Ch.5 THERMOCHEMISTRY Energy, E work, w 1 st Law Thermo Calorimetry Enthalphy, H, heat, q heat of...
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Transcript of Ch.5 THERMOCHEMISTRY Energy, E work, w 1 st Law Thermo Calorimetry Enthalphy, H, heat, q heat of...
Ch.5 THERMOCHEMISTRY
Energy, E work, w1st Law ThermoCalorimetry
Enthalphy, H, heat, q heat of rxn; enthalphyies of formationHess’ Law
∆H, Enthalpy∆HRXN
Specific heat
Thermochemistry
Energy, E capacity to do work or transfer heat
Work, w E or force that causes a in direction or position of an object w = F*d
relationship bet chem rxns & E es due to heat
Heat, q E to cause increase of temp of an objecthotter -----> colder
sys ----> surr exothermic, sys losses qsurr ----> sys endothermic, sys gains q
ENERGY
PE: potential E stored E, amt E sys has available
E in motion, Ek = 0.5 mv2KE: kinetic E
2 objects mass1 > mass2 @ same speed which more Ek?1 object v1 < v2 @ same mass which more Ek?
Internal E total KE + PE of system
∆E = ∑Ef - ∑Ei = ∑Epdt - ∑Ereact
∆Esys = -∆Esurr
∆E always => system surr
Transfer of E results in work &/or heat
H2
O2
NOW, think of atoms & molecules in random motion colliding!!!!!!
What kindof Energieswould beinvolved?
E UNITSJoule, J EK = 0.5(2 Kg)(1 m/s)2 = 1 Kg-m2/s2 = 1 J
calorie, cal E needed to raise 1 g H2O by 1oC 1 cal = 4.184 J
1 Cal (food) = 1000 cal= 1 kcal
Transfer of E results in work &/or heat
System – Surroundings Defined as ………….? What???
System: a defined region
Surroundings: everything that will ∆ by influences of the system
OPEN: matter & E ex w/ surr
CLOSED: ex E, not matter w/ surr
E
2 mol O2
1 mol CH4
∆PE
2 mol H2O1 mol CO2
E released to surroundings as Heat
∑PE(H2O + CO2) < ∑PE(O2 + CH4)
system
∆PE
2 mol NO
E1 mol N2
1 mol O2
Heat absord from surroundings
∑PE(NO) > ∑PE(O2 + N2)
system
Determine the sign of DH in each process under 1 atm; eno or exo?
1. ice cube melts2. 1 g butane gas burned to give CO2 & H2O
1. ice is the sys, ice absorbs heat to melt, DH “+”, ENDO
2. butane + O2 is the sys, combustion gives off heat, DH “-”, EXO
must predict ifheat absorbedor released
1st Law of Thermodynamics: total E of universe is constant - E is neither created nor destroyed but es form
Conservation of E
q: Heat, Internal H E transfer bet sys & surr w/ T diff
w: work, other form E transfer mechanical, electrical,
∆E = q + w sum of E transfer as heat &/or work
∆E = q + w + + + - - - + - + : sys gain E; w > q - + - : sys lost E |w| > |q|
What is ∆E when a process in which 15.6 kJ of heat and 1.4 kJ of work is done on the system?
∆E = q + w 15.6 + 1.4 kJ = 17.0 kJ
State FunctionsProperty of variable depends on current state; not how that state was obtained
T, H, E, V, P use CAPITAL letters to indicate state fcts
∆ : state fcts depend on initial & final states
∆H Enthalpy
Must measure q & w2 types: electrical, PV - movement of charged particles - w of expanding gas
w = -P∆V@ constant P∆H = ∆E + P∆V q + w
3 Chemical Systems
#1 no gas involveds, l, ppt, aq phases have little or not V change; P∆V ≈ 0, then ∆H ≈ ∆E
#2 amt of gas no changeH2 (g) + I2 (g) -- 2 HI (g)
P∆V ≠ 0, then ∆H ≈ ∆E∆E mostly transfer as Heat
P∆V = 0, then ∆H = ∆E
#3 amt of gas does changeN2 (g) + 3 H2 (g) -- 2 NH3 (g)
∆H Enthalpy Changes∆Hcomb ∆Hf ∆Hfus ∆Hvap
combines w/ O2 cmpd formed subst melts subst vaporizes s -- l l -- g
PV WorkCalculate the work associated with the expansionof a gas from 46 L to 64L @ 15 atm.
w = -P∆Vw = -(15 atm)(18 L) = -270 L-atm
NOTE: “PV” work - P in P∆V always refers to external P - P that causes compression or resists expansion
A balloon is inflated by heating the air inside. The vol changes from4.00*106 L to 4.5*106 L by the addition of 1.3*108 J of heat. Find ∆E,assuming const P = 1.0 atm
Heat added, q = + P = 1.0 atm1 L-atm = 101.3 J∆V = 5.0*105 L
∆E = q + ww = -P∆Vw = -(1.0 atm)(5.0*105 L) = -5.0*105 L-atm
(-5.0*105 L-atm)(101.3 J / 1 L-atm) = -5.1*107 J
∆E = q + w = (1.3*108 J) + (-5.1*107 J) = 8*107 JMore E added by heating than gas expanding,net increase in q, ∆E “+”
REVIEW
PE - KE J - cal 1st LawEnthalpy sys - surr PV STATE Fcts
Measure of Heat flow, released or absorded, @ const P & V
CALORIMETRYHeats of Reaction
Heat Capacity, C Specific Heat, Cs
T when object absorbs heat C of 1 g of subst
Solar-heated homes use rocks to store heat. An increase of 120Cin temp of 50.0 Kg of rocks, will absorb what quantity of heat?Assume Cs = 0.82 J/Kg-K. What T would result in a releaseof 450 kJ?
Not as simple as: ∆Hfinal - ∆Hinitial
q = Cs*m*T
+q or -q?gains lossendo exo
KgJ
T*m
q
)T - (T*g
heat trans C
if
s
How much Heat is transferred when 720 g of antifreeze cools 25.5 oC?Cs = 2.42 J/g-K
q = Cs * mass * ∆T ∆ T = -25.5 ?
T?
K or oC? K = oC
THN IK!!!!
q = (2.42 J/g-K) * (720 g) * (-25.5K) = -44400 J or -44.4 kJ
HESS’S LAWHeat Summation
Rxn are multi-step processesCalculate H from tabulated values
Hess’ states:overall H is sum of individual steps
REACTS ======> PDTS
Figure 05.22
- reverse rxn?
- had 3X many moles?
What effect H if
Figure 05.22
THN IK!!!!
Calculate HRXN for Ca(s) + 0.5 O2(g) + CO2 (g) --------> CaCO3(s) HRXN = ? given the following steps: Ca (s) + 0.5 O2 (g) -----> CaO (s) Ho
f = - 635.1 kJ CaCO3 (s) -----> CaO (s) + CO2 (g) Ho
f = + 178.3 kJ
Note: to obtain overall rxn ==> (1st rxn) + (-2nd rxn)
Ca (s) + 0.5 O2 (g) -----> CaO (s) Hof = - 635.1 kJ
CaO (s) + CO2 (g) -----> CaCO3 (s) Hof = - 178.3 kJ
Ca(s) + 0.5 O2(g) + CO2 (g) ------> CaCO3(s) Horxn = - 813.4 kJ
Ho?“o”??
Enthalpies of
Formation
STANDARD STATESSet of specific conditions
- gas: 1 atm, ideal behavior - aq solution: 1 M (mol/L) - pure subst: most stable form @ 1 atm & Temp T usually 25oC - forms 1 mole cmpd; kJ/mol
Use superscript “o” indicates Std States
parts elemental its
fromsubst of formation
0rxn
0f H H
Individual ∆Hf 0 values from book table, appendix C, pg 1100
NOTE: look at state
Ca (s) + 0.5 O2 (g) -----> CaO (s) H = - 635.5 kJ
0.0 H Ca 0f(s)
0.0 H O 0.5 0f(g)2
635.5- H CaO 0f(s)
Orxn
Of
Of H (Reacts) H - (Pdts) H
-635.5 kJ - (0.0 + 0.0)kJ = -635.5 kJ
What is the change in enthalpy for the reaction of sulfur dioxideand oxygen to form sulfur trioxide. All in gas form.Is this endo- or exo-thermic?
2 SO2 (g) + O2 (g) -----> 2 SO3 (g)
Solar-heated homes use rocks to store heat. An increase of 120Cin temp of 50.0 Kg of rocks, will absorb what quantity of heat?Assume Cs = 0.82 J/Kg-K. What T would result in a releaseof 450 kJ?
q = (0.82 J/g-K)*(5.0*104 g)*(12.0 K) =4.9 * 105 J
q = Cs*m*T
T = q/[Cs*m]
T = (4.5*105 J)/[(0.82 J/g-K)*(5.0*104 g)] = 11O decrease
H = Hf Pdts - Hf reacts
H = (-792.0 kJ) - (-593.6 kJ) = -198.4 kJ
Exothermic, -H What if rxn were reversed?????
Find Hof per mole in tables (kJ/mol)
SO2 = -296.8 SO3 = -396.0 O2 = 0.0 free element
Sum Hf reactants using stoich coeff & also pdts
kJ 792.0- mol 1
kJ 0.396SO mol 2 3
kJ 593.6- mol 1
kJ 0.0O mol 1 2
mol 1
kJ 8.296SO mol 2 2
Write balanced eqn for the formation of 1 mol of NO2 gas fromnitrogen monoxide gas and oxygen gas. Calculate DHO
rxn
1 NO(g) + .5 O2(g) ---> NO2(g)
DHOf : 90.3 kJ + .5(0) kJ ---> 33.2 kJ
(33.2 kJ) - (90.3 + 0)kJ = -57.1 kJ
Find the overall rxn, CH3OH(l) + H2O(l) ---> CO2(g) + 3 H2(g),
from the given steps: H2(g) + CO(g) ---> CH3OH(l)
CO(g) + H2O(l) ---> CO2(g) + H2(g)
Calculate DHrxn for each step and find the overall DHrxn
1 CH3OH(l) ---> 2 H2(g) + 1 CO(g)
1 CO(g) + 1 H2O(l) ---> 1 CO2(g) + 1 H2(g)
DHf: -238.6 0 -110.5 DHrxn = 128.1 kJ DHf: -110.5 -285.8 -393.5 0 DHrxn = 2.8 kJ
1 CH3OH(l) + H2O(l) ---> CO2(g) + 3 H2(g) DHrxn = 130.9 kJ
Toss a ball upward. a. Does KE increase or decrease
b. As ball goes higher, want effect to PE
Definea. System b. Closed system c. Not part of system
Explaina. 1st Law b. Internal E c. How internal E of closed system increase
Decrease, KE converts to PE
Increases
Region of study w/ E changes exchange E not mass surroundings
E not created nor destroyed, changes formTotal E of system, KE + PESystem absorbs heat or work done on system
Calculate E of system, is endo- or exo- thermica. Balloon cooled, remove 0.655 kJ heat, shrinks, & atmosphere does 382 J work on
b. 100 g metal bar gains 25oC, absorbs 322 J of heat. Vol is constant
c. Surroundings do 1.44 kJ work compressing gas in perfectly insulated container
q “-” w “+” E =-0.655 kJ + 0.382 kJ = -0.273 kJ EXO
q “+” w = 0 E = +322 J ENDO
q = 0 (perfectly insulated) w “+” E = +1.44 kJ ENDO
Ca(OH)2(s) ----- CaO(s) + H2O(g)
Requires addition of 109 kJ of heat per mol of Ca(OH)2
a. Write balanced thermochemistry equation
b. Draw enthalpy diagram
Ca(OH)2(s) ----- CaO(s) + H2O(g) H = 109 kJ
CaO(s) + H2O(g)
Ca(OH)2(s)
H = 109 kJ