Heating Curves

2. Heating Curves H. L. McLean Integrated First Year Curriculum in Science, Engineering and Mathematics

3. Heating Curve for Water • What process(es) is (are) at constant temperature? • What process(es) is (are) at constant pressure? • What process(es) is (are) at constant volume? • What assumptions are utilized for discussion purposes?

4. 500 400 300 Temperature (ºC) 200 100 0 -100 0 100 200 300 400 500 600 700 800 900 1000 Heat Added (calories) Heating Curve of WaterIdentify DE, PDV, Process & Value for Each Step 1.00 gram of water at 1.00 atm

5. Conclusions • What is the system? • What phases are present? • What extensive property is being depicted? Is the gram specific heat capacity an extensive property? Why or Why not? • What is the significance of the slope of the heating curve? • Any other good questions? TBC

6. 500 400 Heating Curve of Water 1.00 gram of water at 1.00 atm 300 Temperature (ºC) 200 100 0 -100 0 100 200 300 400 500 600 700 800 900 1000 Heat Added (calories) No temperature change; ice and water have same T H2O(s,0 ºC) H2O(l,0 ºC) Heating Curve of WaterIdentify Process, Value, PDV and DE for Each Step DHfus,H2O = qp/m = 80. cal / 1.00 g = 80 cal/g Water is more dense than ice; volume decreases PDV < 0 qp - PDV = DE > 80 cal/g

7. 500 400 Heating Curve of Water 1.00 gram of water at 1.00 atm 300 Temperature (ºC) 200 100 0 -100 0 100 200 300 400 500 600 700 800 900 1000 Heat Added (calories) H2O(s,-100 ºC) H2O(s,0 ºC) Heating Curve of WaterIdentify Process, Value, PDV and DE for Each Step Cp,H2O(s) = q/m·DT = 50. cal / [1.00 g · (0 - (-100) ºC)] = 0.50 cal/g·K Assume density of ice is constant; no volume change PDV = 0 ; DE = 50 cal/g

8. Thermodynamic term value DE PDV Heating Curve of Water a) Cp,H2O (s) 0.50 cal/g·K 0 > 0 b) DHfus, H2O 80 cal/g < 0 > 0 c) Cp,H2O (l) 1.00 cal/g·K 0 > 0 d) DHvap, H2O 540 cal/g > 0 > 0 e) Cp,H2O (g) 0.50 cal/g·K > 0 > 0

9. Synopsis • On the water heating curve summary sheet, fill-in the blanks (study guide) • Other ideas may include Chemical system (balanced equation) Type of process (vaporization, sublimation, fusion, heating or cooling) Conditions (isothermal, isobaric, isochoric, adiabatic, or T,  V,  ngas)

10. Calorimetry E. A. Mottel, Author Integrated, First-Year Curriculum in Science, Engineering and Mathematics Modified by H. L. McLean

11. Please be sure you have a copy of Calorimetry lecture notes Calorimetry homework (suggested review assignment, be prepared to ask questions during the next presentation period / will not officially collect and grade)

12. Topics • This lecture describes the experimental methods by which the enthalpy of a reaction can be determined.

13. Types of Calorimeters • Constant Pressure Calorimeter • Constant Volume Calorimeter • Calorimeters are employed to develop standard tables of changes in enthalpy values (H)and provide information about changes in internal energy (E)

14. Calorimeter • An insulated device in which reactants are mixed together, and the change in temperature of the products and the calorimeter is used to calculate the heat of reaction. q system+q surroundings = 0 GOAL: limit heat gain of surroundings to zero

15. Thermometer Vacuum or insulated space q Constant Pressure Calorimeter System: reactants and part of calorimeter touching the reactants Adiabatic calorimeter q = 0 Surroundings: everything else No heat flow between system and surroundings Define system appropriately

16. 0 Energy Balance q system+q surroundings = 0 q solution + q calorimeter = 0

17. Constant Pressure Calorimeter • The change in temperature, mass of resulting solution, heat capacity of the solution, and the calorimeter constant (calorimeter and thermometer combined heat capacity) are all involved in determining the heat flow. (msolution . Cp,solution · DT) q solution + (Ccalorimeter · DT) q calorimeter = 0

18. Constant Pressure Calorimeter • If a reaction is exothermic, all the heat released is trapped by the solution and calorimeter unit. • No heat is transferred to the surroundings. The heat flow in this process is equal to which thermodynamic term, qp or qv?

19. Constant Pressure CalorimeterExample • A steel bolt (17.93 g) is heated in a Bunsen burner flame for four minutes. • The bolt is placed in 209 mL of water at 19.1 °C. • The temperature of the water rises to 26.0 °C. • The heat capacity at constant pressure for iron is 0.444 J·g–1 ·°C–1. • What is the temperature of the hot bolt?

20. Constant Volume Calorimeter • Oxygen Bomb Calorimeter • A thick walled device in which a reactant is combusted in air or oxygen. • Heat flows from the device to its immediate surroundings, and the temperature change of these surroundings is used to determine the heat of reaction.

21. Ignition source Thermometer water Oxygen Bomb Calorimeter

22. Constant Volume Calorimeter • Oxygen Bomb Calorimeter • Allows heat to flow to calibrated surroundings which behave as a large heat sink. What is a heat sink? • The bomb and immediate surroundings are responsible for the calorimeter constant, also called the "water equivalent". • q = m · Cp · DT (calibration) • m is the water equivalent of the calorimeter • Cp is the heat capacity of water

23. Constant Volume Calorimeter • Often the surroundings are at constant pressure, even though the reaction is performed at constant volume. • Because pressure may be building up, it is advantageous to keep DT small. • Lid is clamped and should prevent DV

24. Constant Volume CalorimeterExample • 2.40 grams of methane (MW =16.0) is burned in a bomb calorimeter with a water equivalent of 9000. g. (Recall this is a calibrated value! • The initial temperature of the calorimeter was 22.3 °C, the final temperature was 25.9 °C. • Assume the heat capacity of water is 1.00 cal·g–1·K–1. • Calculate the heat evolved. (Recall the lab.) The heat flow in this process is equal to which thermodynamic term, qp or qv?

25. Review Questions When is qv pertinent? How are qp and qv related? Why and how is Cp used for qv measurements? What is the significance of the water equivalent mass? What is an extensive property of matter?