Measuring and Expressing Enthalpy Changes

1. Measuring and Expressing Enthalpy Changes Prentice Hall Chapter 17.2 Dr. Yager

2. Objectives • Describe how calorimeters are used to measure heat flow. • Construct a thermochemical equation. • Solve for enthalpy changes in chemical reactions by using heats of reaction.

3. A burning match releases heat to its surroundings in all directions. How much heat does this exothermic reaction release? You will learn to measure heat flow in chemical and physical processes by applying the concept of specific heat.

4. Calorimetry • Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. • Incalorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings.

5. The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

6. Constant-Pressure Calorimeters • The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system. The change inenthalpyis represented by ∆H.

7. Constant-Volume Calorimeters (∆U) • Calorimetry experiments can be performed at a constant volume using a bomb calorimeter. • ∆H = ∆U + ∆(PV)

11. A small pebble is heated and placed in a foam cup calorimeter containing 25.0 mL of water at 25.0oC. The water reaches a maximum temperature of 26.6oC. How many calories of heat were released by the pebble? qsys= ∆H = -qsurr = -m x C x ∆T

12. Thermochemical Equations • In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. • A chemical equation that includes the enthalpy change is called a thermochemical equation. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ

13. The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. CaO(s) + H2O(l) → Ca(OH)2(s) ∆H = -65.2kJ Note: the sign changes because the heat is given off to the surroundings

14. 17.2 Exothermic Reaction

15. Endothermic Reaction

19. The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.40 mol of Fe2O3 reacts with an excess of CO? Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) + 26.3 kJ For every mole of Fe2O3, 26.3 kJ are given up to the surroundings . ∆H = -26.3 kJ Heat produced = 26.3 kJ/mol x 3.40 mol = 89.3 kJ DH = -89.3 kJ Heat absorbed by surroundings = 89.3 kJ DH = 89.3 kJ

20. The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.30 mol of CO reacts with an excess ofFe2O3? Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g) + 26.3 kJ For every 3 moles of CO, 26.3 kJ are given up to the surroundings . ∆H = -26.3 kJ Heat produced = 26.3 kJ x 3.30 mol CO = 28.9 kJ 3 mol CO Heat absorbed by surroundings = 28.9 kJ

21. The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.

23. The change in temperature recorded by the thermometer in a calorimeter is a measurement of • the enthalpy change of the reaction in the calorimeter. • the specific heat of each compound in a calorimeter. • the physical states of the reactants in a calorimeter. • the heat of combustion for one substance in a calorimeter.

24. The change in temperature recorded by the thermometer in a calorimeter is a measurement of • the enthalpy change of the reaction in the calorimeter. • the specific heat of each compound in a calorimeter. • the physical states of the reactants in a calorimeter. • the heat of combustion for one substance in a calorimeter.

25. 2. For the reaction: CaO(s) + H2O(l)  Ca(OH)2(s), H = 65.2 kJ. This means that 62.5 kJ of heat is __________ during the process. • absorbed • destroyed • changed to mass • released

26. 2. For the reaction: CaO(s) + H2O(l)  Ca(OH)2(s), H = 65.2 kJ. This means that 62.5 kJ of heat is __________ during the process. • absorbed • destroyed • changed to mass • released

27. 3. How much heat is absorbed by 325 g of water if its temperature changes from 17.0°C to 43.5°C? The specific heat of water is 4.18 J/g°C. • 2.00 kJ • 3.60 kJ • 36.0 kJ • 360 kJ

28. 3. How much heat is absorbed by 325 g of water if its temperature changes from 17.0°C to 43.5°C? The specific heat of water is 4.18 J/g°C. • 2.00 kJ • 3.60 kJ • 36.0 kJ • 360 kJ

29. 4. Which of the following is a thermochemical equation for an endothermic reaction? • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890 kJ • 241.8 kJ + 2H2O(l) 2H2(g) + O2(g) • CaO(s) + H2O(l)  Ca(OH)2(s) 65.2 • 2NaHCO3(s) + 129 kJ Na2CO3(s) + H2O(g) + CO2(g)

30. 4. Which of the following is a thermochemical equation for an endothermic reaction? • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + 890 kJ • 241.8 kJ + 2H2O(l) 2H2(g) + O2(g) • CaO(s) + H2O(l)  Ca(OH)2(s) 65.2 • 2NaHCO3(s) + 129 kJ Na2CO3(s) + H2O(g) + CO2(g)

31. 5. Oxygen is necessary for releasing energy from glucose in organisms. How many kJ of heat are produced when 2.24 mol glucose reacts with an excess of oxygen? C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + 2808 kJ/mol of glucose • 4.66 kJ • 9.31 kJ • 1048 kJ • 6290 kJ

32. 5. Oxygen is necessary for releasing energy from glucose in organisms. How many kJ of heat are produced when 2.24 mol glucose reacts with an excess of oxygen? C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + 2808 kJ/mol of glucose • 4.66 kJ • 9.31 kJ • 1048 kJ • 6290 kJ