Section I Getting H 1. Some automobiles and buses are equipped to burn propane gas, C 3H 8, as a fuel.

Transcription

1 Pretest version Section I Getting H 1. Some automobiles and buses are equipped to burn propane gas, C 3H 8, as a fuel. The complete combustion of propane is shown by the following chemical equation: C 3 H 8 ( g ) 5 O 2 ( g ) 3 CO 4 H O 2 ( g ) 2 ( g ) H Given the following heats of formation. 1 H 2(g) + O 2(g) H 2O (g) 2 H = C (s) + O 2(g) CO 2(g) H = a) What is the heat of formation of propane? 3 C (s) + 4 H 2(g) C 3H 8(g) H =? Reverse equation (1): 3 CO H 2O (g) C 3H O 2 H = /e Multiply equation (2) by 4: 4 H 2(g) + 2 O 2(g) 4 H 2O (g) H = 242.0(4) Multiple equation (3) by 3: 3C (s) + 3O 2(g) 3CO 2(g) H = 393.5(3) Add them up: 3 C (s) + 4 H 2(g) C 3H 8(g) H = -104 b) Express your answer in (a) in /e of C H = -104 /3e C = / C

2 c) How much heat would be either released or absorbed (specify) if 4.4 g of propane were formed from carbon and hydrogen? 4.4g /(44 g/e) = 0.10 e C 3H 8(g) 0.10 e C 3H 8(g) ( -104 /e) =-10 (released) 2. There is a ΔH (heat of vaporization) associated with the evaporation of CCl 4(l). a) Write a simple equation to represent the evaporation of CCl 4(l). CCl 4(l) CCl 4(g). b) Use the following thermochemical equations to calculate the heat of vaporization of CCl 4(g): C (s) + 2 Cl 2(g) CCl 4(l) C (s) + 2 Cl 2(g) CCl 4(g) ΔH = ΔH = Reverse 1 st equation and add it to the second one: ΔH = 36.3 (notice that the sig fig rule for addition applies; there was no multiplication or division, so we preserve decimal places.) c) Plot the enthalpy versus progress of reaction graph and locate CCl 4(l) and CCl 4(g) on the graph. Clearly label both axes. Draw it like 2(b) with CCl 4(l) as a reactant at the bottom of the hill. 3. Consider the following combustion reaction of ethanol: C 2H 5OH (l) + 3 O 2(g) 2 CO 2(g) + 3 H 2O (l) Bond Energy (/) Bond Energy (/) H H 436 C = O 805 H O 460 C C 347 H C 413 C = C 607 C O 358 C C 839 O O 204 Calculate the energy of the double bond between oxygen (O) atoms in the diatomic oxygen gas (O 2) ecule.

3 H bb = 3x +5 C-H +1 C-C + 1 C-O + 1 O-H = 3x + 5(413) = 3x Products: H bf = 4(C=O) + 6(O-H) = 4(-805) + 6(-460) = H bb + H bf = H 3x = ( see original equation) X = 460. per O 2 bond ( an approx. because throughout interecular bonds are ignored and because we assume that all bonds of one type are identical) Section II: Routine Calorimetry Problems(One of each type) 1. You only have to burn grams of octane(c8h18-found in gasoline) to raise the temperature of grams of water from to o C. Calculate the ar heat of combustion of octane. = 1000 g(4.19 J/g o C)( ) o C = J Reactant: H = -Q = n = 1.000g/(114 g/e) = es H/n = / es = /e

4 2. Nacho chips are very calorie-intensive: their H is per gram. If only 6.00 grams of chips are burned in a calorimeter containing 5.00 L originally at o C, what maximum temperature will the water attain? Reactant H = /gram (6.00 grams) = Q = - H = = J J = 5000 ml(1.0g/ml)(4.19 J/g o C)(x ) o C x = O C 3. When a calorimeter was filled with 20.0 ml of 3.00 /L hydrochloric acid, HCl(aq), and 50.0 ml of 1.20 /L sodium hydroxide, NaOH(aq), the temperature rose from 22.4 C to 29.8 C. What was the ar heat of neutralization of HCl(aq)? (Assume the density and specific heat for all solutions to be equal to that of water, and ignore the water created by the reaction.) = ( )ml (1.0 g/ml) (4.19 J/g o C)( ) o C = J Reactant : H = -Q = J= n for HCl = CV = 3.0 es/l (0.020 L) = es H/n = /0.060 es /e 4. Specific Heat Capacities of Some Common Substances (J/g C) Silver Lead Ice (solid) 2.06 Iron Nickel Glass 0.84 Copper Gold Someone had to find the identity of an unknown solid. The investigator placed this solid in C water and allowed it to heat up to the temperature of the water. She then placed the hot metal in a calorimeter containing cool water and collected the data recorded in the data table below.

5 Mass of the unknown solid Temperature of the hot water bath Initial temperature of the water in the calorimeter Final temperature of the water in the calorimeter Volume of the water in the calorimeter 52.8 g C 21.3 C 26.0 C 90.0 ml Using the data given and the table of Specific Heat Capacities, determine the identity of the unknown solid. (modified from June 1993) -Qhot = Qcold - (mc T)hot = (mc T)cold -52.8c(26-100) = 90(4.19)( ) c = J/g o C the unknown is iron. Section III Problems With a Twist or Two 5. Pure C alcohol (density g/ml; specific heat 2.4 J/g o C ) is added to 350 ml of 5.0 o C water to create a mixture with a temperature at C. What will be the % of alcohol (by volume) in such a mixture? -Qhot = Qcold - (mc T)hot = (mc T)cold - x (2.4 J/g o C)( ) =350 ml (1.0g/ml) (4.19 J/g o C)(18-5) o C x = 3177 g = 3177 g (0.791 g/ml) = 4017 ml % alcohol = 4017/( )*100 % = 92% g of 1010 o C copper and g of 935 o C iron were added to a 2.0 L bucket of ice cold water(0.0 o C). How hot did the water get? Copper s specific heat = 0.39 J/g o C. Iron s specific heat = 0.45 J/g o C. -Qhot 1 + -Qhot 2 = Q cold - (mc T)hot1 (mc T)hot2 = (mc T)cold -500*0.39*(x -1010) 400*0.45(x 935) = 2000*4.19*(x 0) x = 41.7 o C For flashback topics, see strike package.