SIR MIELANGELO REFALO SIXT FORM alf-yearly Exam 2016 Subject: hemistry ADV 1 ST Time: 3 hours Answer 6 questions. All questions carry equal marks. You are reminded of the importance of clear presentation in your answers, and the use of good English. A copy of the Periodic Table of Elements is printed on the last page of this booklet. Important information: The molar gas constant R = 8.31 Question 1: This question is about energetics. (a) State ess s Law. (b) Use the following data to calculate the standard enthalpy of formation, Δf, of butane, 410, whose structural formula is: 410 (g) + 7½ O2 (g) 4O2 (g) + 52O (l) 2 (g) + ½ O2 (g) 2O (l) (graphite) + O2 (g) O2 (g) Give a 3-significant figure answer. Page 1 of 8 Δ combustion = 2878 kj mol 1 Δ combustion = 286 kj mol 1 Δ combustion = 394 kj mol 1 (3 marks) (c) A theoretical value for the standard enthalpy of formation, Δf, of butane can be calculated from bond energy terms for and and appropriate standard enthalpies of atomisation. Draw a properly labelled ess cycle that shows how this can be done. (3 marks) (d) A similar exercise using data as in question (c) to determine the standard enthalpy change of formation, Δf of methylpropane gives the same value for Δf as obtained for butane. Explain why this is so and discuss why the two enthalpy changes should, in fact, be different. The structural formula of methylpropane is as shown on the right. (e) Write a thermochemical equation, including state symbols, for each of the following enthalpy changes: (i) The enthalpy of solution of Lil (ii) The enthalpy of solvation of Li + (iii) The lattice enthalpy of Lil (f) Dissolving 1.50g of Lil in 100cm 3 of water results in a temperature change from 18.8 to 21.9. alculate the standard enthalpy of solution of Lil, in kj mol 1. Assume that the specific heat capacity of the solution is 4.18 J cm 3 1. (3 marks)
Question 2: This question is about gases. (a) State the units for all variables in the ideal gas equation, and use them to find the units of the molar gas constant R. (3 marks) (b) The combustion of methylhydrazine, 3NN2, in oxygen produces steam, carbon dioxide and nitrogen gas according to the following equation: 23NN2 (g) + 5O2 (g) 62O (g) + 2O2 (g) + 2N2 (g) 0.500 mol of methylhydrazine are ignited in 2.00 mol of oxygen gas. (i) alculate the total volume of the mixture of gases, in m 3 obtained when the reaction is complete. Include the volume of any excess reactant, if any is present, in your final answer. Assume that the temperature of the mixture is kept at 500 K and that its pressure is 100 000 Pa. (4 marks) (ii) alculate the partial pressure that is exerted by the nitrogen gas present in the mixture of gases, at 500K. (iii) What will the partial pressure of nitrogen become if the gaseous mixture is allowed to stand overnight in the presence of some solid KO pellets? (c) (i) State the physical conditions under which real gases deviate from ideal behaviour. (ii) Give two reasons that explain why gases do not exhibit ideal behaviour under the conditions listed as your answer for (c) (i). (iii) Sketch a graph which represents the distribution of molecular speeds at two different temperatures, T1 and T2, where T2 = T1 + 10 K. Question 3: This question is about redox reactions and volumetric analysis. (a) Write down the oxidation number of the underlined particle: (i) S8 (ii) Na2S (iii) SiO2 (iv) OF2 (b) omplete and balance the following redox reactions: (i) r2o7 2 (aq) + + (aq) + NO2 (aq) r 3+ (aq) + NO3 (aq) + 2O (l) (ii) lo (aq) + Mn 2+ (aq) + O (aq) MnO2 (s) + l (aq) + 2O (l) (c) Iron (II) sulfate crystals have the formula FeSO4.n2O. In an experiment to determine n, 8.1675g of the salt were dissolved and made up to 250 cm 3 of solution with distilled water. A 25.0 cm 3 portion of the solution was acidified by adding some dilute sulfuric(vi) acid and titrated against potassium manganate(vii) solution of concentration 0.0300 mol dm 3, of which 22.5 cm 3 was required. The unbalanced equation for the redox reaction occurring during the titration is as follows: MnO4 (aq) + 8 + (aq) + 5Fe 2+ (aq) Mn 2+ (aq) + 5Fe 3+ (aq) + 42O (l) Page 2 of 8
(i) Identify the oxidising agent and the reducing agent reacting during the titration. (ii) Find the number of moles of potassium manganate(vii) used in 22.5 cm 3. (iii) Find the number of moles of iron(ii) present in 25.0 cm 3 of the salt solution. (iv) alculate the number of moles of iron(ii) that were present in 250.0 cm 3. (v) alculate the value of n in the formula FeSO4.n2O. (d) Define the term disproportionation. Illustrate your answer by a balanced chemical equation of a chemical reaction that involves disproportionation. Question 4: This question is about entropy and free energy. (a) State what is meant by the term spontaneous change when applied to chemical reactions, in terms of the Gibbs free energy. (b) State, without doing any calculations, whether the following reactions will have a positive or negative value of ΔS θ : (i) N4NO3 (s) N2O (g) + 22O (g) (ii) 3O2 (g) 2O3 (g) (½ mark) (½ mark) (c) Natural gas is converted into hydrogen by reaction with steam in the presence of a nickel catalyst. The balanced chemical equation for this reaction is as follows: 4 (g) + 2O (g) O (g) + 32 (g) Substance Δf θ /kj mol 1 S θ /J K 1 mol 1 4 (g) 74.9 186 2O (g) 242 189 O (g) 111 198 2 (g) 131 (i) State and explain the approximate value of Δf θ for 2 (g). (ii) alculate the entropy change, ΔS θ, for the reaction of steam with methane. (iii) alculate the enthalpy change, Δ θ, for the reaction of steam with methane. (iv) Use your answers to (ii) and to (iii) to calculate the temperature at which the reaction will be feasible. (v) Explain whether the reaction of steam with methane becomes more spontaneous or less spontaneous if carried out at a temperature which is higher than that calculated in (c) (iv). (d) At 0, the change 2O (s) 2O (l) is at equilibrium and so ΔG θ is zero. If Δ θ for the forward process is +6.01 kj mol 1, calculate the value of the ΔS θ for the following processes at 0 : (i) ice water (ii) water ice Page 3 of 8
(e) Icing sugar can be hazardous (it may ignite) when it is being produced by powdering sucrose in a factory. Explain why sucrose is stable at room temperature, but its powdering process is hazardous. Question 5: This question is about bonding and structure. (a) Draw a dot-cross diagram, showing the outer electrons only, for Srl2. State the predominant type of bonding present in this compound. (b) Draw a Lewis diagram showing the shape and the type of bonding present in one XeOF4 molecule. Refer to the VSEPR theory to state the shape that this molecule has. (c) (i) Draw Lewis diagrams showing the shape and bonding in the molecules BF3 and N3. (1 mark each) (ii) Indicate the polarities present, if any, in the molecules BF3 and N3, by drawing the symbols δ+ and δ on the diagrams drawn in (d) (i) wherever appropriate. (iii) Explain, by referring to intermolecular bonding, why the boiling temperature of BF3 is much lower than the boiling temperature of N3. (d) Account for the following observations: (i) The ground state electron configuration of carbon shows that 1 carbon atom has 2 unpaired electrons in its valence shell, yet carbon forms 4 and not 2. (3 marks) (ii) Sodium chloride is a high melting point solid while ammonium chloride sublimes upon heating. Question 6: This question is about nuclear chemistry and mass spectrometry. (a) The nucleus of an isotope of uranium, A, contains 92 protons and 146 neutrons. The isotope decays by the emission of a particle B containing 2 protons and producing a nuclide. The half-life of the process is 4.5 10 9 years. The nuclide decays by a process whereby a neutron changes into a proton with the emission of a particle D and a nuclide E. Particle D has a very small mass. (i) Define the terms isotope and half-life. (ii) Write a balanced nuclear equation for the decay of A into B and. In your answer represent all particles by their appropriate chemical symbols. (iii) State the name of the particle B. (½ mark) (iv) Write a balanced nuclear equation for the decay of into D and E. In your answer represent all particles by the appropriate chemical symbols. (v) State the name of the particle D. (½ mark) Page 4 of 8
(vi) A sample of uranium ore contains 1.28 10 22 atoms of isotope A. Determine the mass of uranium atoms of isotope A present in the sample 1.35 10 10 years ago. (3 marks) (b) The following are the mass spectrum of ethanal. Ethanal is an organic molecule whose chemical formula is 24O. (i) What is the name given to the peak at m/z = 29? (ii) Draw the displayed structural formula of the chemical species that are responsible for the peaks at m/z =15 and at m/z = 29. (iii) By referring to your answers to (e) (ii), draw the displayed structural formula of ethanal. State the m/z value of the peak that supports your answer. (iv) A small peak appears further to the right of the molecular ion peak in the mass spectra of many organic compounds. Give an explanation for this common occurrence. (c) The relative atomic mass of l is 35.5. alculate the percentage abundance of the 37 l isotope in the element chlorine given that this element has the isotopes 35 l and 37 l. Question 7: This question is about dynamic equilibria. (a) Write an expression for Kc of the following equilibrium and give the units for Kc: u 2+ (aq) + 4N3 (aq) [u(n3)4] 2+ (aq) (b) Write an expression for Kp of the following equilibrium and give the units for Kp: P4 (s) + 6l2 (g) 4Pl3 (g) Page 5 of 8
(c) The copper mineral malachite is principally composed of copper(ii) carbonate. On heating in a closed container the following equilibrium is established forming copper(ii) oxide and carbon dioxide, as shown by the following equation: uo3 (s) uo (s) + O2 (g) Explain briefly what would happen to the amount of uo3 that is decomposed if these changes are applied separately to the equilibrium mixture: (i) O2 is added to the mixture. (ii) A few drops of concentrated NaO are added to the mixture. (iii) The volume of the container is increased. (d) Nitrogen and oxygen react at high temperatures to form NO. N2 (g) + O2 (g) 2NO (g) Δ θ = +180 kj mol 1 The value of Kc at 600 is 3.3 10 10. (i) State the sign of the enthalpy change of reaction of the reverse process. (ii) alculate the fraction of N2 that is converted to NO when an equimolar mixture of N2 and O2 are heated to 600, in closed container. (4 marks) (iii) For the reaction of N2 and O2, sketch and label on the same set of axes, graphs to show changes in (I) concentration of the reactants with time and (II) concentration of the product with time. Indicate on the sketch the point at which equilibrium is reached. (iv) Explain qualitatively what would happen to Kc when the temperature is increased at constant volume. (v) Use the value of Kc given for the formation of NO to calculate Kc of the equilibrium reaction by which NO decomposes, as shown by the equation below: 2NO (g) N2 (g) + O2 (g) Question 8: This question is about atomic structure. (a) Give a value for the following, for the species (i) atomic number (ii) mass number (iii) neutron number (iv) electron configuration, in terms of orbitals. (b) Explain what is meant by the following statement: l is a p-block element while Fe is a d- block element. (c) The successive ionisation energies of an element X are shown in the following table: Page 6 of 8
Process I1 I2 I3 I4 I5 Ionisation energy (kj mol 1 ) 787 1577 3232 4356 16091 (i) Write a thermochemical equation, including state symbols and the enthalpy change value, for the third ionisation energy of element X. (ii) Explain why the difference between the 4 th and the 5 th ionisation energies is much greater than that between any two other successive values. (iii) The first ionisation energy of the other elements in the group in which X lies in the periodic table are, in order of increasing value, in kj mol 1 : 709, 716, 762 and 1086. (I) State and explain the electronic configuration, in terms of orbitals, of the outer shell of element X by referring to the ionisation energies given. (II) Use the periodic table provided to suggest a possible identity for element X. (d) (i) Sketch a graph of the first ionisation energies against atomic numbers of the elements in period 3. (ii) Explain the general trend that appears in the graph. (iii) Explain any particular exceptions to the general trend that appears in the graph. Page 7 of 8
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