Lecture 334/25/05

1 st Law of Thermodynamics revisited ∆E = q + w Change in Energy content heat work

work (w) = - F x d w = - (P x A) x d w = - P∆V if ∆V = 0, then no work

State function property of a system whose value depends on the final and initial states, but not the path driving to Mt Washington route taken vs. altitude change ∆E is a state function q and w are not

Change in Enthalpy (∆H or q p ) equals the heat gained or lost at constant pressure ∆E = q p + w ∆E = ∆H + (-P∆V) ∆H = ∆E + P∆V

∆E vs. ∆H Reactions that don’t involve gases 2KOH (aq) + H 2 SO 4 (aw)  K 2 SO 4 (aq) + 2H 2 O (l) ∆V ≈ 0, so ∆E ≈ ∆H Reactions in which the moles of gas does not change N 2 (g) + O 2 (g)  2NO (g) ∆V = 0, so ∆E = ∆H Reactions in which the moles of gas does change 2H 2 (g) + O 2 (g)  2H 2 O (g) ∆V > 0, but often P∆V << ∆H, thus ∆E ≈ ∆H

Enthalpy is an extensive property Magnitude is proportional to amount of reactants consumed H 2 (g) + ½ O 2 (g)  H 2 O (g) ∆H = KJ 2H 2 (g) + O 2 (g)  2H 2 O (g) ∆H = KJ Enthalpy change for a reaction is equal in magnitude (but opposite in sign) for a reverse reaction H 2 (g) + ½ O 2 (g)  H 2 O (g) ∆H = KJ H 2 O (g)  H 2 (g) + ½ O 2 (g) ∆H = KJ Enthalpy change for a reaction depends on the state of reactants and products H 2 O (l)  H 2 O (g) ∆H = 88 KJ

Constant pressure calorimetry (cofee cup calorimetry) heat lost = heat gained Measure change in temperature of water 10 g of Cu at 188 °C is added to 150 mL of water in a cofee cup calorimeter and the temperature of water changes from 25 °C to 26 °C. Determine the specific heat capacity of copper.

Bomb calorimetry Mainly for combustion experiments ∆V = 0 q rxn + q bomb + q water = 0 Often combine q bomb + q water into 1 calorimeter term with q cal = C cal ∆T combustion chamber

Bond enthalpies

Enthalpies of formation

Hess’ Law

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?