lecture 334/21/06. quiz 1.a 12.3 g sample of iron requires heat transfer of 41.0 j to raise its...
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Lecture 33 4/21/06
QUIZ
1. A 12.3 g sample of iron requires heat transfer of 41.0 J to raise its temperature from 17.3 ºC to 24.7 ºC. Calculate the specific heat capacity of iron.
2. Which requires more energy: a) warming the iron sample in question 1 or b) the same increase in temperature for an equal mass of water?
KgJ 184.4O)C(H
Hm q T Cmq
2
change phase
Constant pressure calorimetry(coffee cup calorimetry)
heat lost = heat gained
Measure change in temperature of water
Bomb calorimetry
Mainly for combustion experiments ∆V = 0 qrxn + qbomb + qwater = 0
combustion chamber
Bomb calorimeter math
Book: qrxn + qbomb + qwater = 0
qrxn = - [Cbomb∆T + Cwatermwater∆T]
In the lab: qrxn + qcalorimeter = 0
qrxn = - [Ccalorimeter∆T]
Example
Sulfur (2.56 g) was burned in a bomb calorimeter with excess O2. The temperature increased from 21.25 ºC to 26.72 ºC. The bomb had a heat capacity of 923 J/ºC and the calorimeter contained 815 g of water. Calculate the heat evolved per mole of SO2 formed.
S(s) + O2 (g) SO2 (g)
Standard heat of reaction (∆H°rxn)
Same standard conditions as before:
Standard heat of formation (∆H°f)
Enthalpy change for the formation of a substance from its elements at standard state
Na(s) + ½ Cl2 (g) NaCl (s) ∆H°f = -411.1 kJ
Key points
Using ∆H°f to get ∆H°rxn
2 ways to look at the problem
Calculate ∆H°rxn for:
C3H8 (g) + 5 O2 3 CO2 (g) + 4 H2O (l)
Given:
3 C(s) + 4 H2 (g) C3H8 (g) ∆H°f = -103.85 KJ/mol
C(s) + O2 (g) CO2 (g) ∆H°f = -393.5 KJ/mol
O2 (g) + 2 H2 (g) 2H2O (l) ∆H°f = -285.8 KJ/mol
Spontaneity
Some thought that ∆H could predict spontaneity
Sounds great BUT . . . . .
Entropy (Measurement of disorder)
Related to number of microstates
∆Suniverse = ∆Ssystem + ∆Ssurroundings
2nd Law of Thermodynamics Entropy of the universe increases with spontaneous
reactions
Reversible reactions vs. Irreversible reaction
3rd Law of thermodynamicsS = O at O K
S° - entropy gained by converting it from a perfect crystal at 0 K to standard state conditions
Degrees of freedom
translational motion molecules in gas > liquid > solid
vibrational motion movement of a atom inside a molecule
rotational motion rotation of a molecule
Entropy trends
Entropy increases: with more complex molecules with dissolution of pure gases/liquids/solids with increasing temperature with increasing volume with increasing # moles of gases
Which has higher entropy?
dry ice or CO2
liquid water at 25°C or liquid water at 50°C
pure Al2O3(s) or Al2O3 with some Al2+ replaced with Cr3+
1 mole of N2 at 1 atm or 1 mol of N2 at 10 atm
CH3CH2CH2CH3 (g) or CH3CH3 (g)
Is the reaction spontaneous?
Gibbs Free Energy (∆G)
∆G° = ∆H° - T∆S°
∆G = ∆H - T∆S
∆G° = Σn∆Gf°(products) - Σn∆Gf°(reactants)
Gibbs Free Energy ∆G = ∆H - T∆S
∆H ∆S -T∆S ∆G spontaneous
? example
- + 2O3 (g) 3O2 (g)
+ - 3O2(g) 2O3 (g)
- - H2O (l) H2O (s)
+ + H2O (s) H2O (l)
Gibbs Free Energy (∆G) and equilibrium
QlnRTGG
R = 8.314 J/mol-K
Example
A hot plate is used to heat two 50-mL beakers at the same constant rate. One beaker contains 20.0 grams of graphite (C=0.79 J/g-K) and one contains 10 grams of ethanol (2.46 J/g-K).
Which has a higher temperature after 3 minutes of heating?