ENTROPY &
FREE ENERGY
A guide for A level students
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS
ENTHALPY CHANGES
INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may
be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available
from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either
or
clicking on the grey arrows at the foot of each page
using the left and right arrow keys on the keyboard
ENTROPY
CONTENTS
• Spontaneous changes
• Free enthalpy and entropy
• Second Law of Thermodynamics
• Change of state
• Is a reaction spontaneous? - worked examples
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
ENDOTHERMIC
Energy is put in to
overcome the
electrostatic
attraction between
ions
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
ENDOTHERMIC
EXOTHERMIC
Energy is put in to
overcome the
electrostatic
attraction between
ions
Energy is released
as the ions are
attracted to polar
water molecules
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
ENDOTHERMIC
EXOTHERMIC
If the energy released when the ions dissolve is less than that put in to break up the lattice, the
overall process will be ENDOTHERMIC and the temperature of the solution will drop.
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
This must mean that energy has to be put in for the reaction to take place
FREE ENERGY & ENTROPY
SPONTANEOUS CHANGES
- occur in one particular direction and not the other
- take place without the need for work
Exothermic reactions are usually spontaneous
- this is because they go from higher to lower enthalpy
However ...
Why should reactions with a positive DH value take place spontaneously ?
(some salts dissolve readily in water and the solution gets colder, not hotter)
This must mean that energy has to be put in for the reaction to take place
The answer is that enthalpy change DH does not give the full story.
Free energy changes, DG, give a better picture.
FREE ENERGY & ENTROPY
FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative DG indicates a reaction capable of proceeding of its own accord
FREE ENERGY & ENTROPY
FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative DG indicates a reaction capable of proceeding of its own accord
DG < 0 (- ive)
DG > 0 (+ ive)
DG = 0
Spontaneous reaction
Non-spontaneous reaction
(will be spontaneous in reverse direction)
The system is in equilibrium
FREE ENERGY & ENTROPY
FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative DG indicates a reaction capable of proceeding of its own accord
DG < 0 (- ive)
DG > 0 (+ ive)
DG = 0
Spontaneous reaction
Non-spontaneous reaction
(will be spontaneous in reverse direction)
The system is in equilibrium
ENTROPY (S)
• Entropy (symbol S) is a measure of the disorder of a system
• The more the disorder, the greater the entropy
• If a system becomes more disordered, the value of DS is positive
• Values tend to be in JOULES - not kJ
DS = Sfinal - Sinitial
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
‘all chemical and physical changes
involve an overall increase in entropy’
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
‘all chemical and physical changes
involve an overall increase in entropy’
Entropy increases when • solids melt
•
•
•
•
liquids boil
ionic solids dissolve in water
the number of gas molecules increases
the temperature increases
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
Entropy increases when
‘all chemical and physical changes
involve an overall increase in entropy’
SOLIDS MELT
Regular arrangement
of particles in solids
Less regular arrangement;
more disorder in liquids
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
Entropy increases when
‘all chemical and physical changes
involve an overall increase in entropy’
LIQUIDS BOIL
Irregular arrangement;
some disorder in liquids
Random nature and disorder
of particles in a gas
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
Entropy increases when
‘all chemical and physical changes
involve an overall increase in entropy’
IONIC SOLIDS DISSOLVE IN WATER
Regular arrangement in
an ionic crystal lattice
Ions dissociate in water;
there is less order
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
Entropy increases when
‘all chemical and physical changes
involve an overall increase in entropy’
THE MOLES OF GAS INCREASE
Particles in gases move in a
random way. The more gas
molecules there are, the greater
the degree of randomness.
THE SECOND LAW
The Second Law of Thermodynamics is based on entropy and states that…
‘Entropy tends to a maximum’
This infers that...
Entropy increases when
‘all chemical and physical changes
involve an overall increase in entropy’
THE TEMPERATURE INCREASES
Lower temperature; less
energy
Higher temperature; more
energy and more disorder
FREE ENERGY & ENTROPY
Free energy, enthalpy and entropy are related ...
Special
case
DG = DH - TDS
For a reversible reaction at equilibrium the value of DG is zero
If
DG = ZERO
then
and
DH = T DS
DS = DH
T
This occurs during changes of state (melting, boiling etc)
FREE ENERGY & ENTROPY
Free energy, enthalpy and entropy are related ...
Special
case
DG = DH - TDS
For a reversible reaction at equilibrium the value of DG is zero
If
DG = ZERO
then
and
DH = T DS
DS = DH
T
Worked Example
Calculate the entropy change when water turns to steam at 100°C (373K).
The enthalpy of vaporisation of water is +44 kJ mol-1
DS
=
DH
T
=
+ 44 kJ mol -1
373 K
= + 118 J K -1 mol -1
(+ive as gases have more disorder)
Entropy change values are much smaller than enthalpy
change values; they tend to be in Joules rather than kJ
IS A REACTION SPONTANEOUS?
A reaction should be spontaneous if DG is negative, so ...
• Work out if it is exothermic (DH -ive) or endothermic (DH +ive)
• Is there an increase in disorder ?
If YES then DS will be positive.
• Is the temperature high or low ?
This can affect the value of TDS°
IS A REACTION SPONTANEOUS?
A reaction should be spontaneous if DG is negative, so ...
• Work out if it is exothermic (DH -ive) or endothermic (DH +ive)
• Is there an increase in disorder ?
If YES then DS will be positive.
• Is the temperature high or low ?
This can affect the value of TDS°
General examples
• If DH is –ive
and
DS is +ive
then
DG must be negative
• If DH is +ive
and
DS is -ive
then
DG must be positive
IS A REACTION SPONTANEOUS?
Specific examples
DG = DH - TDS
(1)
H2(g)
DH
DS
DG
+
F2(g)
- ive
0
- ive
—>
2HF(g)
highly exothermic process
same number of gas molecules
must be negative
(taking 0 away from a negative number)
IS A REACTION SPONTANEOUS?
Specific examples
DG = DH - TDS
(2)
Na+(g)
+
DH
DS
DG
- ive
- ive
- ive
Cl¯(g)
—>
NaCl(s)
highly exothermic (Lattice Enthalpy)
more order in a solid
mostly due to the high value of lattice enthalpy
IS A REACTION SPONTANEOUS?
Specific examples
DG = DH - TDS
(3)
NH4NO3(s) —> NH4+(aq) + NO3¯(aq)
DH
DS
DG
+ive
+ive
- ive
endothermic (the solution goes colder)
more disorder as lattice breaks up
if T is high or the value of DS is big enough
IS A REACTION SPONTANEOUS?
Specific examples - summary
(1)
(2)
(3)
H2(g)
+
F2(g)
DH
DS
DG
- ive
0
- ive
Na+(g)
+
DH
DS
DG
- ive
- ive
- ive
—>
2HF(g)
highly exothermic process
same number of gas molecules
must be negative (taking 0 away from a negative number)
Cl¯(g)
—>
NaCl(s)
highly exothermic (Lattice Enthalpy)
more order in a solid
mostly due to the high value of lattice enthalpy
NH4NO3(s) —> NH4+(aq) + NO3¯(aq)
DH
DS
DG
+ive
+ive
- ive
endothermic (the solution goes colder)
more disorder as lattice breaks up
if T is high or the value of DS is big enough
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reaction ?
Give reasons for your decision.
What is the sign of DG?
a)
CaCO3(s)
—> CaO(s)
+
CO2(g)
DG = DH -
T DS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reaction ?
Give reasons for your decision.
What is the sign of DG?
a)
CaCO3(s)
—> CaO(s)
+
CO2(g)
DG = DH +
Endothermic
T DS
+ +
increase in the
number of gas
molecules
Because DS is positive; TDS must be positive
DH is positive (very endothermic reaction)
For DG to be NEGATIVE, TDS must be bigger than DH
This is more likely with a higher temperature
The reaction is more likely to be spontaneous if heated
Temperature
is ALWAYS
positive
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reaction ?
Give reasons for your decision.
What is the sign of DG?
b)
NH3(g)
+
HCl(g)
—>
NH4Cl(s)
DG = DH -
T DS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reaction ?
Give reasons for your decision.
What is the sign of DG?
b)
NH3(g)
+
HCl(g)
—>
NH4Cl(s)
DG = DH -
T DS
Temperature
is ALWAYS
positive
+
Exothermic
decrease in
the number of
gas molecules
Because DS is negative; TDS must be negative
DH is negative
For DG to be negative TDS must be less negative than DH
This is more likely if the value of T is lower
The higher the temperature the less likely that the reaction will proceed
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
c)
Na(s)
—> Na(g)
(Equivalent to Enthalpy of Atomisation)
DG = DH -
T DS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
c)
Na(s)
—> Na(g)
(Equivalent to Enthalpy of Atomisation)
DG = DH +
Endothermic
T DS
+ +
increase in the
number of gas
molecules
Because DS is positive; TDS must be positive
DH is positive
For DG to be NEGATIVE, TDS must be bigger than DH
This is more likely with a higher temperature so the reaction
Solids are more likely to become gases if heated
Temperature
is ALWAYS
positive
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
d)
C6H12(l)
+
9O2(g)
—> 6CO2(g)
DG = DH -
+
6H2O(g)
T DS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
d)
C6H12(l)
+
9O2(g)
—> 6CO2(g)
DG = DH -
+
6H2O(g)
T DS
Temperature
is ALWAYS
positive
+ +
COMBUSTION
is exothermic
increase in the
number of gas
molecules
Because DS is positive; TDS must be positive
DH is negative
Taking a +ive value away from a –ive value must give a –ive value
DG must be NEGATIVE so the reaction is SPONTANEOUS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
e) C(s)
+ O2(g)
—> CO2(g)
DG = DH -
T DS
IS A REACTION SPONTANEOUS?
What is the sign of the entropy change in the following reactions ?
Give reasons for your decision.
What is the sign of DG?
e) C(s)
+ O2(g)
—> CO2(g)
DG = DH -
T DS
+
COMBUSTION
is exothermic
Temperature
is ALWAYS
positive
very small
no change in
the number of
gas molecules
DS is very small (could be –ive or +ive)
TDS will therefore not be a large number
DH will be have a relatively large negative value
Taking a small +ive value away from a –ive value must give a –ive value
DG must be NEGATIVE so the reaction is SPONTANEOUS
ANSWERS
What is the sign of the entropy change (DS) in the following reactions ?
Give reasons for your decision.
a) CaCO3(s) —> CaO(s) + CO2(g)
b) NH3(g) + HCl(g) —> NH4Cl(s)
c) Na(s) —> Na(g)
d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)
e) C(s) + O2(g)
—> CO2(g)
ANSWERS
What is the sign of the entropy change (DS) in the following reactions ?
Give reasons for your decision.
a) CaCO3(s) —> CaO(s) + CO2(g)
+
more gas molecules
b) NH3(g) + HCl(g) —> NH4Cl(s)
-
fewer gas molecules
c) Na(s) —> Na(g)
+
goes from solid to gas
d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)
+
more gas molecules
‘0’
‘similar’ moles of gas
e) C(s) + O2(g)
—> CO2(g)
ENTROPY &
FREE ENERGY
The End
© 2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING