Thermochemistry The study of heat transfer in chemical rxns
Dec 26, 2015
ReadingChapter 13 pages 498 - 506Chapter 15 page 591- 602
HW due Friday November 10th
Chapter 13 p 535: #43, 47, 49, 51Chapter 15 p 640: #59
• HW For tonight:
EnthalpyH= q = mcT
Heat flow/change
in a system
m is for mass!
c is for specific heat!
ΔT is for change in temp!
Grammar of Thermochemistry
Exothermic condensation reaction
H2O (g) H2O (l) + 44kJ
H2O (g) H2O (l) ΔHo = -44 kJ
Endothermic evaporation reaction
2 H2O (l) + 88 kJ 2 H2O (g)
2 H2O (l) 2 H2O (g) ΔHo = +88 kJ
Specific Heat (capacity)
cAbility of a specific quantity (1g) of a substance to store heat as its temp rises by 1oC
units Jg * oC
Calorimeter• Device that measures Δ heat• It tries to be an adiabatic
system• In real life, gives
experimental yield
• 3.358 kJ of heat added to the 50.0 g water inside a calorimeter. Twater increases from 22.34oC to 36.74oC. What is the heat capacity of the calorimeter in J/oC?
• cwater = 4.180 J/g * oC• ΔT = (36.74oC – 22.34oC) = 14.40oC• 50.00g * (4.184 J/g * oC) * 14.40oC =
3.012 x 103 J• 3.012 kJ goes into water• 3.358 kJ – 3.012 kJ = .346 kJ absorbed by
calorimeter• .346 kJ = 346 J ÷ 14.40oC = 24.0 J/oC
• 100.0 g of water at 50.0oC is added to a calorimeter that already contains 100.0g of water at 30.0oC. The final temperature is 39oC. What is the heat capacity of the calorimeter?
• cwater = 4.184 J/g * oC
• ΔTadded water = (50.0oC – 39.0oC) = 11oC• 100.0g * (4.184 J/g * oC) * 11oC =
4.60 x 103 J• ΔTcalorimeter water = (39.0oC – 30.0oC) = 9oC• 100.0g * (4.184 J/g * oC) * 9oC =
3.76 x 103 J• 4.60 kJ – 3.76 kJ = .834 kJ absorbed by calorimeter• .834 kJ = 834 J ÷ 9.0oC = 93 J/oC
• Calculate the amount of heat that must be absorbed by 50.0 grams of ice at -12.0oC to convert it to water at 20.0oC.
• cice = 2.09 J/g * oC Hf for ice = 334 J/g
• cwater = 4.184 J/g * oC
• Step 1 – warm the ice to 0oC requires:– (50.0 g) (2.09 J/g * oC) (0oC – (-12oC)) = 0.125 x 104 J
• Step 2 – melt the ice with no Δ in temp:– 50.0 g * 334J/g = 1.67 x 104 J
• Step 3 – warm the liquid to 20.0oC requires:– 50.0 g * 4.18 J/g * oC * (20 oC - 0 oC) = .418 x 104 J
Homework
• Extra Credit Homework AssignmentDue Monday November 13th – # 55 page 535, chapter 13
• Homework due Tuesday November 14th
– Chapter 15, page 641 # 61, 63, 67, 69
A certain calorimeter absorbs 20 J/oC. If 50.0 g of 50oC water is mixed with 50.0 g of 20oC water inside the calorimeter, what will be the final temperature of the mixture?
Heat lost by the hot water will be gained by the cold water and the calorimeter:
ΔHhot water = ΔHcool water + ΔHcalorimeter
ΔHhot water = (50.0 g) (4.180 J/oC*g) (50oC – x)
= 209J/oC (50oC – x)
ΔHcool water = (50.0 g) (4.180 J/oC*g) (x – 20oC)
= 209 J/oC (x – 20oC)
ΔHcalorimeter= 20 J/oC (x – 20oC)
Solve algebraically:
• 209 (50 – x) = 209 (x – 20) + 24 (x – 20)
• 209 (50 – x) = 235 (x – 20)
• 0.889 (50 – x) = x – 20
• 44 – 0.889x = x – 20
• 64 = 1.889x
• x = 33.9oC = 30oC
A certain calorimeter absorbs 24 J/oC. If 50.0 g of 52.7oC water is mixed with 50.0 g of 22.3oC water inside the calorimeter, what will be the final temperature of the mixture?
Heat lost by the hot water will be gained by the cold water and the calorimeter:
ΔHhot water = ΔHcool water + ΔHcalorimeter
ΔHhot water = (50.0 g) (4.180 J/oC*g) (52.7oC – x)
= 209J/oC (52.7oC – x)
ΔHcool water = (50.0 g) (4.180 J/oC*g) (x – 22.3oC)
= 209 J/oC (x – 22.3oC)
ΔHcalorimeter= 24 J/oC (x – 22.3oC)
Solve algebraically:
• 209 (52.7 – x) = 209 (x – 22.3) + 24 (x – 22.3)
• 209 (52.7 – x) = 235 (x – 22.3)
• 0.889 (52.7 – x) = x – 22.3
• 46.87 – 0.889x = x – 22.3
• 69.17 = 1.889x
• x = 36.6oC = 37oC
Heat of ReactionHrxn
Heat/enthalpy change of a chemical reaction
Units J or kJ
Sometimes, units J/mol rxn
Mole of reaction• Depends on how it is given in the problem
(or how you balance your reaction)
• Can say that
O2 (g) + 2 H2 (g) 2 H2O (g) + 45 kJ
• ΔHrxn = 45 kJ/mol rxn
• You can use the following conversion factors:
1 mol O2 2 mol H2 2 mol H2O 1 mol rxn 45 kJ 45 kJ 45 kJ 45 kJ
When X reacts with water the temp in a 1.5 kg
calorimeter containing 2.5 kg water went from 22.5oC to 26.5oC. Calculate Hrxn.
cwater = 4.18 J/g oC ccalorimeter = 2.00 J/g oC
ΔHrxn = ΔHwater + ΔHcalorimeter
Δ T = 26.5oC – 22.5oC = 4oC
Heat absorbed by water: Δ Hwater = mc ΔT– 2.5 kg = 2,500 g – (2,500 g)(4.18J/g*oC)(4oC) = 41,800 J = 41.8 kJ
Heat absorbed by calorimeter: Δ Hcalorimeter = mc ΔT– 1.5 kg = 1,500 g– (1,500 g)(2.00 J/g*oC)(4oC) = 12,000 J = 12 kJ
Total heat added to system = 41.8 + 12 = 53.8 kJ54 kJ
• 80 g NaOH is dissolved with 1.40 L of 0.7 M HCl in a calorimeter. HCl solution has a mass of 1.4 kg or 1,400g.
• Ccalorimeter = 20 J/oC water = 10oC
• cHCl same as cwater = 4.18 J/g*oC
• What is the heat released by the solution
• What is the Hsolution for the reaction:
– NaOH (s) + HCl (aq) NaCl (aq) + H2O (l)
• Heat absorbed by calorimeter:– 20 J/oC * 10oC = 200 J
• Heat absorbed by HCl solution:– 1,400 g * (4.18 J/g*oC) * (10oC) = 58,520 J
• Hsolution = Hcalorimeter + HHCl solution
• 200 J + 58,520 J = 58,720 J• Heat released by solution = 58,720 J = 59 kJGo back and see how many moles of NaOH & HCl
reacted:80 g NaOH is 2 moles – therefore you have 2 moles
rxn Hsolution = 59 kJ/2 mol rxn = 30 kJ/mol rxn
Change! To the HW Due Wednesday
• Chapter 15, page 637: 13 & 15
Due Thursday November 16th• Chapter 15, page 637 – 8: 25, 27, 29, 31
• When 2.61 g of C2H6O is burned at constant pressure,82.5 kJ of heat is given off. What is ΔH for the reaction:
C2H6O (l)+ O2 (g) 2 CO2 (g) + 3 H2O (l)
• 82.5 kJ 46.0 g C2H6O 1 mol C2H6O 2.61 g C2H6O mol C2H6O mol rxn
• ΔH for the reaction = -1450 kJ/mol rxn
• When Al metal is exposed to O2 it is oxidized to form Al2O3. How much heat is released by the complete oxidation of 24.2 g of Al at 25oC and 1 atm?
4 Al (s) + 3 O2 (g) 2 Al2O3 (s) ΔH = -3352 kJ/mol rxn
• 24.2 g Al 1 mol Al 1 mol rxn -3352 kJ 27 g Al 4 mol Al mol rxn
• -751 kJ = 751 kJ of heat are released
Energy of Formation Gf
o
The energy req’d to form 1 mol of a compound from pure elements
units kJ/mole
Calculate H, G, & S when 19.7 kg of BaCO3 is decomposed into BaO + CO2
Cmpd BaCO3 CO2 . BaO
Hf
o -1216.3 -393.5 -553.5
Gf
o -1137.6 -394.4 -525.1
Sf
o 112.1 213.6 70.4
Calculate H, G, & S when 13.6 g of CaSO4 is changed
into CaO + SO2 + O2 at 27oC
Cmpd CaSO4 SO2 CaO
Hf
o -1434.1 -296.8 -635.1
Gf
o -1321.8 -300.2 -604.0
Lab Results: Cup H2O NaOH Thermo
5.0 g 50.0 g 4.0 g 15.0 g Ti = 22.0
oC Tf = 27.0
oC
Cmpd NaOH Na+ OH-
Hf
o -425.6 -240.1 -230.0
Determine: theoretical and experimental heat changes, &
Calculate the potential H, G, & S for the reaction & Sf
o for
O2 when burning 8.8 kg of C3H8 Cpd C3H8 CO2 H2O
Hf
o-103.8 -393.5 -241.8
Gf
o- 23.5 -394.4 -228.6
Sf
o269.9 213.6 188.7
Calculate H, G, & S in the production of 831ML
ammonia at 227oC under 125
kPa pressureCompd NH3
Hfo -46.1
Gfo -16.5
2nd Law Thermodynamics
•Total entropy in a system always increases assuming no energy is added to the system
Calculate Ho, Go, & S when A + BC AC2 + Bat -23oC & Teq
Compd BC AC2
Hfo(kJ/mole) -150 -250
Gfo(kJ/mole) -125 -225
Calculate Htotal, when 40.0 g of
H2O is changed from - 25oC to
125oC. FPw = 0.0oC
BPw = 100.0 oC Hv = 2260 J/gCice = 2.06 (J/g K) Hf = 330 J/gCwater = 4.18 (J/g K)Csteam = 2.02 (J/g K)
Calculate Ho, G
o, & S for
AD2 + BC AC2 + BD
at (-23oC)
Cpd BC AD2 AC2 BD
Hf
o -150 -250 -300 -175
Gf
o -125 -225 -250 -150
Sf
o 75 50 80 ?
Determine Sf
o
BD