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
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.
In calorimetry, 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 in enthalpy is 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. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ
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) kJ

13. CaO(s) + H2O(l) → Ca(OH)2(s) ∆H = -65.2kJ
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
These enthalpy diagrams show exothermic and endothermic processes: a) the reaction of calcium oxide and water and b) the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)?

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) Fe(s) + 3CO2(g) kJ
For every mole of Fe2O3, 26.3 kJ are given up to the surroundings . ∆H = kJ
Heat produced = 26.3 kJ/mol x 3.40 mol = 89.3 kJ
DH = 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 of Fe2O3?
Fe2O3(s) + 3CO(g) Fe(s) + 3CO2(g) kJ
For every 3 moles of CO, 26.3 kJ are given up to the surroundings . ∆H = kJ
Heat produced = kJ x 3.30 mol CO = 28.9 kJ 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
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
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) kJ
241.8 kJ + 2H2O(l) 2H2(g) + O2(g)
CaO(s) + H2O(l)  Ca(OH)2(s) 65.2
2NaHCO3(s) 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) kJ
241.8 kJ + 2H2O(l) 2H2(g) + O2(g)
CaO(s) + H2O(l)  Ca(OH)2(s) 65.2
2NaHCO3(s) kJ Na2CO3(s) + H2O(g) + CO2(g)

31. 5. Oxygen is necessary for releasing energy from glucose in organisms
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
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