Thermochemistry and Calorimetry

Transcription

1 WHY? ACTIVITY 06-1 Thermohemitry and Calorimetry Chemial reation releae or tore energy, uually in the form of thermal energy. Thermal energy i the kineti energy of motion of the atom and moleule ompriing an objet. Heat i thermal energy that i tranferred from one objet to another. The amount of energy releaed or tored by a hemial reation an be determined from the hange in temperature. You need to know how muh energy i either releaed or taken up and tored by a reation in order to determine the utility of that reation. Thi knowledge alo allow you to identify the ondition eential for that reation to our effiiently and afely. LEARNING OBJECTIVES Quantify the relationhip between energy tranferred and the hange in temperature Undertand the utility of the peifi heat apaity Undertand how to determine the heat of hemial reation SUCCESS CRITERIA Corretly ue the peifi heat apaity of ubtane in variou ituation to interrelate the amount of heat and the hange in temperature Determine the energy tranferred in a hemial reation from the hange in temperature PREREQUISITES Ativity 04-1: Balaned Chemial Reation Euation Ativity 04-3: Introdution to Aid Bae Reation INFORMATION Energy i meaured in unit of Joule (J). Thermal energy i tranferred pontaneouly from an objet at a higher temperature to another objet at a lower temperature. Thermal euilibrium i reahed when the two objet are at the ame temperature. The temperature of an objet inreae when it aborb energy; the temperature of an objet dereae when it releae energy. Thi temperature hange an be ued to determine the amount of energy that wa aborbed or releaed. 87

2 Foundation of Chemitry MODEL 1: MEASURING THERMAL ENERGY In an experiment to determine the relationhip between the temperature hange of water and the amount of energy aborbed by the water, an eletrial heater wa ued to inreae the temperature of 1 g of water by 1 C. The energy produed by an eletrial heater over ome period of time an be alulated very aurately: energy eletrial urrent voltage time. The reult of thi experiment are given below. Ma of Water Change in Temperature Energy Ued in Heating the Water g C J KEY QUESTIONS 1. How muh energy wa reuired to inreae the temperature of g of water by C? 2. How muh energy would be reuired to inreae the temperature of g of water by C? 3. How muh energy would be reuired to inreae the temperature of g of water by 50.0 C? 4. The peifi heat apaity of any ubtane i defined a the amount of energy needed to inreae the temperature of 1 g of the ubtane by 1 C. What i the peifi heat apaity of water? Be ure to inlude the unit of J/g C. 5. Baed on your anwer to Key Quetion 1-4, what i the euation that relate the amount of energy aborbed or releaed, by any ubtane, to the hange in temperature of that ubtane? Write thi euation uing the following notation: heat, m ma, T hange in temperature, and peifi heat apaity of the ubtane. 6. If you needed to experimentally determine the peifi heat apaity of a new metal alloy that you developed, what would you need to meaure, and how would you alulate the peifi heat apaity from the meaured value? m T 88 Chapter 6: Energy and Chemial Reation

3 EXERCISES 1. Aluminum ha a peifi heat apaity of J/g C. How muh energy i releaed when 1.0 kg of aluminum ool from 35 C to 20 C? 2. In preparing dinner you need a up of very hot water, whih you prepare on your eletri tove. You ue 80 kj of eletrial energy to heat 250 g of water tarting at 20 C. What i the final temperature of the water? m ΔT ΔT m T T + ΔT f ΔT i 80,000 J -1-1 ( ) (250 g) J g C T f 20 C + 77 C 97 C 77 C GOT IT! 1. At room temperature, eual mae of water and aluminum were eah heated with 1 kj of eletrial energy. The aluminum beame muh hotter than the water. Explain why. T m Ativity 06-1 Thermohemitry and Calorimetry 89

4 Foundation of Chemitry MODEL 2: HEAT OF A CHEMICAL REACTION Solution of hydrohlori aid and odium hydroxide are mixed in a beaker that i urrounded by Styrofoam inulation. The temperature of the olution and beaker inreae from 24 C to 38 C. The volume of the reulting olution i 435 ml. Determine the amount of energy, E total, releaed by thi reation. E total E olution + E beaker E total total energy releaed E olution energy ued to heat the olution E olution m T E beaker energy ued to heat the beaker, inulation, and thermometer E beaker (330 J/ C) T Aume that ine the olution i dilute, the peifi heat of the olution i the ame a that of water. The beaker, inulation, and thermometer have a ombined heat apaity of 330 J/ C. The heat apaity i the amount of energy reuired to hange the temperature of ome objet by 1 C. E total (4.184 J/g C)(435 ml 1.00 g/ml)(14 C) + (330 J/ C)(14 C) 30.1 kj KEY QUESTIONS 7. In Model 2, what thing beome hotter due to the energy that i releaed in the reation of hydrohlori aid and odium hydroxide? 8. How i the energy alulated that i ued in heating the olution in Model 2? 9. How i the energy alulated that i ued in heating the other item in Model 2? 10. How i the total energy releaed in the hemial reation in Model 2 alulated? 11. What i the differene between the peifi heat apaity and the heat apaity of a ubtane or objet? 90 Chapter 6: Energy and Chemial Reation

5 PROBLEMS 1. You mix 100. ml of 1.0 M HCl with 100 ml of 1.0 NaOH, both at 25 C. The temperature of your alorimeter rie by 5.98 C, and it heat apaity i 100 J/ C. How muh energy i releaed per mol of H 2 O(l) formed? 2. An unknown piee of metal weighing 100. g i heated to 90.0 C. It i dropped into 250 g of water at 20.0 C. When euilibrium i reahed, the temperature of both the water and piee of metal i 29.0 C. Determine the peifi heat of the metal uing the fat that the energy lot by the metal mut eual the energy aborbed by the water. Aume that the heat apaity of the ontainer, a Styrofoam up, i negligible. metal - mt mt water J 1.5 J g C (100 g)(-61.0 C) -1-1 Ativity 06-1 Thermohemitry and Calorimetry 91