALO SIXT FORM alf-yearly Exam 2014 Name: CEMISTRY ADV 1 ST 3 hrs ANSWER ANY 7 QUESTIONS. All questions carry equal marks. You are reminded of the importance of clear presentation in your answers, and of the use of good English. Important Information: A Periodic Table is printed on the last page of this exam paper. 1 mole of gas at s.t.p. occupies a volume of cm 3 R = 8.31 J K 1 mol 1 K w = mol 2 dm 6 at 298 K 1. This question is about energetics and standard solutions. (a) Define the term standard enthalpy of formation of a substance. (b) Use the following information to draw the Born-aber cycle for calcium chloride, CaCl 2. Include state symbols and the enthalpy values in each step of the cycle, and calculate the enthalpy of formation of CaCl 2. (5 marks) Process Δ θ (kj mol -1 ) Enthalpy of atomisation of calcium +193 First ionisation energy of calcium +590 Second ionisation energy of calcium Enthalpy of atomisation of chlorine +121 First electron affinity of chlorine 348 Lattice enthalpy of calcium chloride 2237 (c) Magnesium is found above calcium in the periodic table. Would you expect the lattice enthalpy of magnesium chloride to be more negative or less negative than that of calcium chloride? Explain your answer. (d) When 2.00 g of anhydrous calcium chloride are dissolved in 100 cm 3 of water, the temperature rises by 3.5 C. The solution formed has a density of 1.00 g cm 3 and a heat capacity of 4.20 J g 1 K 1. (i) Write a thermochemical equation for the enthalpy of solution of CaCl 2. (ii) Calculate the standard enthalpy of solution of CaCl 2. (e) Anhydrous calcium chloride is a compound which is not suitable to be used as a primary standard in quantitative analysis, because it is deliquescent. (i) What is a standard solution? (ii) Explain why deliquescent compounds should not be used as primary standards. (iii) Give the name of one compound that may be used to prepare an alkaline standard solution. Page 1 of 8

2 2. This question is about Redox Reactions (a) Write down the oxidation number of the underlined particle: 2 + (i) Br (ii) CrO 3 (iii) 2 O 2 (iv) SO 4 (v) N 4 (vi) NO 3 (3 marks) (b) Complete and balance the following equation for a redox reaction that occurs in an alkaline medium: ClO + Mn 2+ MnO 2 + Cl (c) Complete and balance the following equation for a redox reaction that occurs in an acidic medium: IO 3 + I + + I O (d) A solution of sodium thiosulfate was standardised in the following way g of potassium iodate(v), KIO 3, were dissolved in cm 3 of aqueous solution. A cm 3 portion of this solution was placed in a conical flask; then excess aqueous sulfuric(vi) acid and excess potassium iodide solution were added to it. The iodine liberated was titrated with cm 3 of sodium thiosulfate solution. (i) Find the number of moles present in g of KIO 3. (ii) Find the number of moles of KIO 3 in the cm 3 used in the titration. (iii) Use the balanced equation in (c) to find the number of moles of iodine that were produced when excess acid and potassium iodide solution where added to the KIO 3 (aq) sample in the conical flask. (iv) Balance the following equation for the redox reaction that occurs during the titration: S 2 O 2-3 (aq) + I 2 (aq) S 4 O 2-6 (aq) + I - (aq) (v) Use the balanced equation in (d) (iv) to find the number of moles of sodium thiosulfate present in cm 3. (vi) Calculate the molar concentration of the sodium thiosulfate solution. 3. This question is about Gases. (a) Describe briefly TWO characteristics that distinguish the gaseous state from the liquid state and from the solid state. (b) Describe briefly TWO assumptions that the Kinetic Theory of Gases makes about the behaviour of a gas when it is said that a gas is behaving ideally. (c) A sample of an ideal gas had a volume of 50.0 cm 3 measured at 20.0 C and Pa. What is its volume at 0 C and at 100 kpa? (d) Define the term partial pressure of a gas in a mixture of gases. (e) (i) A mixture of gases at N m -2 has the composition 40% N 3, 25% 2 and 35% N 2. Calculate the partial pressure of N 3 in the mixture. (ii) What will the partial pressures of the other gases become if the ammonia is removed by the addition of some solid phosphorus(v) oxide? (f) g of a liquid was vaporised. The vapour formed occupied 100 cm 3 at 20 C and Pa. (i) Name the apparatus in which this vaporisation must have been carried out. (ii) Calculate the molar mass of the liquid, if its vapour form behaves ideally. Page 2 of 8

3 (g) A g sample of impure calcium carbonate was heated. The calcium carbonate that it contained decomposed according to the following equation: CaCO 3 (s) CaO (s) + CO 2 (g). The decomposition produced 160 cm 3 of CO 2 measured at S.T.P. Calculate the mass of CaCO 3 that was present in the impure sample used. 4. This question is about bonding. (a) Explain what is meant by an ionic bond, giving an example of a compound that has pure ionic bonding among its particles. (b) Explain what is meant by a covalent bond, giving an example of a compound that has pure covalent bonding among its particles. (c) (i) Write the electron configuration, in terms of orbitals, for the carbon atom and for the hydrogen atom, both in their ground state configuration (Proton number of C = 6, = 1) (ii) In the molecule C 4, carbon forms 4 bonds by overlapping its orbitals with other atoms. Explain how this is possible in view of your answer to (c) (i). (2 ½ marks) (d) Copy molecule A and molecule B (shown below) on your foolscap and answer the following questions about them: 1 N 2 O 1 2 N O Molecule A O 3 4 C C Molecule B (i) Use the VSEPR theory to estimate the size of the bond angles marked 1 to 4. (Proton Number of N = 7, C = 6, O = 8, = 1) (ii) Use the symbols δ+ and δ to indicate strong polarity wherever it may occur on each of the molecules drawn. (iii) Give the name of the strongest intermolecular attractive force that may form in between molecules of A and molecules of B when substances A and B are mixed together. (½ mark) (e) Discuss briefly each of the following statements. (i) The bond enthalpy term of the C-C bond in graphite is more endothermic than that of the C-C bond in diamond. owever, diamond is harder than graphite. (ii) At 0 C, hydrogen fluoride is a liquid while all other hydrogen halides are gases. (iii) The sulfur dioxide molecule is a polar molecule while the carbon dioxide molecule is a non-polar molecule. 5. This question is about atomic structure. (a) Distinguish between the terms orbital and energy level. (b) Sketch and label, on a separate set of axes, the shape of an s-orbital and the shape of a p-orbital. A graph paper is NOT required to answer this question. Page 3 of 8

4 (c) Briefly describe what would be happening to the electrons in metal ions when these ions produce a flame colour when inserted into a Bunsen flame. (d) Consider the first 8 successive ionisation energies for an element X: Process I 1 I 2 I 3 I 4 I 5 I 6 I 7 I 8 Ionisation energy (kj mol ) (i) Use the values in the table given above to state which group of the periodic table element X is found. Briefly explain your answer. (ii) Write down a thermochemical equation representing the second ionisation energy of element X. (e) Consider the following graph, which shows the first ionisation energies of the elements Li to Ne: Page 4 of 8 (i) Explain fully why there is a general increase in the first ionisation energy of the elements in going across period 2. (ii) Explain why the first ionisation energy decreases on going from Be to B. (iii) Explain why the first ionisation energy decreases on going from N to O. (f) State and explain how the atomic radii vary down a group of elements of the periodic table. (g) (i) Write down the electronic configuration, in terms of orbitals, for the following species: F -, O 2-, S 2-, Cl (ii) State which two among the three species mentioned in (g) (i) are isoelectronic, and briefly explain your choice. 6. This question is about atomic and nuclear chemistry. (a) Write the atomic number, the mass number, and also the number of neutrons and the number of electrons in the species e. (b) Distinguish between the terms relative atomic mass and molar mass. In your answer state the units of each (if any). (c) Magnesium has three isotopes and, with percentage abundances of 78.6, 10.1 and 11.3 respectively. Calculate the relative atomic mass of this sample of magnesium, giving your answer to three significant figures.

5 (d) Draw a well-labelled diagram of a mass spectrometer, and describe briefly how this apparatus operates. (3 ½ marks) (e) The following is the mass spectrum of bromoethane. The labels are m/z values: C C Above: The structural formula of bromoethane Br (i) Suggest the identity of the chemical species responsible for the peaks at m/z = 29 and at m/z = 108. (ii) Bromine has a relative atomic mass of 80 and has two isotopes. Use the mass spectrum to state the ratio of the relative abundance of these two isotopes. (iii) What is the name given to the peak at m/z = 29? (½ mark) (iv) What is the name given to the peak at m/z = 110? (½ mark) (f) (i) What is the half-life of a radioactive substance? (ii) a has a half-life of 15 hours. What fraction of a remains after 60 hours? (g) Copy and complete the following two equations each representing a nuclear reaction. Use the periodic table and the information given in the nuclear equations below to find the actual symbols of the nuclei X and Y. Use the actual symbols as part of your answer. (i) (ii) (2 ½ marks) 7. This question is about chemical equilibria. (a) Consider the equation labelled as Equation 1: A + B r f C + D r Use Equation 1 to distinguish between the terms reversible reaction b and dynamic equilibrium. (b) Consider Equation 2. This equation shows the decomposition of sulfur trioxide into sulfur dioxide and oxygen: r f Equation 2: 2SO 3 (g) 2 SO 2 (g) + O 2 (g) Δ = kj mol 1 r b 2.97 moles of SO 3 (g) are introduced into a 5.00 dm 3 container at 1100 K. At equilibrium, 0.47 mol of O 2 (g) have been formed. Page 5 of 8

6 (i) Explain the effect of compressing the contents of the container, while keeping the temperature constant, on the equilibrium position and on the yield of O 2. (ii) Explain the effect of raising the temperature to 1200 K, while keeping the pressure constant, on the equilibrium position and on the yield of O 2. (iii) Write an expression for the equilibrium constant K c for Equation 2. (iv) Calculate the value of K c for the reaction at 1100 K. (3 marks) (v) Use the ideal gas equation to calculate the total pressure inside the container at 1100 K and hence use your answer to calculate the partial pressure of SO 3 gas in the same container at the same temperature. (c) Sulfuric(VI) acid is manufactured by the Contact Process. One stage of such process involves the oxidation of sulfur dioxide according to Equation 3: Equation 3: 2 SO 2 (g) + O 2 (g) r f r b 2SO 3 (g) Compare Equation 3 to Equation 2 found in question 7 (b) in order to be able to: (i) state the value of the enthalpy change of the forward process of Equation 3; (ii) calculate the value of K c for the equilibrium reaction shown by Equation 3, when the reaction that Equation 3 shows is also carried out at 1100 K. (d) What is the effect of the introduction of a catalyst on (i) the position of equilibrium, and (ii) the rate of attainment of equilibrium? Give one reason that supports both of your answers. (3 marks) 8. This question is about chemical thermodynamics and about sparingly soluble salts. (a) Define the term entropy of a chemical system. (b) Predict whether the entropy change for the following reaction will be significantly positive, significantly negative or approximately zero and explain your reasoning: 2C (graphite) + SiO 2 (s) 2CO (g) + Si (s) (c) Write down an equation that relates the free energy change to the enthalpy change and the entropy change of a chemical reaction occurring at a specific temperature T. (d) Consider the following information about the precipitation reaction of lead(ii) sulfate, which proceeds as the following equation shows: Pb 2+ (aq) + SO 4 2- (aq) r f r b PbSO 4 (s) θ formation (kj mol -1 ) S θ (J K -1 mol -1 ) Pb 2+ (aq) SO 2-4 (aq) PbSO 4 (s) (i) Calculate the value of θ for the precipitation of PbSO 4 (s) and state its units. (ii) Calculate the value of θ for the precipitation of PbSO 4 (s) and state its units. (iii) Calculate the value of G θ for the precipitation reaction at 298 K and comment on the solubility of lead(ii) sulfate at this temperature. (3 marks) Page 6 of 8

7 (e) The solubility product of lead(ii) sulfate in water at 298 K is mol dm -3. (i) Explain the meaning of the term molar solubility of a sparingly soluble substance. (ii) Write an expression for the solubility product, K sp, of lead(ii) sulfate. (iii) Write an expression that relates K sp to the molar solubility, s, of lead(ii) sulfate. (½ mark) (iv) Calculate the molar solubility of lead(ii) sulfate and state its units. (½ mark) (v) Will the value of the molar solubility of lead(ii) sulfate in a solution of mol dm 3 solution of Na 2 SO 4 be more or less than that calculated in (e) (iv)? Explain your answer qualitatively and in a brief way. (vi) Refer to your answer for (d) (i) to explain the effect that an increase in temperature may have on the value of K sp for lead(ii) sulfate. 9. This question is about the chemistry of acids, bases and soluble salts. (a) Define p. (b) Calculate the p of mol dm 3 aqueous solution of sulfuric(vi) acid. Give your answer to three significant figures. (c) N 3 is a weak alkali. The pk b of an aqueous solution of ammonia at 298 K is (i) Write a balanced equation showing the dissociation of ammonia in water. (½ mark) (ii) Explain the terms base and conjugate acid as used in the Brønsted-Lowry theory by referring to an aqueous solution of ammonia in your answer. (iii) Write an expression for the base dissociation constant of ammonia, and state its units. (½ mark) (iv) Calculate the p of a mol dm 3 N 3 solution at 298 K. Give your answer to three significant figures. (3 marks) (v) Methanamine, C 3 N 2, is a weak base having a K b of mol dm 3. Using the appropriate calculation(s), state whether methanamine or ammonia is the weaker base. (vi) Calculate the p of a buffer solution containing mol dm 3 N 3 and mol dm 3 N 4 Cl in aqueous solution at 298 K. Give your answer to three significant figures. (d) Sketch a p vs. volume of alkali titration curve for the titration of Cl (aq) with N 3 (aq) and estimate the value of the p of the solution obtained at the equivalence point. (1½ marks) (e) Use the titration curve in (d) to explain whether phenolphthalein or methyl orange would be a suitable indicator for the titration of hydrochloric acid with ammonia solution. State the colour change that is observed at the end point of this titration. (2 ½ marks) Page 7 of 8

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SIR MICHELANGELO REFALO CENTRE FOR FURTHER STUDIES VICTORIA GOZO

SIR MICHELANGELO REFALO CENTRE FOR FURTHER STUDIES VICTORIA GOZO Page 1 of 7 Half Yearly Exam 2013 Subject: Chemistry 1 st Year Level: Advanced Time: 3 hrs Answer SEVEN (7) questions. All questions carry