Chemistry 30: Thermochemistry. Practice Problems

Transcription

1 Name: Period: Chemistry 30: Thermochemistry Practice Problems Date: Heat and Temperature 1. Pretend you are doing a scientific study on the planet Earth. a. Name three things in the system you are studying. b. What is the surroundings? c. Explain if Earth is an open system, a closed system or an isolated system. 2. Distinguish between kinetic and potential energy in the following examples: two separated magnets; an avalanche of snow; temperature, books on library shelves; a mountain stream; a stock car race; separation of charge in a battery, chemical bonds. 3. A piece of steel is heated up, then submerged in an isolated system filled with water. The water heats up as the steel cools. Explain how this models the conservation of energy. 4. How are the molecules different in a solid, a liquid and a gas? 5. How does heat depend on temperature? Specific Heat Capacity 6. Could another liquid be used as effectively as water in a calorimeter? Why or why not? 7. You have two lawn chairs, one made of iron and one made of aluminum. Both are painted the same colour. On a sunny day, which would be hotter to sit in, assuming they have both been in the sun for the same amount of time? Why? 8. The temperature of 335 g of water changed from 24.5 C to 26.4 C. How much heat did this sample absorb? 9. How much heat, in kilojoules, has to be removed from 225 g of water to lower its temperature from 25.0 C to 10.0 C? 10. A calorimeter has a heat capacity of 1265 J/ C. A reaction causes the temperature of the calorimeter to change from C to C. How many joules of heat were released in this process? 11. What is the specific heat of silicon if it takes 192 J to raise the temperature of 45.0 g by 6.0 C? 12. Assuming that Coca Cola has the same specific heat as water, calculate the amount of heat in kj transferred when one can (about 350 g) is cooled from 25 C to 3 C.

2 13. An insulated cup contains 75.0 g of water at C. A g sample of metal at C is added. The final temperature of the water and metal is C. What is the specific heat of the metal? 14. A 2.88 g piece of silver at C is dropped into a calorimeter of water. The initial water temperature is 15.2 C. The final temperature of both the water and silver is 18.1 C. What volume of water is in the calorimeter? ml of ethanol with an initial temperature of 62.4 C is poured into ml of water with an initial temperature of 19.9 C. What is the final temperature of the liquid mixture? Thermochemical Equations 16. The complete combustion of acetic acid (CH 3COOH) releases 871 kj of heat per mole of acid. Write a thermochemical equation for the reaction. 17. Exactly 332 kj of heat is required for the decomposition of aluminum oxide into its elements. Write a thermochemical equation for the reaction. 18. What information needs to be in a thermochemical equation? 19. When magnesium metal combines with oxygen from the air, 1204 kj of heat energy is released. a. Write a thermochemical equation for the reaction. b. Calculate the amount of heat transferred if 1.3 g of magnesium reacts. c. What mass of magnesium oxide is produced during an enthalpy change of -96 kj? 20. When solid sodium hydroxide is dissolved into water, forming aqueous sodium and hydroxide ions, 445 kj/mol of energy is released. a. Write the thermochemical equation for this process. (Hint: water is not included in the equation.) b. How much heat, in kj, is released when 30 g of sodium hydroxide is dissolved? 21. The decomposition of baking soda is represented by the following thermochemical equation: 2 NaHCO 3 (s) kj Na 2CO 3 (s) + H 2O (l) + CO 2 (g) a. What is the value for ΔH? b. How much heat is needed to decompose 10.0 g of baking soda?

3 Heat of Reaction and Solution 22. You mix ml of M RbOH with ml of M HBr in a calorimeter. If the temperature of each of the two solutions was C before mixing, and the temperature rose to C, what is the enthalpy of the reaction? RbOH (aq) + HBr (aq) RbBr (aq) + H 2O (l) ΔH =? 23. In an experiment, g of solid lithium hydroxide is mixed with ml of M solution of nitric acid (HNO 3) in a coffee cup calorimeter. A neutralization (double displacement) reaction occurs and the temperature of the nitric acid goes from 24.5 C to 31.4 C. a. Write the balanced chemical equation for the reaction that occurs. Include the states of all reactants and products. b. How many moles of nitric acid are present in the solution before the reaction? c. Calculate the molar enthalpy (kj/mol) of reaction ( H) per mole of lithium hydroxide. d. State two assumptions that have been made in this calorimetry experiment g of potassium chlorate (KClO 3) is dissolved in 65.0 ml of water. Determine: a. The amount of heat produced or absorbed by the reaction b. The temperature change of the water 25. A student uses an aluminum calorimeter to determine the enthalpy of solution for solid ammonium nitrate. The student assumes that the calorimeter neither gains nor loses heat during the experiment, that the density and specific heat capacity of the final solution are the same as water and that the final mass of the solution is g. The data were collected and recorded as follows: Mass of aluminum calorimeter g Mass of aluminum calorimeter and contents g Mass of ammonium nitrate 1.68 g Initial temperature of calorimeter and contents C Final temperature of calorimeter and contents C What is the experimental value of ΔH sol for ammonium nitrate? Latent Heat 26. What is happening to the molecules in a substance when the state is changing? 27. What is happening to the molecules in a substance when the temperature is changing? 28. How can you identify the melting and boiling points on a heating curve?

4 29. Use the following heating curve to answer the questions: a. In what part of the curve is the substance a solid? b. In what part of the curve is the substance a gas? c. In what part of the curve is the substance both a solid and a liquid? d. What is the melting point of the substance? e. What is the boiling point of the substance? f. In what part(s) of the curve would kinetic energy be increasing? g. In what part(s) of the curve would potential energy be changing? h. At what temperature are the molecules moving the slowest? i. At what temperature would the molecules be furthest apart? 30. Your mom asks you to boil a pot of water, and to leave it to boil for five minutes so it will be good and hot. What is wrong with this statement? 31. A substance has a heat of fusion of 100 J/g and a heat of vaporization of 1000 J/g. Which takes more energy: melting or vaporization? Explain why, using what you know about the molecules in solids, liquids and gases. 32. Sketch a cooling curve for ethanol as it is cooled from 90 C to Ethanol condenses at 78 C and freezes at -114 C.

5 Hess Law 33. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target PCl 5 (g) PCl 3 (g) + Cl 2 (g) H =? 1 P 4 (s) + 6Cl 2 (g) 4PCl 3 (g) H = kj 2 4PCl 5 (g) P 4 (s) + 10Cl 2 (g) H = 3438 kj 34. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target 2CO 2 (g) + H 2O (g) C 2H 2 (g) + 55 O2 22 (g) H =? 1 C 2H 2 (g) + 2H 2 (g) C 2H 6 (g) H = kj 2 H 2O (g) H 2 (g) + 1 O2 2 (g) H =71.2 kj 3 C 2H 6 (g) + 7 O2 2 (g) 2CO2 (g) + 3H2O (g) H = -283 kj 35. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target H 2SO 4 (l) SO 3 (g) + H 2O (g) H =? 1 H 2S (g) + 2O 2 (g) H 2SO 4 (l) H = kj 2 H 2S (g) + 2O 2 (g) SO 3 (g) + H 2O (l) H = -207 kj 3 H 2O (l) H 2O (g) H = 44 kj 36. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target N 2 (g) + 2O 2 (g) 2NO 2 (g) H =? 1 N 2 (g) + 3H 2 (g) 2NH 3 (g) H = -115 kj 2 2NH 3 (g) + 4H 2O (l) 2NO2 (g) + 7H2 (g) H = kj 3 H 2O (l) H 2 (g) + 1 O2 2 (g) H = kj 37. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target N 2H 4 (l) + CH 4O (l) CH 2O (g) + N 2 (g) + 3H 2 (g) H =? 1 2NH 3 (g) N 2H 4 (l) + H 2 (g) H = kj 2 2NH 3 (g) N 2 (g) + 3H 2 (g) H = 57.5 kj 3 CH 2O (g) +H 2 (g) CH 4O (l) H = 81.2 kj 38. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target ½ H 2 (g) + ½ Cl 2 (g) HCl (g) H =? 1 COCl 2 (g) +H 2O (l) CH 2Cl 2 (l) + O 2 (g) H = 47.5kJ 2 2HCl (g) + ½ O 2 (g) H 2O (l) + Cl 2 (g) H = 105 kj 3 CH 2Cl 2 (l) + H 2 (g) + 3 O2 2 (g) COCl2 (g) + 2H2O (l) H = kj

6 39. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target HCl (g) + NaNO 2 (s) HNO 2 (l) + NaCl (s) H =? 1 2NaCl (s) + H 2O (l) 2HCl (g) + Na 2O (s) H = 507 kj 2 NO (g) + NO 2 (g) +Na 2O 2NaNO 2 (s) H = -427 kj 3 NO (g) + NO 2 (g) N 2O (g) + O 2 (g) H = -43 kj 4 2HNO 2 (l) N 2O (g) + O 2 (g) + H 2O (l) H = 34 kj 40. Determine the enthalpy of the target equation using Hess Law, then determine if the reaction is Target Zn (s) + 11 S8 88 (s) + 2O2 (g) ZnSO4 (s) H =? 1 Zn (s) + 1 S8 8 (s) ZnS (s) H = kj 2 2ZnS (s) + 3O 2 (g) 2ZnO (s) + 2SO 2 (g) H = kj 3 2SO 2 (g) + O 2 (g) 2SO 3 (g) H = kj 4 ZnO (s) + SO 3 (g) ZnSO 4 (s) H = kj Standard Enthalpy of Formation 41. Write H f for each of the following compounds: a. CuSO 4 (s) b. SO 3 (g) c. Hg (l) 42. Find H f for ammonia. What does the negative of this number represent? 43. Write the formation equation for each of the following: a. N 2O 4 c. C 2H 5OH b. CO d. Ba(OH) 2 e. NH 4NO The next four steps are all connected: a. Write the formation equations for NO (g), NH 3 (g) and H 2O (g), including H f for each. b. Use Hess Law to find H for the target equation: 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2O (g) c. Check your answer from (c) using Summation of Standard Enthalpies. d. Is this an endothermic or exothermic equation? 45. Use Summation of Standard Enthalpies to find H for the following reactions: a. Fe 2O 3 (s) + 3 CO (g) 2 Fe (s) + 3 CO 2 (g) b. 2 CO (g) + O 2 (g) 2 CO 2 (g) c. NH 3 (g) + HCl (g) NH 4Cl (s) d. CaO (s) + 2HCl (g) CaCl 2 (s) + H 2O (l)

7 46. Given the following equation: CaCO 3 (s) CaO (s) + CO 2 (g) a. Find H for the reaction. b. How much energy is used if 55.4 g of carbon dioxide is formed? 47. Given that the ΔH rxn for the equation below is kj, use this value and heats of formation to calculate the value of ΔH f of Mg(NO 3) 2 (aq). 2 HNO 3 (l) + Mg(OH) 2(s) Mg(NO 3) 2 (aq) + 2 H 2O (l) 48. Pretend that the standard enthalpy of formation for one mole of solid mercury(ii) nitrate is kj/mol. a. Write the formation equation for solid mercury(ii) nitrate as a thermochemical equation. b. Calculate the amount of energy absorbed or released when 18.1 g of this compound is formed. Bond Enthalpy and Lewis Structures 49. Draw Lewis structures for each of the following simple molecules: a. CH 4 c. H 2O b. NH 3 d. SiF 4 e. NCl Draw Lewis structures for each of the following molecules that contain multiple bonds: a. H 2CO c. CO e. O 2 b. HCN d. CO 2 f. N Draw Lewis structure for each of the following hydrocarbons: a. C 2H 6 c. C 2H 5OH b. C 3H 8 d. C 2H 4 e. C 2F Estimate the change in enthalpy using average bond energy: a. b. c. d.

8 e. 53. Draw Lewis structures for the reactants and products, then estimate the enthalpy change for the reaction using bond energies a. H 2 (g) + CO 2 (g) H 2 O (g) + CO (g) b. 2 H 2 O 2 (g) 2 H 2 O(g) + O 2 (g) c. CO(g) + 2 H 2 (g) CH 3 OH(g) d. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) e. H 2 (g) + C 2 H 4 (g) C 2 H 6 (g) f. 2 C 2 H 6 (g) + 7 O 2 (g) 4 CO 2 (g) + 6 H 2 O(g) g. CH 4 (g) + 3 Cl 2 CHCl 3 (g) + 3 HCl(g) h. HCN(g) + 2H 2 (g) CH 3 NH 2 (g) 54. Why does using average bond energy not give the same value for the enthalpy of a reaction as using standard enthalpy of formation?