What is a spontaneous reaction?

1
What is a spontaneous reaction?

• One, that given the necessary activation energy,
  proceeds without continuous outside assistance

2
Why do some reactions occur spontaneously
others do not?

• Atoms react to achieve greater stability
• Therefore products are generally more
  energetically stable than reactants
• In general, exothermic reactions (-DH) tend to
  proceed spontaneously

3
EXCEPTIONS

• Some endothermic reactions and those that produce
  less energetically stable products proceed
  spontaneously
• EXAMPLES
• Ba(OH)2(aq) 2 NH4NO3(aq) Ba(NO3)2(aq) 2
  NH4OH(l)
• NH4NO3(s) NH4 (aq) NO3 -(aq)

4
Entropy, S

• - a measure of the disorder of a system or the
  surroundings

5
Entropy of The Universe
6
The Universe
The Surroundings
The System
7

• 1st law of thermodynamics
• The total energy of the universe is constant
• (The best you can do is break even)
• 2nd law of thermodynamics
• The entropy of the universe is increasing
• (You cant break even)

8
Low entropy is less probable
9

• DSuniverse DSsystem DSsurroundings
• If DSuniverse gt 0, reaction is spontaneous
• If DSuniverse lt 0, reaction is nonspontaneous

10
How does the system impacts the DSsurr?
DH lt 0
Ssurr increases!
heat

• heat

11
Entropy is a State Function

• DS Sfinal - Sinitial
• path taken is irrelevant
• rate of change is irrelevant

12
DS gt 0 for

• - melting
• - vaporizing
• - making a solution
• - a reaction that produces
• an increased number of moles
• - heating a substance

13
H2O(s) H2O(l)

ordered, low S
DS gt 0
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DS gt 0 for

• - melting
• - vaporizing
• - making a solution
• - a reaction that produces
• an increased number of moles
• - heating a substance

15
H2O(l) H2O(g)
high entropy
low entropy
16
DS gt 0 for

• - melting
• - vaporizing
• - making a solution
• - a reaction that produces
• an increased number of moles
• - heating a substance

17

• low entropy
• high entropy

Benzene
Toluene
Very unlikely!
More likely!
18
DS gt 0 for

• - melting
• - vaporizing
• - making a solution
• - a reaction that produces
• an increased number of moles
• - heating a substance

19

• Ba(OH)28H2O(s) 2 NH4NO3(s)
• Ba(NO3)2(aq) 2 NH3(aq) 10 H2O(l)
• DH 80.3 kJ (unfavorable)
• 3 moles 13 moles
• DS gt 0 (favorable)

20
DS gt 0 for

• - melting
• - vaporizing
• - making a solution
• - a reaction that produces
• an increased number of moles
• - heating a substance

21
G
DSvaporization
Entropy
L
DSfusion
S
Temperature
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Entropy tends to increase

• In general, a system will increase in entropy (DS
  gt 0) if
• the volume of a gaseous system increases
• the temperature of a system increases
• the physical state of a system changes from solid
  to liquid to gas
• the number of moles in a system increases

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Calculating DS for a reaction

• DSrxn Snp Soproducts - Snr Soreactants

standard entropy in J/K i.e. (_at_ SATP)
stoichiometric coefficient
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for example,

• C8H18(g) 12.5 O2(g) 8 CO2(g) 9 H2O(g)
• 13.5 moles 17 moles
• (expect DS gt 0)
• DSrxn Sn Soproducts - Sn Soreactants

25
for example,

• 8(213.6) 9(188.6) 463.2 12.5(204.8)
• 383.0 J K-1 mol-1

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• Temperature and pressure are strongly connected
  to ideas of enthalpy and entropy. (Remember that
  -?H and ?S are favourable).
• Consider the following three examplesFor each
  reaction, identify the sign of ?H and ?S.
  Indicate whether the reaction is likely to be
  spontaneous.
• Zn (s) 2 HCl (aq) ? ZnCl2 (aq) H2 (g)
• 3 C (s) 3 H2 (g) ? C3H6 (g)
• 2 Pb(NO3)2 (s) ? 2 PbO (s) 4 NO2 (g) O2 (g)
• In a case where both ?H and ?S are favourable, we
  consider the reaction to be spontaneous and very
  likely to occur. What about in cases where only
  one is favoured?

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Gibbs Free Energy

• DSuniv DSsys DSsurr

-DHsys
and, DSsurr
T
-DHsys
thus, DSuniv DSsys
T
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now multiply through by -T
-TDSuniv -TDSsys DHsys
-TDSuniv DHsys -TDSsys
or,
or,
DGsys DHsys -TDSsys
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• DG DH -TDS

Gibbs energy change or the free energy change
30
DG and spontaneity

• recall that DGsys -TDSuniv
• since DSuniv gt 0 for a spontaneous change,
• DGsys lt 0 for a spontaneous change

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Whats free about free energy?
the energy used up creating disorder
the free energy left over

• DG DH -TDS

the energy transferred as heat
32
When is DG lt 0?

• DHo DSo DGo Spontaneous?
• - - always
• - never
• - - or - at lower T
• or - at higher T

33
G is a state function

• DG Gfinal - Ginitial
• path is irrelevant
• rate of reaction is irrelevant

34
How do we find DG values?

• 1. Calculate DH, DS values, then use DG DH -
  TDS
• 2. Look up DGof values

35
for example,

• Will this reaction proceed at 25oC?
• 4 KClO3(s) 3 KClO4(s) KCl(s)

36
4 KClO3(s) 3 KClO4(s) KCl(s)

• DHrxn SnpHoproducts - SnrHoreactants
• 3 DHof (KClO4(s)) DHof (KCl (s))
• - 4 DHof (KClO3(s))
• 3(-432.8) (-436.7) - 4(-397.7)
• -144.3 kJ mol-1

37
4 KClO3(s) 3 KClO4(s) KCl(s)

• DSrxn Snp Soproducts - Snr Soreactants
• 3 So(KClO4(s)) So(KCl (s)) - 4 So(KClO3(s))
• 3(151.0) (82.6) - 4(143.1)
• - 36.8 J K-1 mol-1

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4 KClO3(s) 3 KClO4(s) KCl(s)

• DG DH - TDS
• -144.3 kJ mol-1 - 298 K (-0.0368 kJ K-1mol-1)
• -133.3 kJ mol-1

DG lt 0, thus reaction proceeds spontaneously
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4 KClO3(s) 3 KClO4(s) KCl(s)

• DG DH - TDS
• -144.3 kJ mol-1 - 298 K (-0.0368 kJ K-1mol-1)
• -133.3 kJ mol-1

25oC
N.B. conversion to kJ!
40
How do we find DG values?

• 1. Calculate DH, DS values, then use DG DH -
  TDS
• 2. Look up DGof values (standard free energies of
  formation)

41
4 KClO3(s) 3 KClO4(s) KCl(s)

• DGrxn SnpGoproducts) - Snr Goreactants
• 3 Go(KClO4(s)) Go (KCl(s))
• - 4 Go (KClO3(s))
• 3(-303.2) (-409.2) - 4(-296.3)
• - 133.6 kJ

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