2. Calculate the heat change in joules when 45.2 g of steam at C condenses to water at the same temperature.!

Transcription

1 Chem L3 Worksheet:Thermochemistry: Chapter 11 Page 1 Water Heating Cooling curve Problems ( Stairstep ): pp , How much heat is released when 65.0 g of steam cools from C to C?! (2392 J) 2. Calculate the heat change in joules when 45.2 g of steam at C condenses to water at the same temperature.! (102,000 J, to 3 sig figs!) 3. How many g of ice (at 0.00 C) can be melted by the addition of 5000 joules of heat?! (15.0 g) 4. Calculate the heat change when 90.0 g of ice at 30.0 C melts to water at 0.0 C. (2 steps)!! (35,700 J, 3 sig figs) 5. Calculate the heat required to change 75.0 g of water at 25.0 C to steam at C. (3 steps!)!! (200,000 J, 3 sig figs) 6. Calculate the heat released when 50.0 g of water at 35.0 C freezes to form ice at 20.0 C.!! (26,100 J, 3 sig figs) 7. You re going to do an experiment to determine the heat of fusion of ice g of ice (at 0.00 C) are added to 100 g water (at 35.0 C) in a foam cup. The ice and water are stirred until all the ice melts and the final temp. of water + melted ice is 0.00 C. (p ) a. How much heat did the water lose?! (14,630 J) b. Calculate the experimental heat of fusion (per gram) of the ice.! (318 J/g)

2 Thermochemical Equations:! (p )! Page 2! Exothermic = H, heat is a product, release heat! Endothermic = + H, heat is a reactant, absorb heat 8. Two substances are put into water in a glass beaker to react. The water and the beaker become very hot. a. Is this reaction exothermic or endothermic. Explain.! (ans below)! b. If the two substances are defined as the system, what is the surroundings? 9. Classify these reactions as exothermic or endothermic.! a. discharging a battery! c. burning alcohol! b. photosynthesis! d. baking a potato 10. a. Is this reaction exothermic or endothermic?!caco 3 (s) kj > CaO(s) + CO 2 (g) b. What is H for this reaction? c. Which is at Higher enthalpy (energy), the products or reactants? ( uphilll or downhill?) d. Would this reaction: absorb heat or release heat? e. Sketch the energy diagram for this reaction. ( uphilll or downhill?) Heat Soichiometry Problems 10f. Calculate the heat required to decompose 5.20 mol of CaCO 3.! (915 kj) 11. Is this reaction exothermic or endothermic? 2 Mg(s) + O 2 (g) > 2 MgO(s) kj a. What is the heat change when 2.90 g of Magnesium is burned in oxygen?! (71.8 kj) b. What is the heat change when 5.00 g of MgO is formed?! (74.6 kj) c. How many grams of Mg are burned when 500 kj of heat are released?! (20.2 g) (8.exo, b. water, beaker, air etc; 9, exo=a,c, endo=b, d; 10 endo, H= +176, prod, absorb, uphill; 11. exo)

3 Chemistry Worksheet: Enthalpy ( H) and Hf (p ) Page 3 H reaction = H f (products) H f (reactants) 1. Calculate H for these reactions. Is each exothermic ( H) or endothermic (+ H)? a. 2 CO 2 (g) > 2 CO(g) + O 2 (g)! (+566 kj) b. CH 4 (g) + 2 O 2 (g) > CO 2 (g) + 2 H 2 O(l)! ( kj) 2. a. Balance this equation for the complete combustion of butane:! (2, 13, 8, 10) C 4 H 10 (l) + O 2 (g) ----> CO 2 (g) + H 2 O(g) b. Calculate H for the reaction using H f values. Is it exothermic or endothermic?! ( 5271 kj) c. A butane lighter contains about 25 grams of butane. How much heat would be produced by burning the 25.0 grams of C 4 H 10 in the lighter?! ( 1136 kj) d. How many grams of ice (at 0.0 C) could you melt with the heat from part c? (watch units!!)! (3401 g) e. How many grams of water (at C) could you boil with the heat from part c?! (503 g) grams of NaOH is mixed into 100 ml (100 g) of 1 M HCl. The temperature of the HCl goes from 25.0 C to 38.5 C. The reaction is NaOH(aq) + HCl(aq) > NaCl(aq) + H 2 O(l) a. Is the reaction exothermic or endothermic? b. How much heat did the HCl ( mostly water ) absorb? (Assume Cp = 4.18 J/g C)! (+5643 J) c. Calculate H/mol NaOH for the reaction of the NaOH and the HCl.! ( 113 kj)

4 Entropy: = a measure of the disorder of a system (p ) and Spontaneity! Page 4 High Entropy = Mixed up ; Low Entropy = NOT mixed up, Orderly 4. For each choice, which system has the higher entropy (is more mixed up)? (answers below)! a. completed jigsaw puzzle! or separate jigsaw pieces?! b. 50 g of liquid water! or 50 g of ice?! c. 10 g of NaCl crystals! or a solution containing 10 g of NaCl?! d. water at 25 C! or water at 75 C? 5. What two factors together determine whether a process is spontaneous? 6. Do the following reactions have S (become less mixed up, go to lower entropy) ;! or + S (become more mixed up, go to higher entropy)?! a. gases from two separate containers mix together! b. liquid water freezes! c. CaCO 3 (s) > CaO(s) + CO 2 (g)! d. NH 3 (g) + HCl(g) > NH 4 Cl(s)! e. H 2 O(l) > H 2 O(g) Spontaneity: G = H T S : If G is, process is spontaneous 7. Consider steam condensing to form liquid water. Does this process;! a. Absorb energy (+ H) or release energy ( H)?! b. Have + S or S?! c. Why does this process occur spontaneously at 25 C but not at 150 C? 8. Are the following reactions: a. always spontaneous, b. never spontaneous,! c. spontaneous only at low temps., d. spontaneous only at high temps., or! e. you can t tell with the information given. Explain each answer!! I. The reaction is exothermic! II. The entropy is increased in the reaction! III. The entropy decreases and the reaction is endothermic! IV. The reaction is exothermic and the entropy decreases! V. 2 O 3 (g) > 3 O 2 (g)! H = 284 kj! VI. CaCO 3 (s) kj > CaO(s) + CO 2 (g)! VII. I 2 (s) > I 2 (g) (Hint: determine H and S) (answers: 4a: separate, b. liquid, c. solution, 75 C; 6. a. +, b., c. +, d., e. +; 7. H, S, so G {spont} at low Temp but not high; 8I. e, II. e, III. b, IV. c, V. a, VI. d, VII. d)

5 Chemistry Worksheet: Thermochemistry: Review Questions Page 5 1. Equal masses of two substances absorb the same amount of heat. The temperature of substance A increases twice as much as the temp. of substance B. Which substance has the higher specific heat? Explain.! 2. On a sunny day, why does the concrete deck around an outdoor swimming pool become hot, while the water stays cool? 3. You calculate the T of a sample of liquid water to be 135 C. What s wrong with that answer? 4. How much heat is released when 40.0 g of steam at 150 C condenses to liquid water at 30 C?! (106,000 J) 5. Consider this thermochemical reaction: 2 NO 2 (g) > N 2 (g) + 2 O 2 (g) kjoule a. Is this reaction exothermic or endothermic?! b. Is it Uphill or Downhill in enthalpy? c. Would this reaction: absorb heat or release heat? d. What is H for this reaction? (Hint: no calculations needed!!) e. Is S + or for this reaction? Explain. f. Is this reaction spontaneous, non-spontaneous, or spontaneous only at high or low temperatures? Why? g. Sketch the reaction energy diagram for this reaction: (uphill or downhill?) (5. exo, reactants, release, 67.6 kj, + S {go to more mol gas}, always spont, downhill)

6 !! Page 6 6. The Manchester airport melts snow instead of plowing. During the Feb 5, 2014 snowstorm, assume 10 inches of wet, heavy snow fell (about 25 cm). Assume the runway area is 500 m by 100 m. The density of the wet snow is about 0.20 g/cm 3. a. How many grams of snow fell on the airport? (change m to cm, use density)! (2.5 x 10 9 g) b. How much Heat would be required to melt the snow (starting at 10.0 C), and form liquid water at 15.0 C? (You need the answer to part a. If you can t do part a, look at the anwer!)! (1.04 x J) c. Here is the reaction for the combustion of liquid propane fuel. Balance the equation.! C 3 H 8 (l) + O 2 (g) > CO 2 (g) + H 2 O(l)! (see ans. in part e below) d. Calculate H for the balanced reaction.! ( kj) e. If the airport burns liquid propane fuel to provide the heat to melt the snow, how many grams of propane would you need to burn to produce the amount of heat you calculated in part b? Then change the grams to kilograms.! (ans to c. 1, 5, 3, 4) (2.08x10 7 g)! f. If LP (liquid propane) fuel cost $2.50 per gallon, that s about $0.75 per kg. How much did the fuel cost to melt the snow at the airport for the storm?! ($15,600)