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?