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ENTROPY & ENTROPY & FREE ENERGYFREE ENERGY
A guide for A level studentsA guide for A level students
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008
SPECIFICATIONSSPECIFICATIONS
ENTHALPY CHANGESENTHALPY 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 clicking on the grey arrows at the foot of each page
or using the left and right arrow keys on the keyboard
ENTROPYENTROPY
CONTENTS• Spontaneous changes
• Free enthalpy and entropy
• Second Law of Thermodynamics
• Change of state
• Is a reaction spontaneous? - worked examples
FREE ENERGY & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 H 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 & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 H 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
EXOTHERMIC
Energy is released as the ions are
attracted to polar water molecules
FREE ENERGY & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 H 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 & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 H 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 & ENTROPYFREE ENERGY & ENTROPY
SPONTANEOUS CHANGESSPONTANEOUS 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 H 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 Hdoes not give the full story.
Free energy changes, G, give a better picture.
FREE ENERGY & ENTROPY FREE ENERGY & ENTROPY
FREE ENERGY (G)FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative G indicates a reaction capable of proceeding of its own accord
FREE ENERGY & ENTROPY FREE ENERGY & ENTROPY
FREE ENERGY (G)FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative G indicates a reaction capable of proceeding of its own accord
G < 0 (- ive) Spontaneous reaction
G > 0 (+ ive) Non-spontaneous reaction(will be spontaneous in reverse direction)
G = 0 The system is in equilibrium
FREE ENERGY & ENTROPY FREE ENERGY & ENTROPY
FREE ENERGY (G)FREE ENERGY (G)
A reaction is only spontaneous if it can do work - it must generate free energy
A negative G indicates a reaction capable of proceeding of its own accord
G < 0 (- ive) Spontaneous reaction
G > 0 (+ ive) Non-spontaneous reaction(will be spontaneous in reverse direction)
G = 0 The system is in equilibrium
ENTROPY (S)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 S is positive• Values tend to be in JOULES - not kJ
S = Sfinal - Sinitial
THE SECOND LAWTHE 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 changesinvolve an overall increase in entropy’
THE SECOND LAWTHE 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 changesinvolve 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 LAWTHE 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 changesinvolve an overall increase in entropy’
Entropy increases when SOLIDS MELT
Regular arrangement of particles in solids
Less regular arrangement; more disorder in liquids
THE SECOND LAWTHE 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 changesinvolve an overall increase in entropy’
Entropy increases when LIQUIDS BOIL
Irregular arrangement; some disorder in liquids
Random nature and disorder of particles in a gas
THE SECOND LAWTHE 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 changesinvolve an overall increase in entropy’
Entropy increases when IONIC SOLIDS DISSOLVE IN WATER
Regular arrangement in an ionic crystal lattice
Ions dissociate in water; there is less order
THE SECOND LAWTHE 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 changesinvolve an overall increase in entropy’
Entropy increases when 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 LAWTHE 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 changesinvolve an overall increase in entropy’
Entropy increases when THE TEMPERATURE INCREASES
Lower temperature; less energy
Higher temperature; more energy and more disorder
FREE ENERGY & ENTROPYFREE ENERGY & ENTROPY
Free energy, enthalpy and entropy are related ... G = H - TS
Specialcase For a reversible reaction at equilibrium the value of G is zero
If G = ZERO thenH = T S
and S = H T
This occurs during changes of state (melting, boiling etc)
FREE ENERGY & ENTROPYFREE ENERGY & ENTROPY
Free energy, enthalpy and entropy are related ... G = H - TS
Specialcase For a reversible reaction at equilibriumreversible reaction at equilibrium the value of G is zero
If G = ZERO thenH = T S
and S = H T
Worked ExampleCalculate the entropy change when water turns to steam at 100°C (373K).The enthalpy of vaporisation of water is +44 kJ mol-1
S = H = + 44 kJ mol -1 = + 118 J K -1 mol -1
T 373 K (+ive as gases have more disorder)
Entropy change values are much smaller than enthalpychange values; they tend to be in Joules rather than kJ
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
A reaction should be spontaneous if A reaction should be spontaneous if G is negative, G is negative, so ...
• Work out if it is exothermic (H -ive) or endothermic (H +ive)
• Is there an increase in disorder ? If YES then S will be positive.
• Is the temperature high or low ? This can affect the value of TS°
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
A reaction should be spontaneous if A reaction should be spontaneous if G is negative, G is negative, so ...
• Work out if it is exothermic (H -ive) or endothermic (H +ive)
• Is there an increase in disorder ? If YES then S will be positive.
• Is the temperature high or low ? This can affect the value of TS°
General examples
• If H is –ive and S is +ive then G must be negative
• If H is +ive and S is -ive then G must be positive
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
(1)(1) H2(g) + F2(g) —> 2HF(g)
H - ive highly exothermic processS 0 same number of gas moleculesG - ive must be negative
(taking 0 away from a negative number)
Specific examplesSpecific examples
G = H - TS
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
(2)(2) Na+(g) + Cl¯(g) —> NaCl(s)
H - ive highly exothermic (Lattice Enthalpy)S - ive more order in a solidG - ive mostly due to the high value of lattice enthalpy
Specific examplesSpecific examples
G = H - TS
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
(3)(3) NH4NO3(s) —> NH4+(aq) + NO3¯(aq)
H +ive endothermic (the solution goes colder)S +ive more disorder as lattice breaks upG - ive if T is high or the value of S is big enough
Specific examplesSpecific examples
G = H - TS
IS A REACTION SPONTANEOUS?IS A REACTION SPONTANEOUS?
(1)(1) H2(g) + F2(g) —> 2HF(g)
H - ive highly exothermic processS 0 same number of gas moleculesG - ive must be negative (taking 0 away from a negative number)
(2) Na+(g) + Cl¯(g) —> NaCl(s)
H - ive highly exothermic (Lattice Enthalpy)S - ive more order in a solidG - ive mostly due to the high value of lattice enthalpy
(3)(3) NH4NO3(s) —> NH4+(aq) + NO3¯(aq)
H +ive endothermic (the solution goes colder)S +ive more disorder as lattice breaks upG - ive if T is high or the value of S is big enough
Specific examples - summarySpecific examples - summary
IS A REACTION SPONTANEOUS?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 G?
a) CaCO3(s) —> CaO(s) + CO2(g)
G = H - T S
IS A REACTION SPONTANEOUS?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 G?
a) CaCO3(s) —> CaO(s) + CO2(g)
increase in the number of gas
molecules
Endothermic
Because S is positive; TS must be positive
H is positive (very endothermic reaction)
ForG to be NEGATIVE, TS must be bigger than H
This is more likely with a higher temperature
The reaction is more likely to be spontaneous if heated
++
Temperature is ALWAYS
positive
+G = H - T S
IS A REACTION SPONTANEOUS?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 G?
b) NH3(g) + HCl(g) —> NH4Cl(s)
G = H - T S
IS A REACTION SPONTANEOUS?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 G?
b) NH3(g) + HCl(g) —> NH4Cl(s)
decrease in the number of gas molecules
Exothermic
Because S is negative; TS must be negative
H is negative
For G to be negative TS must be less negative than H
This is more likely if the value of T is lower
The higher the temperature the less likely that the reaction will proceed
+
Temperature is ALWAYS
positiveG = H - T S
IS A REACTION 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 G?
c) Na(s) —> Na(g) (Equivalent to Enthalpy of Atomisation)
G = H - T S
IS A REACTION 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 G?
c) Na(s) —> Na(g) (Equivalent to Enthalpy of Atomisation)
increase in the number of gas
molecules
Endothermic
Because S is positive; TS must be positive
H is positive
For G to be NEGATIVE, TS must be bigger than H
This is more likely with a higher temperature so the reaction
Solids are more likely to become gases if heated
++
Temperature is ALWAYS
positive
+G = H - T S
IS A REACTION 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 G?
d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)
G = H - T S
IS A REACTION 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 G?
d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)
increase in the number of gas
molecules
COMBUSTIONis exothermic
Because S is positive; TS must be positive
H is negative
Taking a +ive value away from a –ive value must give a –ive value
G must be NEGATIVE so the reaction is SPONTANEOUS
++
Temperature is ALWAYS
positiveG = H - T S
IS A REACTION 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 G?
e) C(s) + O2(g) —> CO2(g)
G = H - T S
IS A REACTION 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 G?
e) C(s) + O2(g) —> CO2(g)
no change in the number of gas molecules
COMBUSTIONis exothermic
S is very small (could be –ive or +ive)
TS will therefore not be a large number
H will be have a relatively large negative value
Taking a small +ive value away from a –ive value must give a –ive value
G must be NEGATIVE so the reaction is SPONTANEOUS
very small+
Temperature is ALWAYS
positiveG = H - T S
ANSWERSANSWERS
What is the sign of the entropy change (What is the sign of the entropy change (S) in the following reactions ?S) 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)
ANSWERSANSWERS
What is the sign of the entropy change (What is the sign of the entropy change (S) in the following reactions ?S) 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)
+
-
+
+
‘0’
more gas molecules
fewer gas molecules
goes from solid to gas
more gas molecules
‘similar’ moles of gas