AP Chapter 6: Thermochemistry Name

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1 AP Chapter 6: Thermochemistry Name Warm-Ups (Show your work for credit) Date 1. Date 2. Date 3. Date 4. Date 5. Date 6. Date 7. Date 8.

2 AP Chapter 6: Thermochemistry 2 Warm-Ups (Show your work for credit) Date 1. Date 2. Date 3. Date 4. Date 5. Date 6. Date 7. Date 8.

3 AP Chapter 6: Thermochemistry 3 Warm-ups and problems will be collected before you take the test. Read Chapter 6: Thermochemistry Answer the following problems in the space provided. For problems involving an equation, carry out the following steps: 1. Write the equation. 2. Substitute numbers and units. 3. Show the final answer with units. There is no credit without showing work. Energy 1. Distinguish between thermal energy and chemical potential energy. 2. In a certain chemical reaction, the chemical potential energy of the products is higher than that of the reactants. Is this reaction endothermic or exothermic? Is heat flowing into or out of the reaction? What forms of energy are being exchanged during this reaction? First Law of Thermodynamics 3. Explain each symbol and the sign conventions in the equation: E = q + w. 4. If gas in a balloon is heated, it expands. Calculate the work done by the gas in a balloon when it expands from 2.3 L to 4.7 L in a room at 20 o C and 1.0 atm. 5. Calculate the work done when 50.0 g of tin dissolves in excess acid at 1.00 atm at 25 o C. Sn(s) + 2H + (aq) Sn 2+ (aq) + H 2(g) 6. Calculate the change in (internal) energy when one mole of nitrogen is oxidized at 25 o C according to the following reaction. N 2(g) + 2O 2(g) 2NO 2(g) H = 66.4 kj Specific Heat (Capacity) 7. A piece of copper weighing 6.22 kg is heated from 20.5 o C to o C. Calculate the heat absorbed by the copper.

4 AP Chapter 6: Thermochemistry 4 Phase Change 8. a. What is happening at the molecular level when ice melts? b. What is happening at the molecular level when water boils? c. Why is the heat of vaporization greater that the heat of fusion for all substances? d. Would you expect the heat of vaporization of water to be greater or less than the heat of vaporization of methane? Explain. 9. What is the heat change when 56.4 g of water is frozen at 0 o C? Enthalpy 10. In writing thermochemical equations, why is it important to indicate the physical state of each substance? 11. Consider the following reaction: 2CH 3OH(l) + 3O 2(g) 4H 2O(l) + 2CO 2(g) H = kj What is the value of H if: (a) the equation is multiplied throughout by 2 (b) the direction of the reaction is reversed so that the products become the reactants and vice versa (c) water vapor instead of liquid water is formed as the product? 12. What is the heat change when 76.3 g of carbon reacts with excess hydrogen to form ethane? 2C(s) + 3H 2(g) C 2H 6(g) H = -84 kj Calorimetry 13. What is the difference between specific heat capacity and heat capacity? What are the units of each? Which is an intensive property and which is an extensive property?

5 AP Chapter 6: Thermochemistry Consider the following data: Metal Al Cu Mass (g) Specific Heat (J/g o C) Temperature ( o C) Circle the option in each set of brackets that make the following a correct statement when the two metals are placed in thermal contact. Heat will flow from (Al or Cu) to (Al or Cu) because (Al or Cu) has a higher (mass or specific heat or heat capacity or temperature) g of water at 35.2 o C is mixed with 127 g of water at 62.3 o C in a perfect insulator. What is the final temperature of the mixture? 16. A quantity of 200. ml of M HC1 is mixed with 200. ml of M Ba(OH) 2 in a constantpressure calorimeter that has a heat capacity of 453 J/ C. The initial temperature of the HC1 and Ba(OH) 2 solutions is the same at C. For the process: H + (aq) + OH - (aq) H 2O(l) the heat of neutralization is kj. What is the final temperature of the mixed solution? 17. When 50.0 ml of 1.50 M NaOH is mixed with 50.0 ml of 1.50 M NH 4Cl in a perfect insulator, the temperature of the solution rises from o C to o C. Write the chemical reaction and calculate the molar heat of this reaction?

6 AP Chapter 6: Thermochemistry 6 Standard Enthalpies (use Appendix 3 in the text where necessary) 18. Predict the value of H o f (greater than, less than, or equal to zero) for these elements at 25 C: a. Br 2(g) b. Br 2 (l) c. I 2 (g) d. I 2 (s) 19. The standard enthalpies of formation of ions in aqueous solutions are obtained by arbitrarily assigning a value of zero to H + ions; that is, H o f[h + (aq)] = 0. a. Calculate the enthalpy for the reaction: HCl(g) H + (aq) + Cl - (aq) b. Calculate the enthalpy of neutralization when one mole of a strong monoprotic acid (such as HC1) is titrated by one mole of a strong base (such as KOH) at 25 C. 20. Calculate the heat of combustion for the following reaction from the standard enthalpies of formation. C 2H 4(g) + 3O 2(g) 2CO 2(g) + 2H 2O(l) 21. When 16.2 g of cadmium metal is reacted with excess oxygen gas to form cadmium oxide, 36.7 kj of heat are given off. What is the heat of formation of cadmium oxide? Compare your result to the value in Appendix The first step in the industrial recovery of zinc from the zinc sulfide ore is roasting, that is, the conversion of ZnS to ZnO by heating: 2ZnS(s) + 3O 2(g) 2ZnO(s) + 2SO 2(g) H o rxn= -879kJ Calculate the heat evolved (in kj) per gram of ZnS roasted.

7 AP Chapter 6: Thermochemistry 7 Hess s law 23. From the following data: C(graphite) + O 2(g) CO 2(g) H 2(g) + ½ O 2(g) H 2O(l) H o rxn = kj H o rxn = kj 2C 2H 6(g) + 7O 2(g) 4CO 2(g) + 6H 2O(l) H o rxn = kj calculate the enthalpy change for the reaction 2C(graphite) + 3H 2(g) C 2H 6(g). Lattice Energy 24. a. Define lattice energy. b. Why is the lattice of a solid always a positive quantity? c. Why is the hydration of ions always a negative quantity? d. What is the relationship (equation) between lattice energy, heat of hydration and heat of solution? Miscellaneous problems 25. Portable hot packs are available for skiers and people engaged in other outdoor activities in a cold climate. The air-permeable paper packet contains a mixture of powdered iron, sodium chloride, and other components, all moistened by a little water. The exothermic reaction that produces the heat is a very common one, the rusting of iron: 4Fe(s) + 3O 2(g) 2Fe 2O 3(s) When the outside plastic envelope is removed, O 2 molecules penetrate the paper, causing the reaction to begin. A typical packet contains 250 g of iron to warm your hands or feet for up to 4 hours. How much heat (in kj) is produced by this reaction? 26. Some people use natural gas to heat water. Assuming natural gas is methane costing $1.30 for 15 ft 3 measured at 20 o C and 1.0 atm, calculate the cost of heating a cup (200 ml) of water from 20 o C to 99 o C. Assume only 50.% of the heat from burning methane is used to heat the water, with the remainding lost to the surroundings due to inefficiencies.