6. 6
PPPPooootttteeeennnnttttiiiiaaaallll &&&& KKKKiiiinnnneeeettttiiiicccc EEEEnnnneeeerrrrggggyyyy
Potential
energy ——
energy a
motionless
body has by
virtue of its
position.
7. 7
PPootteennttiiaall EEnneerrggyy
oonn tthhee AAttoommiicc SSccaallee
• Positive and
negative particles
(ions) attract one
another.
• Two atoms can
bond
• As the particles
attract they have a
lower potential
energy
NNaaCCll —— ccoommppoosseedd ooff
NNaa++ aanndd CCll-- iioonnss..
8. 8
PPootteennttiiaall EEnneerrggyy
oonn tthhee AAttoommiicc SSccaallee
• Positive and
negative particles
(ions) attract one
another.
• Two atoms can
bond
• As the particles
attract they have a
lower potential
energy
12. 12
IInntteerrnnaall EEnneerrggyy ((EE))
• PPEE ++ KKEE == IInntteerrnnaall eenneerrggyy ((EE oorr UU))
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
14. 14
TThheerrmmooddyynnaammiiccss
• Thermodynamics is the science of heat
(energy) transfer.
HHeeaatt eenneerrggyy iiss aassssoocciiaatteedd
wwiitthh mmoolleeccuullaarr mmoottiioonnss..
Heat transfers until thermal equilibrium is
established.
15. 15 DDiirreeccttiioonnaalliittyy ooff HHeeaatt TTrraannssffeerr
• Heat always transfer from hotter object to
cooler one.
• EXOthermic: heat transfers from SYSTEM to
SURROUNDINGS.
TT((ssyysstteemm)) ggooeess ddoowwnn
TT((ssuurrrr)) ggooeess uupp
16. 16 DDiirreeccttiioonnaalliittyy ooff HHeeaatt TTrraannssffeerr
• Heat always transfer from hotter object to
cooler one.
• ENDOthermic: heat transfers from
SURROUNDINGS to the SYSTEM.
TT((ssyysstteemm)) ggooeess uupp
TT ((ssuurrrr)) ggooeess ddoowwnn
20. 20
HHEEAATT CCAAPPAACCIITTYY
The heat required to raise an
object’s T by 1 ˚C.
WWhhiicchh hhaass tthhee llaarrggeerr hheeaatt ccaappaacciittyy??
21. SSSSppppeeeecccciiiiffffiiiicccc HHHHeeeeaaaatttt CCCCaaaappppaaaacccciiiittttyyyy 21
How much energy is
transferred due to T
difference?
The heat ((qq)) ““lost”” or ““gained””
is related to
a) sample mass
b) change in T and
c) specific heat capacity
Specific heat capacity =
heat lost or gained by substance (J)
(mass, g)(T change ,K)
27. 27
HHeeaatt TTrraannssffeerr aanndd
CChhaannggeess ooff SSttaattee
Requires energy
(heat).
This is the reason
a) you cool down after
swimming
b) you use water to put
out a fire.
+ energy
Liquid ---> Vapor
29. 29
HHHHeeeeaaaatttt &&&& CCCChhhhaaaannnnggggeeeessss ooooffff SSSSttttaaaatttteeee
What quantity of heat is required to
melt 500. g of ice and heat the
water to steam at 100 oC?
Heat of fusion of ice = 333 J/g
Specific heat of water = 4.2 J/g••K
Heat of vaporization = 2260 J/g
Heat of fusion of ice = 333 J/g
Specific heat of water = 4.2 J/g••K
Heat of vaporization = 2260 J/g
++333333 JJ//gg ++22226600 JJ//gg
30. 30
HHHHeeeeaaaatttt &&&& CCCChhhhaaaannnnggggeeeessss ooooffff SSSSttttaaaatttteeee
How much heat is required to melt 500. g of
ice and heat the water to steam at 100 oC?
1. To melt ice
q = (500. g)(333 J/g) = 1.67 x 105 J
2. To raise water from 0 oC to 100 oC
q = (500. g)(4.2 J/g••K)(100 - 0)K = 2.1 x
105 J
3. To evaporate water at 100 oC
q = (500. g)(2260 J/g) = 1.13 x 106 J
4. Total heat energy = 1.51 x 106 J =
1510 kJ
31. CChheemmiiccaall RReeaaccttiivviittyy 31
What drives chemical reactions? How do they
occur?
The first is answered by TTHHEERRMMOODDYYNNAAMMIICCSS
and the second by KKIINNEETTIICCSS.
Have already seen a number of ““driving
forces”” for reactions that are PRODUCT-FAVORED.
•• formation of a precipitate
•• gas formation
•• H2O formation (acid-base reaction)
•• electron transfer in a battery
32. CChheemmiiccaall RReeaaccttiivviittyy 32
But energy transfer also allows us to predict
reactivity.
IInn ggeenneerraall,, rreeaaccttiioonnss tthhaatt ttrraannssffeerr eenneerrggyy
ttoo tthheeiirr ssuurrrroouunnddiinnggss aarree pprroodduucctt--
ffaavvoorreedd..
So, let us consider heat transfer in chemical processes.
33. 33
HHeeaatt EEnneerrggyy TTrraannssffeerr iinn
aa PPhhyyssiiccaall PPrroocceessss
CO2 (s, -78 oC) ---> CO2 (g, -78 oC)
Heat transfers from surroundings to system in endothermic process.
34. 34
HHeeaatt EEnneerrggyy TTrraannssffeerr iinn
aa PPhhyyssiiccaall PPrroocceessss
• CO2 (s, -78 oC) --->
CO2 (g, -78 oC)
• A regular array of
molecules in a
solid
-----> gas phase
molecules.
• Gas molecules
have higher kinetic
energy.
40. 40
EENNTTHHAALLPPYY
ΔΔH = Hfinal - Hinitial
If Hfinal > Hinitial then ΔΔH is positive
Process is ENDOTHERMIC
If Hfinal > Hinitial then ΔΔH is positive
Process is ENDOTHERMIC
If Hfinal < Hinitial then ΔΔH is negative
If Hfinal < Hinitial then ΔΔH is negative
Process is EXOTHERMIC
Process is EXOTHERMIC
41. 41
UUSSIINNGG EENNTTHHAALLPPYY
Consider the formation of water
H2(g) + 1/2 O2(g) --> H2O(g) + 241.8
kJ
Exothermic reaction —— heat is a ““product””
and ΔΔH = –– 241.8 kJ
42. 42
UUSSIINNGG EENNTTHHAALLPPYY
Making liquid H2O from H2
+ O2 involves two
exothermic steps.
H2 + O2 gas
H2O vapor Liquid H2O
44. HHeessss’’ss LLaaww 44
&& EEnneerrggyy LLeevveell DDiiaaggrraammss
Forming H2O can occur in a
single step or in a two
steps.
ΔHtotal is the same no matter
which path is followed.
45. HHeessss’’ss LLaaww 45
&& EEnneerrggyy LLeevveell DDiiaaggrraammss
Forming CO2 can occur in a
single step or in a two steps.
ΔHtotal is the same no matter
which path is followed.
47. 47
SSttaannddaarrdd EEnntthhaallppyy VVaalluueess
Most ΔΔH values are labeled ΔΔHo
Measured under ssttaannddaarrdd ccoonnddiittiioonnss
P = 1 bar = 105 Pa = 1 atm /
1.01325 Concentration = 1
mol/L
T = usually 25 oC
with all species in standard states
e.g., C = graphite and O2 = gas
48. EEnntthhaallppyy VVaalluueess 48
Depend on how the reaction is written and on phases
of reactants and products
HH22((gg)) ++ 11//22 OO22((gg)) ---->> HH22OO((gg))
ΔΔHH˚ == --224422 kkJJ
22 HH22((gg)) ++ OO22((gg)) ---->> 22 HH22OO((gg))
ΔΔHH˚ == --448844 kkJJ
HH22OO((gg)) ------>> HH22((gg)) ++ 11//22 OO22((gg))
ΔΔHH˚ == ++224422 kkJJ
HH22((gg)) ++ 11//22 OO22((gg)) ---->> HH22OO((lliiqquuiidd))
ΔΔHH˚ == --228866 kkJJ
49. 49
SSttaannddaarrdd EEnntthhaallppyy VVaalluueess
NIST (Nat’’l Institute for Standards and
Technology) gives values of
ΔΔHHoo ff
== ssttaannddaarrdd mmoollaarr eenntthhaallppyy ooff
ffoorrmmaattiioonn
—— the enthalpy change when 1 mol of
compound is formed from elements
under standard conditions.
See Table 6.2
50. ΔΔHH oo,, ssttaannddaarrdd mmoollaarr
50 ff
eenntthhaallppyy ooff ffoorrmmaattiioonn
Enthalpy change when 1 mol of
compound is formed from the
corresponding elements under
standard conditions
HH22((gg)) ++ 11//22 OO22((gg)) ---->> HH22OO((gg))
ΔΔHHff
oo ((HH22OO,, gg))== --224411..88 kkJJ//mmooll
BByy ddeeffiinniittiioonn,,
ΔΔHHf
oo
== 00 ffoorr eelleemmeennttss iinn tthheeiirr ssttaannddaarrdd
ssttaatteess..
51. UUssiinngg SSttaannddaarrdd EEnntthhaallppyy VVaalluueess 51
Use ΔΔH˚’’s to calculate enthalpy change
for
H2O(g) + C(graphite) --> H2(g) + CO(g)
(product is called ““water gas””)