Todays goals

1
Todays goals

• Calorimetry
• Exo/endothermic

2
Admin stuff

• Next OWLs due Monday
• Next Exam Monday, Dec. 11

3
Calculating heats
The following information is given for manganese
at 1atm
What is the change in energy of manganese when a
47.1 gram solid sample is melted?
q 12.5 kJ
4
PRS

• In order to calculate the energy lost in cooling
  H2O from 150 C to 50 C, which physical
  constant(s) are needed?
• Specific heat of water
• DHvap
• DHfus
• Csp(l), Csp(g)
• Csp(l), Csp(g), DHvap
• Csp(l), Csp(g), DHfus
• Csp(l), Csp(g), DHfus

5
What is the energy change on cooling 20.0g of
water from 150.0 C to 50.0 C?
A
C
B
D
A -gt B -1,840 J B -gt C -45,120 J Total -51.1
kJ C -gt D -4,180 J
6
Measuring heat energy
7
What heat energies should we measure/know?
Enthalpies -- DH

• How much energy does ice absorb when it
  melts (DHfus)?
• How much energy is released when a hydrocarbon
  burns (octane rating of gas - DHrxn)?
• How much energy is heat is absorbed when ammonium
  nitrate dissolves? (cold packs - DHsol)

physical
chemical
chemical
8
Direction of heat energy transfer

• Endothermic process heat energy is absorbed
  (taken in) by the system under consideration
  like when ice melts, and the surroundings get
  colder. For these, q is positive!
• Examples melting a solid, vaporizing a liquid
• Exothermic process heat energy is given out by
  the system under consideration see Gummi bear,
  6.2 on CD, and the surroundings get warmer. For
  these, q is negative!
• Examples combustion, freezing a liquid,
  condensing a gas

9
We use calorimeters to measure enthalpies of
reactions (heat energy)
DH DE
Bomb
DH
Coffee cup
q
operates at constant pressure
operates at constant volume
10
Understanding coffee cup calorimetry
Virtually perfect thermal barrier
qrxn qwat 0
q 0
system
X
water bath
rest of universe
qrxn qwat 0
Measure qwat - gt learn qrxn
Measure Twat ( Tsys)
11
Coffee cup calorimetry

• In the laboratory a student uses a "coffee cup"
  calorimeter to determine the specific heat of a
  metal. She heats 19.4 grams of silver to
  99.34C and then drops it into a cup containing
  79.2 grams of water at 22.98C. She measures the
  final temperature to be 24.00C. Assuming that
  all of the heat is transferred to the water, what
  is the specific heat of silver?

0.231 J / g C
12
Bomb calorimetry (DE DH)
Virtually perfect thermal barrier
universe
water bath
X
bomb
system
qrxn qwat qbomb 0
Measure qwat but qbomb must be calibrated!
Measure Twat ( Tbomb Tsys)
13
Bomb

• A sample of octane is burned in a bomb
  calorimeter giving off 20.9 kJ of heat. The
  temperature of the water bath (1155 grams)
  increases by 3.68C. Calculate the bomb
  calorimeters heat capacity. The specific heat of
  water is 4.184 J/gC.

Burning octane is (A) Endothermic (B)
Exothermic (C) Neither
14
What are appropriate units for heat capacity

• J
• g
• C
• J / g
• J / C
• J / g C
• none of the above

15
Bomb

• A sample of octane is burned in a bomb
  calorimeter giving off 20.9 kJ of heat. The
  temperature of the water bath (1155 grams)
  increases by 3.68C. Calculate the bomb
  calorimeters heat capacity. The specific heat of
  water is 4.184 J/gC.

847 J / C
16
Bomb calorimetry

• 1.45g of glucose are burned in the bomb
  calorimeter previously calibrated (847 J / C),
  which contained 1120g of water. The temperature
  is observed to increase by 7.64 C. What is the
  enthalpy of combustion of glucose (in kJ / g)?

DH -29.2 kJ/g
17
Transfer of heat, heat balance

• I decide to heat up a mug for tea containing 50.0
  g of water at 20.0 C by dropping in a hot (90.0
  C) copper block weighing 40.0 g. What will be
  the equilibrium temperature of the water?

Cwater 4.184 J / g C CCu 0.385 J / g C
Can use heat balance to minimize algebra
24.8 C