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Unformatted text preview: Thermochemistry: Energy Flow & Chemical Change 1. Hydrogen cyanide is used in the manufacture of clear plastics such as Lucite or Plexiglas. It is prepared from ammonia (NH 3 ) and natural gas (CH 4 ). 2NH 3 (g) + 3O 2 (g) + 2CH 4--> 2HCN(g) + 6H 2 O(g) The reaction evolves 939 kJ of heat per 2 mol HCN formed. What is the value of q for this reaction, and is the reaction endothermic or exothermic? a. +469.5 kJ ; endothermic b. +939 kJ ; endothermic c.-939 kJ ; exothermic d.-469.5 kJ ; exothermic e.-1879 kJ ; exothermic 2. The enthalpy change for the reaction between W(s) and C(graphite) is difficult to measure directly because the reaction occurs at 1400 o C: W(s) + C(graphite) --> WC(s) However, the heats of combustion of the elements and of tungsten carbide can be measured and are given below: 2W(s) + 3O 2 (g) --> 2WO 3 (s) H = -1680.6 kJ C(graphite) + O 2 (g) --> CO 2 (g) H = -393.5 kJ 2WC(s) + 5O 2 (g) --> 2WO 3 (s) + 2CO 2 (g) H = -2391.6 kJ The enthalpy of reaction, H, for the formation of tungsten carbide, WC(s), from the elements is: a.-3.8 kJ b.-38.0 kJ c.-380.0 kJ d. +380.0 kJ e. none of these 3. Suppose you have a balloon of given volume, V 1 , containing a gas at temperature, T 1 . When you place the balloon in a colder room at temperature, T 2 , the balloon's temperature starts to drop. What are the signs of the system's q, w, and E for this process? a. +q, +w, + E b.-q, -w, - E c.-q, -w, + E d. +q, -w, - E e.-q, +w, - E 4. When burned in oxygen, 10.0 g of phosphorus generated enough heat to raise the temperature of 2950 g of water from 18.0 o C to 38.0 o C. The heat of formation of P 4 O 10 from P 4 (s) and O 2 (g) is: a.-3.06 kJ mol-1 b.-30.6 kJ mol-1 c.-306 kJ mol-1 d.-3060 kJ mol-1 e.-6120 kJ mol-1 5. Calculate the heat of combustion per mole of glucose(s), C 6 H 12 O 6 (s). a.-2813 kJ/mole b.-1273 kJ/mole c.-281.3 kJ/mole d.-17.01 kJ/mole e.-4.00 kJ/mole 6. The melting of ice at body temperature is an endothermic process: H 2 O(s) --> H 2 O(l) ; H = +6.0 kJ/mole Thus eating ice counteracts the exothermic processes of metabolizing food. How much ice (in grams) would you have to eat to counteract the energy gained by eating 1.00 oz of peanuts (13 kJ/g)? [1 oz = 28.3 g] a. 61 g b. 78 g c. 110 g d. 370 g e. 1100 g 7. A gas is allowed to expand at constant temperature from a volume of 1.0 L to 10.1 L against an external pressure of 0.50 atm. If the gas absorbs 250 J of heat from the surroundings, what are the values of q, w, and E? q w E (a) 250 J-460 J-210 J (b)-250 J-460 J-710 J (c) 250 J 460 J 710 J (d)-250 J 460 J 245 J (e) 250 J-4.55 J 245 J 8. In the reaction, CH 4 (g) + 2O 2 (g) --> 2H 2 O(l) + CO 2 (g) ; H o rxn = 890.4 kJ At 1.0 atm and 273 K (STP), how much work is involved per mole of CH 4 (g) with the volume change that occurs upon reaction. Ignore the small volume of the liquid....
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This note was uploaded on 02/16/2012 for the course CHEM 125 taught by Professor Na during the Fall '11 term at Purdue.
- Fall '11