ASSOCIATE DEGREE IN ENGINEERING TECHNOLOGY RESIT EXAMINATIONS SEMESTER 2 July, 2012 COURSE NAME: CODE: CHEMISTRY FOR ENGINEEERS CHY-1008 GROUP: ADET 2 DATE: July 2, 2011 TIME: DURATION: 1:00 pm 2 HOURS INSTRUCTIONS: 1. This paper consists of SIX questions. 2. Candidates must attempt ANY FOUR questions on this paper. 3. All working MUST be CLEARLY shown. 4. Keep all parts of the same question together. 5. The use of non-programmable calculators is permitted. DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO Instructions: Answer any FOUR (4) questions. 1
Question 1 The safety air bags used in many vehicles are inflated on impact by a gas that is produced from the rapid decomposition of sodium azide (NaN 3 ). (a) Assuming that the gas produced behaves as an ideal gas under these conditions: (i) State TWO properties of an ideal gas. (ii) Use the ideal gas equation to calculate the number of moles of gas produced in an air bag of volume 4.0 x 10-5 m 3 at a pressure of 2.0 x 10 5 N.m -2 and a temperature of 298 K. (4 marks) (iii) Given that the mass of gas produced in the air is 87.5g, use the answer from (a) (ii) to calculate the molar mass of the gas. (3 marks) (b) State the conditions of temperature and pressure under which gases deviate from ideality. (c) The foul odour of dirty socks may be attributing to caproic acid, an organic acid made of 62% carbon, 10.3% hydrogen and 27.6% oxygen. RAM: C = 12; H = 1; O = 16 (i) Distinguish between empirical formula and molecular formula, using suitable examples. (4 marks) (ii) Calculate the empirical formula of caproic acid (4 marks) (iii) Calculate the molecular formula of caproic acid, given that its molar mass is 116g.mol -1. (3 marks) (d) Describe the movement of particles in steam, water and in ice. (3 marks) [25 MARKS] Question 2 2
a) The initial rates listed were determined for the reaction: 2 NO (g) + Cl 2(g) 2 NOCl (g) Experiment Initial [NO] / M Initial [Cl 2 ] / M Intial rate of decomposition of Cl 2 / M s -1 1 0.13 0.20 1.0 10-2 2 0.26 0.20 4.0 10-2 3 0.13 0.10 5.0 10-3 i. Write the rate law for the reaction. (State how you obtained your law by showing calculations or by explaining your reasoning. (6) ii. Determine the value of the rate constant. (4) iii. Calculate the intial rate when both reactants have a concentration of 0.05M. (2) b. Using an energy profile diagram illustrate how a catalyst alters the rate of a chemical reaction. (5) c. Explain how any TWO factors besides a catalyst affect the rate of a chemical reaction. (4) d. Define the terms (i) (ii) (iii) Rate of reaction Rate constant Order of reaction (iv) Half life (4) Question 3 3
A. Consider the following gaseous equilibrium system: Equation 1: N 2 O 4(g) 2NO 2(g) H = + 57.2 kj (colourless) (red-brown) (a) State TWO characteristics of a chemical system in equilibrium. (b) Write the expression for the reaction in this equilibrium system. (1 mark) (c) Calculate the value of Kc and Kp at 303K for the reaction in Equation 1 if at the equilibrium the concentrations are: 0.038 mol.dm -3 of NO 2 and 0.011 mol.dm -3 of N 2 O 4. (5 marks) (d) Using Le Chatelier s principle explain the effect of EACH of the following on the equilibrium position of the reaction in Equation 1. (i) (ii) An increase in pressure An increase in temperature (iii) An increase in concentration of NO 2 (iv) Adding a catalyst (8 marks) B. (a) Define the terms (i) ph (ii) Weak acid (b) The aqueous hydrogen ion concentration in human blood is 4 X 10-8 mol.dm -3. What is the ph of blood in human blood? (c) The measured ph of a solution of an aqueous base is 12.0. Calculate the corresponding hydroxide ions concentration. (5 marks) [25 MARKS] (Question 4) 4
a) What is a chemical bond? Why do elements participate in bonding? (b) Distinguish between an ionic and a covalent bond. Give two (2) examples of compounds characteristic of each bond type. (2 + 6) c) Explain the trend in atomic radius and ionization across a period and down a group. [6] d) i. Use potassium as an example, write an equation to illustrate first ionization energy. [1] ii. Write electron configuration for Cr, Ca 2+ and N 3-. [3] iii. What is meant by first ionisation energy? [1] e. Compare Potassium with Sodium in terms of the following properties? i. atomic size [1] ii. number of outer shell electrons [1] iii. ionization energy [1] f. Explain why the Be 2+ ion is smaller than the O 2- ion even though the Be atom is larger than the O atom. [5] g. Which of the following has the higher ionization energy, Flourine or Oxygen? Explain. [ 4] [25 MARKS] (Question 5) A. The manganese (IV) oxide reacts in an acidic medium with chloride ions according to the following equation: MnO 2 (s) + 4 H + (aq) + 2 Cl - (aq) --> Mn 2+ (aq) + 2 H 2 O(l) + Cl 2 (g) (a) Define oxidation and reduction in terms of oxidation number and electron transfer. (4 marks) 5
(b) Identify the role played by the Cl - ion in the redox reaction above and explain your answer in terms of oxidation number. B. Using the following electrochemical data. Cu 2+ / Cu E 0 = + 0.34 V Ag + / Ag E 0 = + 0.80 V (a) Construct a labelled galvanic cell for the combined half-cells. (4 marks) (b) Write the relevant half-equations for the change taking place at the anode and at the cathode. (c) Write the equation for the overall reaction. (1 mark) (d) State at which electrode reduction takes place in the above electrochemical cell. Give ONE reason for your answer. (e) Calculate the cell potential. (3 marks) C. The electrolysis of 2 mol.dm -3 copper (II) sulphate solution using pure copper electrodes was carried out. (a) Name the particles responsible for the passage of current in the wires and in the copper (II) solution (2marks) (b) Write the equation for the reaction at the cathode (1 mark) (c) The masses of the electrodes at the start of the electrolysis were: Anode = 8.760g Cathode = 9.040g At the end of the electrolysis the mass of the cathode was 9.675g i. What is the mass gain of the cathode? (1 mark) ii. What is the final mass of the anode? (1 mark) (d) What was the concentration of the copper (II) sulphate solution at the end of the electrolysis? Underline the correct answer. Less than 2 mol/dm 3 Exactly 2 mol/dm 3 More than 2 mol/dm 3 (1 mark) [25 MARKS] 6
(Question 6) a. Construct a Born-Haber cycle for the formation crystalline solid lithium chloride, LiCl(s). (7) b. The table below shows the values of standard enthalpies for some processes involving lithium and chlorine. Cl(g) Cl - -364 Li(s) Li(g) +161 Li(g) Li + (g) + e - +519 ½ Cl 2(g) Cl (g) +121 ΔH ϴ /kj mol -1 Li + (g) + Cl - (g) LiCl(s) ΔH 6 Li(s) + ½ Cl 2 (g) LiCl 2(s) -409 i. Use the values to calculate ΔH 6. (3) ii. What enthalpy change does the value of ΔH 6 (1) c. Using energy profile diagrams distinguish between enothermic an exothermic reactions. (6) D. i. List 4 assumptions of the Kinetic Theory. [4] ii. 0.35g of a gas was found to occupy a volume of 20cm 3 at 1.09 x 10 5 Pa and 22 0 C. What is the relative molecular mass of the gas? R = universal gas constant = 8.3145 J/mol K [4] 7
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Question 1 (a) (i) SOLUTIONS Paper 2 - The gas molecules are in a constant state of random motion. - The molecules themselves occupy negligible volume when compared to the volume occupied by the gas. - The average kinetic energy of the molecules is directly proportional to the absolute temperature of the gas. - Collisions are perfectly elastic. - The molecules exert no forces on one another. Any two correct answer (1 each one) (ii) n = PV / RT (1) n = 2.0 x 10 5 x 4.0 x 10-3 / 8.314 x 298 (1) n = 0.32 mol (Result and unit) (2) (4 marks) (iii) MM = 0.32 x 87.5 (1) MM = 28 g. mol -1 (Result and unit) (2) (3 marks) (b) Low temperatures (1) High pressures. (1) (c) (i) C 3 H 6 O and C 6 H 12 O 2 or (Any other correct samples) (1 marks) (ii) C H O 62 / 12 = 5.16 (1) 10.3 / 1= 10.3 (1) 27.6 / 16= 1.73 (1) 5.16/ 1.73 = 2.9~3 (1) 10.3/ 1.73 = 5.9~6 (1) 1.73 / 1.73 = 1(1) Empirical formula: C 3 H 6 O (1) (7 marks) (iii) MM(C 3 H 6 O) = (12 x 3) + (1 x 6 ) + 16 = 58 g.mol -1 (1) 116 / 58 = 2 (1) Molecular formula: C 6 H 12 O 2 (1) (3 marks) (d) In steam particles can vibrate and move very freely and randomly in all directions. (1) In water particles can vibrate and move freely throughout the liquid. (1) In ice particles can only vibrate in fixed positions but cannot move away from each other. (1) (3 marks) Question 2 A i. 9
Rate =k [NO] x [Cl 2 ] y (1mark) Using exp. 1 &2 Experiment Initial [NO] /M Initial [Cl 2 ] /M Intial rate of decomposition of Cl 2 / M s -1 2 0.26 0.20 4.0 10-2 1 0.13 0.20 1.0 10-2 [2] x = 2 2 therefore x =2, (2mark) Using exp. 1 &3 Experiment Initial [NO] /M Initial [Cl 2 ] /M Intial rate of decomposition of Cl 2 / M s -1 1 0.13 0.20 1.0 10-2 3 0.13 0.10 5.0 10-3 [2] y = 2 therefore y =1, (2mark) Rate =k [NO] 2 [Cl 2 ] 1 (1mark) ii. Rate =k [NO] 2 [Cl 2 ] 1 1.0 x 10-2 = k[0.13] 2 [0.20] 1 (Using exp. 1) (1mark) k = 1.0 x 10-3 / [0.13] 2 [0.20] 1 (1mark) k = 2.96 mol/dm-3 ( 1 mark ans 1 mark units) iii. Rate = k [NO] 2 [Cl 2 ] 1 Rate = 2.96[0.13] 2 [0.20] 1 (Using exp. 1) (1mark) Rate = 1.0 x 10-2 M -3 (1mark) b) Adding a catalyst has exactly this effect on activation energy. A catalyst provides an alternative route for the reaction. That alternative route has a lower activation energy. Showing this on an energy profile: 10
(5 marks) (c) Concentration an increase in concentration increases the frequency of collisions therefore increasing the rate. Particle Size- smaller or finely powdered reactions have larger surface area and hence higher frequency of collision therefore a faster rate. Temperature temperature increase kinetic energy causing the frequency of collisions to increase, it also increases the number of particles with energy greater that the activation energy therefore increasing the rate of reaction. (any two 4 marks) (d) Rate of reaction - The reaction rate (rate of reaction) or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place. (1 marks) I.Rate constant -. In chemical kinetics a reaction rate constant k or \lambda quantifies the speed of a chemical reaction (1 marks) iii.. Order of reaction - In chemical kinetics, the order of reaction with respect to certain reactant, is defined as the index to which its concentration term in the rate equation is raised. (1 marks) iv. Half-life refers to the time taken for half the concentration the reactants to decrease to half its initial value. (1 marks) Question 3 A. (a) The rate of the forward reaction = The rate of the reverse reaction Microscopic processes (the forward and reverse reaction) continue in a balance which yields no macroscopic changes. (so nothing appears to be happening.) The system is closed and the temperature is constant and uniform throughout. The equilibrium can be approached from the left (starting with reactants) or from the right (starting from products (Any two characteristics). 11
(b) Kc = [NO 2 ] 2 / [N 2 O 4 ] (1 mark) (c) Kc = (0.038) 2 / 0.011 = 0.13 mol.dm -3 (2 value & unit) Kp = Kc (RT) Δn (1) Kp = 0.13 (8.314 x 303) = 327.49 atm (2 value & unit) (5 marks) (d) (i) An increase in pressure will cause the reaction to shift in the left(1), that reduces pressure, that is the side with the fewer number of gas molecules(1). (ii) An increase in temperature, the equilibrium position shifts to de right(1), the equilibrium position shifts to use up the heat by producing more products (endothermic process)(1). (iii) An increase in concentration of NO 2, the equilibrium position shifts to the left (1), use up the added products by producing more reactants (1). (iv) Adding a catalyst will not cause a shift in the equilibrium (1). A catalyst provides an alternative mechanism for the reaction but of lower activation enthalpy (1) (8 marks) B. (a) (i) ph is a measure of the concentration of hydrogen ions in a solution. (1) (ii) A weak acid is one which doesn't ionize fully when it is dissolved in water. (1) (b) ph = - log [H + ] (1) ph = - log 4 x 10-8 = 7.4 (1) (c) ph = - log [H + ] ; [H + ] = 10 -ph ; [H + ] = 10-12 (1) [H + ][OH - ] = 1 x 10-14 (1) [OH - ] = 10-14 10-12 (1)any other correct way should be accepted [OH - ] = 1 x 10-2 mol.dm -3 (2 value & unit ) (5 marks) [25 MARKS] Question 4 (a) A chemical bond is the attractive force between atoms/ions that hold them together in a molecule or compound. Elements will undergo bonding to become more stable like their nearest noble gas. (b) An ionic bond involves the electrostatic interaction occurring between distinct point charges (anion and cation), while a covalent bond arises as a result of the sharing of electrons. (6 marks with any two correct examples) 12
(c) Atomic Radius The atomic radius of an element is half of the distance between the centers of two atoms of that element that are just touching each other. Generally, the atomic radius decreases across a period from left to right and increases down a given group. The atoms with the largest atomic radii are located in Group I and at the bottom of groups. Moving from left to right across a period, electrons are added one at a time to the outer energy shell. Electrons within a shell cannot shield each other from the attraction to protons. Since the number of protons is also increasing, the effective nuclear charge increases across a period. This causes the atomic radius to decrease. Moving down a group in the periodic table, the number of electrons and filled electron shells increases, but the number of valence electrons remains the same. The outermost electrons in a group are exposed to the same effective nuclear charge, but electrons are found farther from the nucleus as the number of filled energy shells increases. Therefore, the atomic radii increase. (3 marks) Ionization Energy The ionization energy, or ionization potential, is the energy required to completely remove an electron from a gaseous atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher its ionization energy will be. The first ionization energy is the energy required to remove one electron from the parent atom. The second ionization energy is the energy required to remove a second valence electron from the univalent ion to form the divalent ion, and so on. Successive ionization energies increase. The second ionization energy is always greater than the first ionization energy. Ionization energies increase moving from left to right across a period (decreasing atomic radius). Ionization energy decreases moving down a group (increasing atomic radius). Group I elements have low ionization energies because the loss of an electron forms a stable octet. (3 marks) (d) i. The equation for the first ionization energy for potassium is: K K + + e - (1 marks) ii. Electronic confiquration: Cr= 1s2,2s2,2p6,3s2,3p6,3d5,4s1 Ca 2+ = 1s2 2s2 2p6 3s2 3p4 N 3- = 1s 2 2s 2 (3 marks) iii. first ionisation energy-the first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+. This is more easily seen in symbol terms. 13
(1 marks) e. ) Compare Potassium with Sodium in terms of the following properties? atomic size- Potassium is larger than sodium number of outer shell electrons-both have one electron on its outer shell ionization energy - potassium first ionization energy is less than sodium (1 marks) (1 marks) (1 marks) f. Be 2+ ions indicates that two electrons have been lost and the remaining electrons are pulled closer to the nucleus making the Be 2+ ion smaller. The O 2- ions indicate that two electrons have been gained increasing repulsion causing the electrons to move further away from the nucleus making the O 2- larger. (5 marks) g. The factors affecting ionization energy are nuclear charge, atomic radius and screening effect. Flourine has a smaller radius and a larger nuclear charge than oxygen so it s electrons will be held more tightly and be more difficult to remove and so fluorine will have the higher ionization energy. (4 marks) (Question 5) A. The manganese (IV) oxide reacts in an acidic medium with chloride ions according to the following equation: MnO 2 (s) + 4 H + (aq) + 2 Cl - (aq) --> Mn 2+ (aq) + 2 H 2 O(l) + Cl 2 (g) (a) Oxidation is the loss of electrons (1) or an increase in oxidation number (1) by a molecule, atom or ion. Reduction is the gain of electrons (1) or a decrease in oxidation number (1) by a molecule, atom or ion. (4 marks) (b) Cl - ion in the redox reaction above is a REDUCING AGENT(1), due the oxidation number increases from 1- to 0 (1). B. (a) Anode ( Cu/ Cu 2+ ) (1) 14
Cathode (Ag + / Ag) (1) Salt bridge (1) Voltmeter (1) (4 marks) (b) anode: Cu(s) Cu 2+ (aq) + 2e (1) cathode: Ag + (aq) + 2e Ag (s) (1) (c) Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag (s) (1 mark) (d) Cathode (1). Electrons are gain/ oxidation number of Ag + (aq) decreases(1) (e) E 0 = E cat - E an (1) = +0.80 (-0.34) E 0 = +1.14V (3 sign, value & unit) (4 marks) C. (a) Electrons in the wire (1), and ions in the solution(1) (b) Cu 2+ (aq) + 2e Cu (s) (1 mark) (c) i. 0.635 g (1 mark) ii. 8.125 g (1 mark) (d) Exactly 2 mol/dm 3 (1 mark) [25 MARKS (Question 6) 15
a) ( same as for LiCl, correctly drawn and labeled 7 marks) B.i -364 + 161 +519 + 121 + ΔH 6 = - 409 (1 mark) ΔH 6 = - 409 (-364 + 161 +519 + 121) (1 mark) = - 846j k mol-1 (1 mark) ii. ΔH 6 is the lattice enthalpy (1 mark) c. (3 + 3 mark) d. I. Kinectic Theory Assumptions 1. Gases are composed of separate, tiny particles called molecules 2. Gas molecules are in constant, rapid, straight line motion (which means that gas molecules have kinetic energy ( KE = ½ mv² ) 3. The collisions between molecules are completely elastic (when molecules collide, there is no exchange of energy 4. The molecules of a gas have no attraction or repulsion for each other 5. Each molecule in a gas has a different velocity (any four valid assumptions 4 marks) dii. PV=nRT PV = (M/ RMM)RT RMM = MRT / PV (1 marks) (1 marks) 16
RMM =[ (0.35g) (8.3145Jmol-1K-1)(305K) ] / (1.09 x 10 5 Pa)(20 x 10-6 m 3 ) units) = 407.ll g/mol (1 mark) (1 mark with correct 17