Enthalpy and Calorimetry ­ solutions
Enthalpy
a state function that is the sum of the internal energy and product of pressure and volume of a system
∆H = ∆E + P∆V
­ at constant P, q = ∆H
If ∆H > 0, it is endothermic. It absorbs heat from the surroundings. It is cooler to the touch.
If ∆H < 0, it is exothermic. It gives off heat to the surroundings. It is warmer to the touch.
Enthalpy and Calorimetry ­ solutions
When solid potassium hydroxide pellets are added to water, the following reaction takes place:
NaOH(s) à NaOH(aq)
For this reaction at constant pressure, ∆H = ­43 kJ/mol. Answer the following questions regarding the addition of 14 g of NaOH to water:
a. Does the beaker get warmer or cooler?
b. Is the reaction exo­ or endothermic?
c. What is the enthalpy change for the dissolution?
Enthalpy and Calorimetry ­ solutions
Calorimetry
An experimental technique used to measure the change in energy associated with a reaction or phase change.
Two Types of Calorimeters
1) Constant pressure
­ often a "coffee cup"
calorimeter
­ ∆H = q
2) Constant volume
­ use of a bomb calorimeter
­ because ∆V = 0 and no work is done, ∆E = q
­ often used to measure energy changes for combustion reactions
Enthalpy and Calorimetry ­ solutions
Heat Capacity
The amount of energy required to change the temperature of a system by 1ºC
Ways to express heat capacity
1) Heat capacity
2) Specific heat capacity
3) Molar heat capacity
Enthalpy and Calorimetry ­ solutions
Specific heat capacity can be used to quantify the relationship between the amount of heat gained or released by a substance and the associated temperature change.
q = m Cp ∆T
Enthalpy and Calorimetry ­ solutions
Suppose you find a penny (minted before 1982, when pennies were almost entirely copper) in the snow. How much heat is absorbed by the penny as it warms from the temperature of the snow, which is ­ 8ºC, to the temperature of your body, 37.0ºC? Assume the penny is pure copper and has a mass of 3.10 g.
Enthalpy and Calorimetry ­ solutions
A piece of unknown metal with mass 17.19 g is heated to 100.00ºC and dropped into 25.00 g of water at 24.50ºC. The final temperature of the system is 30.05ºC. What is the specific heat of the metal?
Enthalpy and Calorimetry ­ solutions
Calorimetry
Problem #1
At constant pressure, ∆H for the dissociation of NaOH(s) is – 43 kJ/mol. If 10.0 g of solid NaOH is added to 1.00 L of water (specific heat capacity = 4.18 J/gºC) at 25.0ºC in a constant pressure calorimeter, what will be the final temperature of the solution? (Assume the density of the final solution is 1.05 g/mL.)
NaOH(s) à NaOH(aq)
Enthalpy and Calorimetry ­ solutions
Problem #2
The heat of combustion of glucose, C6H12O6, is 2800 kJ/mol. A sample of glucose weighing 5.00 g was burned with excess oxygen in a bomb calorimeter. The temperature of the bomb rose 2.4ºC. What is the heat capacity of the calorimeter?
A 4.40 g sample of propane was then burned with excess oxygen in the same bomb calorimeter. The temperature of the bomb increased 6.85ºC. Calculate ∆Ecombustion of propane.