Calorimetry Calorimetry: calor (L) + metry (Gr) The measurement of heat changes. food Calories (C) =...
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Transcript of Calorimetry Calorimetry: calor (L) + metry (Gr) The measurement of heat changes. food Calories (C) =...
Calorimetry
Calorimetry
• Calorimetry:• calor (L) + metry (Gr)
• The measurement of heat changes.• food Calories (C) = 1,000 cal
(1 cal = 4.184 J; we use Joules in chemistry)
• 1 cal = heat required to raise the temperature of 1 gram of water by 1oC.
• q sign is the same as before (‘–’ exothermic)
Calorimetry
• Essential Terms:Calorimetry
• Specific Heat (s) amount of heat required to raise the temperature
of 1 gram of substance by 1oC. intensive property
• Heat Capacity (C) amount of heat required to raise the temperature
of a specific system by 1oC. Comparison:
s = heat needed to raise 1 gram of water by 1oC.C = heat needed to raise a glass of ice, tea, &glass by 1oC.
Calorimetry
• Essential Equations: (q=heat; m=mass; D Temp)Calorimetry
• Specific Heat (s) q = m s DT
• Heat Capacity (C) q = C DT(C = m s)
• Heat Exchange (2 substances: A & B)
mA sA DTA
= – mB sB DTB
Calorimetry
Two Types of Calorimeters:Calorimetry
• Constant-Volume Calorimeter• used for combustion reaction
(e.g., Calories in food)
• Constant-Pressure Calorimeter• used for other reactions
(e.g., heat pack: CaCl2 + H2O)
we use this in class
Calorimetry
Example 6.5
A 394-g sample of water is heated from 10.75°C to 83.20°C. Calculate the amount of heat absorbed (in kilojoules) by the water.
Solution
• Specific Heat & Temperature of a Single Substance
CheckSign (+) means heat was absorbed; water was heated; Reaction was endothermic so ‘q’ should be ‘+’
Calorimetry
Example 6.6
A quantity of 1.274 g of naphthalene (C10H8), a pungent-
smelling substance used in moth repellants, was burned in a
constant-volume bomb calorimeter. Consequently, the
temperature of the water rose from 21.49°C to 26.52°C. If the
heat capacity of the bomb plus water was 10.17 kJ/°C,
calculate the heat of combustion of naphthalene on a molar
basis; that is, find the molar heat of combustion.
• Constant-Volume Calorimetry
Calorimetry
Strategy
Knowing mass, DT, and C:
1. How do we calculate the heat absorbed by the calorimeter (qcal) ?
2. What is the heat generated by the combustion of 1.274 g of naphthalene (qrxn) ?
3. What is the conversion factor between grams and moles of naphthalene?
Example 6.6 – Problem Summarym = 1.274 g of C10H8
DT = 21.49°C to 26.52°C; C = 10.17 kJ/°C; q = ? kJ/mol
Calorimetry
1. What is the heat absorbed by the calorimeter (qcal)?
SolutionThe sum of the heat absorbed by the container and the water is equal to the product of the heat capacity and the temperature change. (Assume no heat loss.)
Calorimetry
2. What is the heat generated by the combustion of 1.274 g of naphthalene?
SolutionThe heat absorbed by the calorimeter was released by the chemical reaction.
energy) ofon conservati of (law 0 rxncal qq
Calorimetry
3. What is the conversion factor for naphthalene between grams and moles?
mol 1
g 128
g 274.1
kJ 16.51xqcombustion
kJ/mol 10 x 148.5 3combustionq
Calorimetry
• Measuring Heat Change:
qsys = qcal + qrxn = 0(conservation of energy)
qcal = heat capacity of calorimeter (assume all q is transferred to water)
qrxn = –qcal
mA sA DTA = –mB sB DTB
Calorimetry
Example 6.7A lead (Pb) pellet having a mass of 26.47 g at 89.98°C was placed in a constant-pressure calorimeter of negligible heat capacity containing 100.0 mL of water. The water temperature rose from 22.50°C to 23.17°C. What is the specific heat of the lead pellet? Strategy Draw the initial and conditions. (It really does help!)
• Constant-Pressure Calorimetry
Calorimetry
Strategy Treat the calorimeter as an isolated system. We know the masses of water and the lead pellet as well as the initial and final temperatures. Assuming no heat is lost to the surroundings, we can equate the heat lost by the lead pellet to the heat gained by the water. Knowing the specific heat of water, we can then calculate the specific heat of lead.
Summary
• Constant-Pressure Calorimetry
mPb sPb DTPb = – mH2O sH2O DTH2O
Calorimetry
• Constant-Pressure Calorimetry
Solution
Fin
Calorimetry