ANSWERS IB Chemistry HL Yr 1 Unit 7 Energetics

Transcription

1 ANSWERS IB Chemistry HL Yr 1 Unit 7 Energetics Review Part 1 Multiple Choice 1 When potassium persulphate, K 2 S 2 O 8, is dissolved in water the solution becomes warm Which of the following statements is true about the dissolving of K 2 S 2 O 8? A The process is endothermic and ΔH is positive B The process is endothermic and ΔH is negative C The process is exothermic and ΔH is positive D The process is exothermic and ΔH is negative 2 For the process NaCl(s) NaCl(l) A ΔH is positive and the process is endothermic B ΔH is negative and the process endothermic C ΔH is positive and the process is exothermic D ΔH is negative and the process is exothermic * endothermic going from a condensed (more packed) state to a less condensed state Energy is required to pry apart particles 3 According to the data below, what is ΔH for the conversion of solid sulphur to gaseous sulphur? ΔH (kj mol -1 ) S(s) + O 2 (g) SO 2 (g) -395 S(g) + O 2 (g) SO 2 (g) -618 A 1013 kj mol -1 B kj mol -1 C -223 kj mol -1 D 223 kj mol -1 H = ( ) ml of 10 M NaOH(aq) is added to 500 ml 10 M of HCl(aq) and the solution is quickly stirred The change in temperature (ΔT 1 ) is measured The experiment is repeated using 100 ml of each solution and the change in temperature (ΔT 2 ) is measured It is found that A ΔT 1 is five times as large as ΔT 2 B ΔT 1 is more than five times as large as ΔT 2 C ΔT 1 is equal to ΔT 2 concentration is the same, relative amounts are the same D ΔT 1 is less than ΔT 2 5 The dissolving process for ammonium nitrate is endothermic What will happen to the temperature of a beaker of water when some solid ammonium nitrate is dissolved in it? A The temperature will increase B The temperature will decrease C The temperature will not change D Not enough information is given to predict changes in the temperature

2 IB Chemistry HL/SL Unit 7 Review ANSWERS -2/5-6 Based upon the diagram below, which one of the following statement is correct? 75 products Enthalpy (kj mol -1 ) 50 reactants 25 Time A The reaction is exothermic B The approximate value of ΔH for the forward reaction is 50 kj mol -1 C The approximate value of ΔH for the forward reaction is -50 kj mol -1 D The approximate value of ΔT for the forward reaction is -50 kj mol -1 7 Sulphur dioxide, SO 2 (g), may be oxidized to sulphur trioxide, SO 3 (g), as shown in reaction 1 below 1 SO 2 (g) + (1/2)O 2 (g) SO 3 (g) ΔH = 2 S(s) + O 2 (g) SO 2 (g) ΔH = kj 3 S(s) + (3/2)O 2 (g) SO 3 (g) ΔH = kj On the basis of the reactions given, the enthalpy change for reaction 1 is A kj B kj C kj D kj H = ( ) 8 Use the bond energies below to calculate ΔH for the reaction, H 2 C=CH 2 + H 2 H 3 C CH 3 Bond energy (kj/mol) C C 348 C==C 612 H H 436 C H 412 A ΔH = -124 kj B ΔH = -48 kj C ΔH = 48 kj D ΔH = 124 kj ΔH = 4(212) (6(412) +348) =

3 IB Chemistry HL/SL Unit 7 Review ANSWERS -3/5-9 For the reaction H 2 + F 2 2HF ΔH = kj Which of the following statements best accounts for this behavior? A The bonds in HF are stronger than those in H 2 and F 2 B HF is an ionic compound whereas H 2 and F 2 are covalent (true but not an explanation) C More bonds are formed than broken D HF is a weak acid 10 Some average bond enthalpies (in kj mol -1 ) are as follows: H-H = 436, Cl-Cl = 242, H-Cl = 431 What is the enthalpy change (in kj) for the decomposition of hydrogen chloride? 2 HCl H 2 + Cl 2 a -184 ΔH = 2(431) ( ) b +184 = c +247 d -247 Part 2 Short Answer Questions ANSWERS 1 Hydrogen chloride, HCl, can be made by heating potassium chloride with concentrated sulphuric acid: H 2 SO 4 (l) + 2 KCl (s) 2 HCl (g) + K 2 SO 4 (s) Given the information: H 2 SO 4 (l) + 2 KOH (s) K 2 SO 4 (s) + 2 H 2 O (l) HCl (g) + KOH (s) KCl (s) + H 2 O (l) ΔH = kj ΔH = kj Calculate the enthalpy change for the formation of hydrogen chloride from potassium chloride and concentrated sulphuric acid [4] H 2 SO 4 (l) + 2KOH (s) K 2 SO 4 (s) + 2H 2 O 2KCl (s) + 2H 2 O (l) 2 HCl (g) + 2KOH (s) H 2 SO 4 (l) + 2KCl (s) K 2 SO 4 (s) + 2HCl (g) ΔH = -342 kj ΔH = 408 kj ΔH = 66kJ 2 N 2 (g) + 2O 2 (g) 2NO 2 (g) ΔH = 41 kj 2NO 2 (g) N 2 O 4 (g) ΔH = -35 kj N 2 (g) + 2O 2 (g) N 2 O 4 (g) ΔH =? H = 41 kj kj = 6 kj 3 CO 2 (g) C(s) + O 2 (g) ΔH = 551 kj 2C(s) + 3H 2 (g) C 2 H 6 (g) H = -119 kj 2CO 2 (g) + 3H 2 O(l) C 2 H 6 (g) + 7/2O 2 (g) ΔH = 2185 kj H 2 O(l) H 2 (g) + 1/2O 2 (g) ΔH =???? 2O 2 (g) + C(s) + 2CO 2 (g) ΔH = 551 kj * 2 = 1102 kj C 2 H 6 (g) 2C(s) + 3H 2 (g) H = -119 kj 3 = +396 kj 2CO 2 (g) + 3H 2 O(l) C 2 H 6 (g) + 7/2O 2 (g) ΔH = 2185 kj 3 = +728 kj H 2 O(l) H 2 (g) + 1/2O 2 (g) ΔH = 334 kj

4 IB Chemistry HL/SL Unit 7 Review ANSWERS -4/5- Substance Air Aluminum Copper Gold Iron Mercury NaCl Water Ice SHC (J/g 0 C) a) Which substance would increase in temperature the least if given a certain amount of energy? [1] Water b) How much energy is needed to increase the temperature of 300g of aluminum by 15 0 C? [1] q = m c t, q = (300g)(0902)(15 ºC) = 4059 J c) Which substance, of mass 150g, would increase in temperature by 12 0 C when given 1818J? [1] q c = = 1818 m t 150 x 12 = 101 the substance is AIR d) What mass of NaCl would increase by C, when given 2500J? [1] q m = = 2500 c t 0864 x 500 = What is the enthalpy change for the reaction (use the enthalpies of formation reference table)? 2 NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O (l) H =??? H = / H products - / H reactants = [(Na 2CO 3) + (CO 2) + (H 2O)] - (2 * NaHCO 3) = [ (- 3935) + (- 2858)] = (2 *- 9508) = 913 kj 6 Draw and label and energy diagram showing an endothermic reaction Label the axis, activation energy, H, reactants/ products and indicate the effect of a catalyst has on the curve [5]

5 IB Chemistry HL/SL Unit 7 Review ANSWERS -5/5-7 A calorimeter, like the one we used in class, contains 550 ml of a solution of copper (II) sulfate at a temperature of 228 C A small amount of zinc powder, also at 228 C, is added to the solution Copper metal is formed, and the temperature of the solution rises to 323 C The copper is collected, dried and weighed, and then found to have a mass of 0324 g a Calculate the total amount of heat energy released in this reaction using the equation: q = m x c x ΔT Assume that the mass of the solution is 550 g [2] b Calculate the enthalpy change for this reaction per mole of copper formed Be sure to use the correct sign to indicate whether the reaction is exothermic or endothermic [3] c In a second experiment, when the same amount of copper (II) sulfate solution, but twice as much zinc powder was used, the temperature change was the same as in the first in the first experiment Assuming no experimental error, explain how this could happen [2] a Q = mcδt assume c for water = 550g x 4181J/g 0 C (323 0 C C) = J or 218kJ is the amount of energy released b 0324 g Cu 5098 x 10-3 mol Cu ΔH = -Q = kj/5098 x 10-3 mol Cu = -428 kj/mol Cu This reaction is exothermic c CuSO 4 is limiting so the excess Zn is not involved in the reaction Thus, ΔT remains the same