Lecture 334/21/06

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?

Constant pressure calorimetry (coffee cup calorimetry) heat lost = heat gained Measure change in temperature of water

Bomb calorimetry Mainly for combustion experiments ∆V = 0 q rxn + q bomb + q water = 0 combustion chamber

Bomb calorimeter math Book: q rxn + q bomb + q water = 0 q rxn = - [C bomb ∆T + C water m water ∆T] In the lab: q rxn + q calorimeter = 0 q rxn = - [C calorimeter ∆T]

Example Sulfur (2.56 g) was burned in a bomb calorimeter with excess O 2. The temperature increased from ºC to 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 SO 2 formed. S(s) + O 2 (g)  SO 2 (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 Cl 2 (g)  NaCl (s) ∆H° f = kJ Key points

Using ∆H° f to get ∆H° rxn 2 ways to look at the problem Calculate ∆H° rxn for: C 3 H 8 (g) + 5 O 2  3 CO 2 (g) + 4 H 2 O (l) Given: 3 C(s) + 4 H 2 (g)  C 3 H 8 (g) ∆H° f = KJ/mol C(s) + O 2 (g)  CO 2 (g) ∆H° f = KJ/mol O 2 (g) + 2 H 2 (g)  2H 2 O (l) ∆H° f = KJ/mol

Spontaneity Some thought that ∆H could predict spontaneity Sounds great BUT.....

Entropy (Measurement of disorder) Related to number of microstates ∆S universe = ∆S system + ∆S surroundings 2 nd Law of Thermodynamics Entropy of the universe increases with spontaneous reactions Reversible reactions vs. Irreversible reaction

3 rd Law of thermodynamics S = 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 orCO 2 liquid water at 25°Corliquid water at 50°C pure Al 2 O 3 (s)orAl 2 O 3 with some Al 2+ replaced with Cr 3+ 1 mole of N 2 at 1 atmor1 mol of N 2 at 10 atm CH 3 CH 2 CH 2 CH 3 (g)or CH 3 CH 3 (g)

Is the reaction spontaneous?

Gibbs Free Energy ( ∆G) ∆G° = ∆H° - T∆S° ∆G = ∆H - T∆S ∆G° = Σn∆G f ° (products) - Σn∆G f ° (reactants)

Gibbs Free Energy ∆G = ∆H - T∆S ∆H∆S-T∆S∆G spontaneous? example -+ 2O 3 (g)  3O 2 (g) +- 3O 2 (g)  2O 3 (g) -- H 2 O (l)  H 2 O (s) ++ H 2 O (s)  H 2 O (l)

Gibbs Free Energy (∆G) and equilibrium R = 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?