Name KEY Period CRHS Academic Chemistry Unit 15 Thermochemistry HOMEWORK Due Date Assignment On-Time (100) Late (70) 15.1 15.2 15.3 15.4 Warm Ups Extra Credit Notes, Homework, Exam Reviews and Their KEYS located on CRHS Academic Chemistry Website: https://cincochem.pbworks.com
Page 2 of 12 Warm-Up Unit 15 Homework #1 #6 #2 #7 #3 #8 #4 #9 #5 #10
HW 15.1 MATTER AND ENERGY Unit 15 Homework Page 3 of 12 1. (TRUE / FALSE) Heat energy cannot be transferred but can be lost. 2. If you place an ice cube in a bowl of hot soup, how will heat flow? The ( ICE CUBE / HOT SOUP ) will transfer ( COLD / HEAT ) energy to the ( ICE CUBE / HOT SOUP). 3. Potential energy is stored in the bonds of molecules. 4. Kinetic energy is related to thermal energy. 5. What are the 3 ways heat can be transferred? Conduction Convection Radiation 6. Conduction, convection, or radiation? a. The heat you feel from a fireplace Conv/Rad b. Boiling water Cond/Conv c. Heat you feel from a hot stove Conv/Rad d. Frying a pancake Cond e. Fast particles colliding with slower particles Conv f. Air travels this way Conv g. Transfer through solids Cond h. Transfer through space Rad i. Moves as a wave Rad _ j. Moves as a current Conv k. Sun rays reaching earth Rad l. Occurs with fluids Conv _ 7. In chemistry lab, you pour 100 ml of vinegar into a beaker. You measure the temperature of the sample and find that it is 20 C. You add a tablespoon of calcium chloride to the vinegar and measure the temperature again. You find that it has risen to 24 C. What type of chemical reaction has taken place? ( EXOTHERMIC / ENDOTHERMIC ) How do you know? Temp of surrounding increases
Page 4 of 12 Unit 15 Homework 8. As water freezes in a freezer, heat is removed from water molecules, slowing their movement enough to allow them to freeze. ( EXOTHERMIC / ENDOTHERMIC ) How do you know? Heat removed from system 9. You sit down on a steel bench in the middle of summer. The sun has been beating down on the bench for hours. Heat in the bench transfers to your skin, causing first degree burns and significant mental anguish. From the point of view of the bench, is this ( EXOTHERMIC / ENDOTHERMIC ) How do you know? Heat flows from bench and heats up the surroundings
HW 15.2 HEAT CALCULATIONS Unit 15 Homework Page 5 of 12 1. What is specific heat capacity? Heat (Energy) needed to raise 1 g of a pure substance 1 o C at constant pressure 2. On a sunny day, why does the concrete deck around an outdoor swimming pool become hot, while the water stays cool? Water has higher Cp, which means it takes more energy from sun to raise its temp. If the amount of sun energy per time is the same then the concrete will get hotter in 1h than will water. 3. Using calories, calculate how much heat 32.0 g of water absorbs when it is heated from 25.0 C to 80.0 C. Look at the chart in your notes for the specific heat capacity of water. q=m x Cp x delta T Cp = 1.00 cal/g o C and delta T = 55.0 o C q = m Cp deltat = 32.0g x 1.00 cal 1760 cal g o C x 55o C = 1760 cal 4. When 435 J of heat is added to 3.4 g of olive oil at 21 C, the temperature increases to 85 C. What is the specific heat of olive oil? q=m x Cp x delta T q m x delta T = Cp = 435 J 3.4 g x 64.0 o C = 1.999J J g o = 2.0 C g o C 2.0 J g o C 5. How much heat is required to raise the temperature of 250.0 g of mercury 52 C? Look at the chart in your notes for the specific heat of mercury. q = m Cp deltat = 250.0g x 0.033 cal g o C x 52o C = 429 cal 429 cal 6. How much heat in kilojoules must be added to 178 g of water to increase the temperature of the water by 5.0 C? q = m Cp deltat = 178g x 4.18 J g o C x 5.0o C = 3720 J 3720 J x 1 kj = 3.72 kj or 3.7 kj 1000 J 3.7 kj
Page 6 of 12 Unit 15 Homework 7. When calcium chloride (CaCl 2) dissolves in water, heat is released. This is called the heat of solution. When 10 grams of CaCl 2 are dissolved in 505 grams of water at 25 C in an insulated container, the temperature of the water elevates to 47.1 C. What is the heat of solution for this mass of CaCl 2? Answer -> 11,100 cal or 46,700 J 8. A calorimeter has 230g of an unknown fluid at 22.0 C. A sample of metal transfers 1109 Joules of heat into the fluid and the temperature rises to 28.3 C. What is the specific heat capacity of the fluid? Answer -> Cp = 0.77 J/g o C 9. A 52.3 gram sample of aluminum is combusted in a bomb calorimeter. The aluminum goes from 24.5 C to a maximum of 325.4 C. The bomb contains 312 grams of water at 24.5 C before the reaction begins. What was the total temperature change of the water? What was the final temperature of the water? The specific heat of aluminum is 0.89 J/g- C. Answer -> dt = 75.3 o C
HW 15.3 WHAT IS THERMOCHEMISTRY? Unit 15 Homework Page 7 of 12 1. What is the definition of thermochemistry - Study of transfers of energy as heat that accompany chemical reactions and changes in physical state 2. What is the System in a chemical reaction? The chemicals involved in the reaction. 3. What is the Surroundings in a chemical reaction? Everything but the chemicals involved in the reaction. 4. Explain the source of heat in an exothermic chemical reaction. Energy contained in the chemical bonds of the reactants is greater than the energy contained in the bonds of the products so the excess energy is given off as heat. 5. Classify these processes as exothermic or endothermic. (Think about whether the object is gaining or releasing heat.) A. Burning rubbing alcohol ( exothermic / endothermic ) B. Cooking an egg ( exothermic / endothermic ) C. Combustion of gasoline ( exothermic / endothermic ) 6. Indicate whether Enthalpy of Reaction (ΔH) is positive (+) or negative ( ) for the following: A. the reactants have > enthalpy than the products ΔH = + B. the products have > enthalpy than the reactants ΔH = - C. the surroundings lose heat as a reaction occurs ΔH = + D. the temperature increases as a reaction occurs ΔH = - 7. Label the graphs as endothermic or exothermic. Exothermic Endothermic
Page 8 of 12 Unit 15 Homework Determine the ΔH for the following reactions. 3. N 2 (g) + O 2 (g) + 43.3 kj 2 NO (g) ΔH = 4. 2 C 2H 6 (g) + O 2 (g) 4 CO 2 (g) + 6 H 2O (l) + 683.5 kj ΔH = 5. C 3H 8 (g) C 3H 8 (l) + 41.8 kj ΔH = Rewrite the following equations by expressing the energy change as a term in the equation: a) H 2O (g) H 2O (l) ΔH = -10.76 kj b) 4 Al + 3 O 2 2 Al 2O 3 ΔH = -803.8 kj c) 2 H 2SO 4 2 SO 2 + 2 H 2O + O 2 ΔH = 130.6 kj HW 15.3 Additional Problems (Slip into HW book between page 8 & 9) 2 Na (s) + Cl 2 (g) - > 2 NaCl (s) H = -450 kj 1a. Calculate the heat change (in kj) for the conversion of 25.0 g of chlorine gas to solid sodium chloride at constant pressure. 25. 0 x 1 mole Cl 2 450 kj x = 159 kj released 70. 90 g Cl 2 1 mol Cl 2
Unit 15 Homework Page 9 of 12 1b. Calculate the heat change (in kj) for the conversion of 25.0 g of solid sodium to solid sodium chloride at constant pressure. 1 mole Na 25. 0 x 22. 99 g Na 450 kj x = 245 kj released 2 mol Na 3 Mg (s) + 2 AlCl 3 (aq) 3 MgCl 2 (aq) + 2 Al (s) H = -851 kj 2. Calculate the heat change (in kj) for the reaction of 50.2 g of Mg with excess AlCl 3 at constant pressure. 1 mole Mg 50. 2 g Mg x 24. 305 g Mg 851kJ x = 586 kj released 3 mol Mg
Page 10 of 12 Unit 15 Homework HW 15.4 DRIVING FORCES FOF CHEMICAL REACTIONS 1. In nature, reactions tend to proceed in a direction that leads to lowest energy state. 2. Explain why some endothermic reactions do occur _randomness of products is much greater than the reactants and this overcomes the fact that the enthalpy of reaction is positive... 3. What is Entropy? A measure of the randomness 4. In nature there is tendency to proceed in direction that INCREASES / DECREASES the randomness of a system. 5. Chemical reactions tend to go in direction of DECREASING/INCREASING order. 6. Predict whether the value of S for each of the following reactions will greater than, less than, or equal to zero. C 3CO 2(s) + 5O 2(g) 3 CO 2(g) + 4 H 2O(g) 3O 2 (g) + P 2 (g) 2 PO 3 (g) NaCl (s) Na + 1(aq) + Cl -1 (aq) 2Ca (s) + Cl 2 (g) -> 2 CaCl 2 (s) > < > < 7. Decide if G can ever be negative and therefore the reaction is spontaneous and fill in 3 rd column with conclusions. H S G - value (exothermic) + value (more random) Always negative and spontaneous - value (exothermic) - value (less random) Negative at lower temp + value (endothermic) + value (more random) Negative at high temp + value (endothermic -value (less ransom) Always positive and never sponstaneous Predict whether the reactions will be spontaneous at 298K by calculating G o from the given enthalpies and entropies. 8. NH 4Cl (s) NH 3 (g) + HCl (g) H o = -+176 kj/mol and S o = +0.2857 kj/(mol*k) G o = 90.9 kcal/mol (not spontaneous) 9. C (s) + O 2 (g) CO 2 (g) + 393 kj S o = 0.00300 kj/(mol*k) G o = - 392 kcal/mol (spontaneous)
Unit 15 Homework Page 11 of 12 Extra Credit - THERMOCHEMISTRY Across 3. in an endothermic reaction heat is 6. 1 calorie = 4.184 8. heat flows from a object to a cooler object 9. accurate and precise measurement of heat change in a reaction 11. type of energy stored in chemical or nuclear bonds 12. heat flows out of system - system warms - negative change in enthalpy 15. type of energy a type of potential energy stored in bonds between atoms in a substance 17. heat of a system at constant pressure 19. type of energy total of all energy of position and movement of the particles in a system 21. type of energy - due to motion Down 1. substance on right side of reaction 2. a equation must be balanced 4. capacity for doing work or supplying heat 5. heat capacity is an property of matter 7. change in enthalpy of a reaction is the same with one step or many steps (answer is HESS S LAW) 10. heat enters the system - system cools - positive change in enthalpy 13. water has a specific heat capacity (c) 14. water can hold a amount of heat 16. substance on left side of reaction 18. energy that transfers from one object to another because of temperature difference 20. In an exothermic reaction heat is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
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