1. Chapter 17 Thermochemistry 17.2 Measuring and Expressing
17.1 The Flow of Energy
17.2 Measuring and Expressing
Enthalpy Changes
17.3 Heat in Changes of State
17.4 Calculating Heats of Reaction
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2. Enthalpy Change in a Calorimetry Experiment
Sample Problem 17.3
Enthalpy Change in a Calorimetry Experiment
When 25.0 mL of water containing mol HCl at 25.0°C is added to 25.0 mL of water containing mol NaOH at 25.0°C in a foam-cup calorimeter, a reaction occurs. Calculate the enthalpy change (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume that the densities of the solutions are 1.00 g/mL and the volume of the final solution is equal to the sum of the volumes of the reacting solutions.
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3. Thermochemical Equations
How can you express the enthalpy change for a reaction in a chemical equation?
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4. Thermochemical Equations
In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product.
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5. Thermochemical Equations
In the equation describing the exothermic reaction of calcium oxide and water, the enthalpy change can be considered a product.
CaO(s) + H2O(l) → Ca(OH)2(s) kJ
Calcium oxide is one of the components of cement.
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6. Thermochemical Equations
A chemical equation that includes the enthalpy change is called a thermochemical equation.
CaO(s) + H2O(l) → Ca(OH)2(s) kJ
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7. Thermochemical Equations
Heats of Reaction
The heat of reaction is the enthalpy change for the chemical equation exactly as it is written.
Heats of reaction are reported as ΔH.
The physical state of the reactants and products must also be given.
The standard conditions are that the reaction is carried out at kPa (1 atm) and 25°C.
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8. Thermochemical Equations
Heats of Reaction
Each mole of calcium oxide and water that reacts to form calcium hydroxide produces 65.2 kJ of heat.
CaO(s) + H2O(l) → Ca(OH)2(s)
ΔH = –65.2 kJ
In exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products.
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9. Thermochemical Equations
Heats of Reaction
Baking soda (sodium bicarbonate) decomposes when it is heated. This process is endothermic.
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g)
The carbon dioxide released in the reaction causes muffins to rise while baking.
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10. Thermochemical Equations
Heats of Reaction
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g)
Remember that ΔH is positive for endothermic reactions. Therefore, you can write the reaction as follows:
2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
ΔH = 85 kJ
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11. Thermochemical Equations
Heats of Reaction
The amount of heat released or absorbed during a reaction depends on the number of moles of the reactant involved.
The decomposition of 2 mol of sodium bicarbonate requires 85 kJ of heat.
Therefore, the decomposition of 4 mol of the same substance would require twice as much heat, or 170 kJ.
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12. Thermochemical Equations
Heats of Reaction
To see why the physical state of the reactants and products must be stated, compare the following two equations.
difference = kJ
H2O(l) → H2(g) + O2(g) ΔH = kJ 1 2
H2O(g) → H2(g) + O2(g) ΔH = kJ
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13. Thermochemical Equations
Heats of Reaction
To see why the physical state of the reactants and products must be stated, compare the following two equations.
difference = kJ
H2O(l) → H2(g) + O2(g) ΔH = kJ 1 2
H2O(g) → H2(g) + O2(g) ΔH = kJ
The vaporization of 1 mol of liquid water to water vapor at 25°C requires 44.0 kJ of heat.
H2O(l) → H2O(g) ΔH = 44.0 kJ
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14. Using the Heat of Reaction to Calculate Enthalpy Change
Sample Problem 17.4
Using the Heat of Reaction to Calculate Enthalpy Change
2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) CO2(g)
Calculate the amount of heat (in kJ) required to decompose 2.24 mol NaHCO3(s).
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15. Thermochemical Equations
Heats of Combustion
The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.
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16. Thermochemical Equations
Heats of Combustion
Small amounts of natural gas within crude oil are burned off at oil refineries.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) kJ
This is an exothermic reaction.
Burning 1 mol of methane releases 890 kJ of heat.
The heat of combustion (ΔH) for this reaction is –890 kJ per mole of methane burned.
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17. Heats of Combustion at 25°C
Interpret Data
Heats of Combustion at 25°C
Substance
Formula
ΔH (kJ/mol)
Hydrogen
H2(g)
–286
Carbon
C(s, graphite)
–394
Methane
CH4(g)
–890
Acetylene
C2H2(g)
–1300
Ethanol
C2H6O(l)
–1368
Propane
C3H8(g)
–2220
Glucose
C6H12O6(s)
–2808
Octane
C8H18(l)
–5471
Sucrose
C12H22O11(s)
–5645
Like other heats of reaction, heats of combustion are reported as the enthalpy changes when the reactions are carried out at kPa and 25°C.
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18. Which of the following thermochemical equations represents an endothermic reaction?
A. Cgraphite(s) + 2 kJ Cdiamond(s)
B. 2H2(g) + O2(g) H2O kJ
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19. Which of the following thermochemical equations represents an endothermic reaction?
A. Cgraphite(s) + 2 kJ Cdiamond(s)
B. 2H2(g) + O2(g) H2O kJ
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