CH 6: Thermochemistry. Renee Y. Becker Valencia Community College CHM 1045. Energy. Energy : is the capacity to do work, or supply heat. Energy = Work + Heat Kinetic Energy : is the energy of motion. E K = 1 / 2 mv 2 (1 Joule = 1 kg m 2 /s 2 ) (1 calorie = 4.184 J)
Renee Y. Becker
Valencia Community College
Energy = Work + Heat
EK = 1/2mv2(1 Joule = 1 kgm2/s2)
(1 calorie = 4.184 J)
Which of the following has the greatest kinetic energy?
DE = Efinal – Einitial = E2 – E1 = q + w
q = heat, w = work
(1 N/m2 = 1 Pa)
(1 atm = 101,325 Pa)
Work = Force x Distance
The expansion in volume that occurs during a reaction forces the piston outward against atmospheric pressure, P. Work = -atmospheric pressure * area of piston * distance piston moves
How much work is done (in kilojoules), and in which direction, as a result of the following reaction?
q = DE + PDV
At constant volume (DV = 0): qv = DE
At constant pressure: qp = DE + PDV = DH
Enthalpy change: DH = Hproducts – Hreactants
The explosion of 2.00 mol of solid TNT with a volume of approximately 0.274 L produces gases with a volume of 489 L at room temperature. How much PV (in kilojoules) work is done during the explosion? Assume P = 1 atm, T = 25°C.
2 C7H5N3O6(s) 12 CO(g) + 5 H2(g) + 3 N2(g) + 2 C(s)
Enthalpy is a state function, the enthalpy change from solid to vapor does not depend on the path taken between the two states.
Hsubl = Hfusion + Hvap
Endothermic:Heat flows into the system from the surroundings and DH has a positive sign.
Exothermic:Heat flows out of the system into the surroundings and DH has a negative sign.
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l) DH = –2219 kJ
3 CO2(g) + 4 H2O(l) C3H8(g) + 5 O2(g) DH = +2219 kJ
3 [C3H8(g) + 15 O2(g) 9 CO2(g) + 12 H2O(l)] DH = 3(-2219) kJ
DH = -6657 kJ
a) Burning of 15.5 g of propane: C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(l)
DH = –2219 kJ/mole
b) Reaction of 4.88 g of barium hydroxide octahydrate with ammonium chloride:
Ba(OH)2·8 H2O(s) + 2 NH4Cl(s) BaCl2(aq) + 2 NH3(aq) + 10 H2O(l)
DH = +80.3 kJ/mole
Is an endothermic reaction a favorable process thermodynamically speaking?
3 H2(g) + N2(g) 2 NH3(g) DH° = –92.2 kJ
(1) 2 H2(g) + N2(g) N2H4(g) DH°1 = ?
(2) N2H4(g) + H2(g) 2 NH3(g) DH°2 = –187.6 kJ
(3) 3 H2(g) + N2(g) 2 NH3(g) DH°3 = –92.2 kJ
DH°1 + DH°2 = DH°reaction
Then DH°1 = DH°reaction - DH°2
DH°1 = DH°3 – DH°2 = (–92.2 kJ) – (–187.6 kJ) = +95.4 kJ
CH4(g) + 2 Cl2(g) CH2Cl2(g) + 2 HCl(g)
Use the following data to calculate DH° (in kilojoules) for the above reaction:
CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)
DH° = –98.3 kJ
CH3Cl(g) + Cl2(g) CH2Cl2(g) + HCl(g)
DH° = –104 kJ
DH°f = 0 for an element in its standard state
DH° = DH°f (Products) – DH°f (Reactants)
aA + bB cC + dD
DH° = [cDH°f(C) + dDH°f(D)] – [aDH°f(A) + bDH°f(B)]
CO( subtracted to determine the overall equation.g)
-1131Standard Heats of Formation
Some Heats of Formation, Hf° (kJ/mol)
Calculate DH° (in kilojoules) for the reaction of ammonia with O2 to yield nitric oxide (NO) and H2O(g), a step in the Ostwald process for the commercial production of nitric acid.
Calculate DH° (in kilojoules) for the photosynthesis of glucose and O2 from CO2 and liquid water, a reaction carried out by all green plants.
Which of the following would indicate an endothermic reaction? Why?
Calculate the heat of vaporization, Hvap of water, using standard enthalpies of formation
H2O(g) -241.8 kJ/mol
H2O(l) -285.8 kJ/mol
Constant Pressure subtracted to determine the overall equation.
Specific Heat:The amount of heat required to raise the temperature of 1.00 g of substance by 1.00°C.
q = s x m x t
q = heat required (energy)
s = specific heat
m = mass in grams
t = Tf - Ti
q = MH x n x t
q = heat required (energy)
MH = molar heat
n = moles
t = Tf - Ti
What is the specific heat of lead if it takes 96 J to raise the temperature of a 75 g block by 10.0°C?
How much energy (in J) does it take to increase the temperature of 12.8 g of Gold from 56C to 85C?