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ENTROPY & FREE ENERGY A guide for A level students. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. ENTHALPY CHANGES. INTRODUCTION

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ENTROPY & FREE ENERGY

A guide for A level students

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING


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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 clicking on the grey arrows at the foot of each page

orusing the left and right arrow keys on the keyboard


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ENTROPY

  • CONTENTS

  • Spontaneous changes

  • Free enthalpy and entropy

  • Second Law of Thermodynamics

  • Change of state

  • Is a reaction spontaneous? - worked examples


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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


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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


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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

EXOTHERMIC

Energy is released as the ions are attracted to polar water molecules


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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.


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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


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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 DHdoes not give the full story.

Free energy changes, DG, give a better picture.


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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


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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)Spontaneous reaction

DG > 0 (+ ive)Non-spontaneous reaction

(will be spontaneous in reverse direction)

DG = 0The system is in equilibrium


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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)Spontaneous reaction

DG > 0 (+ ive)Non-spontaneous reaction

(will be spontaneous in reverse direction)

DG = 0The 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


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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’


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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


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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

Regular arrangement of particles in solids

Less regular arrangement; more disorder in liquids


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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 LIQUIDS BOIL

Irregular arrangement; some disorder in liquids

Random nature and disorder of particles in a gas


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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 IONIC SOLIDS DISSOLVE IN WATER

Regular arrangement in an ionic crystal lattice

Ions dissociate in water; there is less order


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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 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.


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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 THE TEMPERATURE INCREASES

Lower temperature; less energy

Higher temperature; more energy and more disorder


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FREE ENERGY & ENTROPY

Free energy, enthalpy and entropy are related ... DG = DH - TDS

Special

case For a reversible reaction at equilibrium the value of DG is zero

If DG = ZERO thenDH = T DS

and DS = DH

T

This occurs during changes of state (melting, boiling etc)


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FREE ENERGY & ENTROPY

Free energy, enthalpy and entropy are related ... DG = DH - TDS

Special

case For a reversible reaction at equilibrium the value of DG is zero

If DG = ZERO then DH = T DS

and 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 = + 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 enthalpy

change values; they tend to be in Joules rather than kJ


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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°


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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 andDS is +ivethenDG must be negative

• If DH is +ive and DS is -ive thenDG must be positive


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IS A REACTION SPONTANEOUS?

Specific examples

DG = DH - TDS

(1)H2(g) + F2(g) —> 2HF(g)

DH - ivehighly exothermic process

DS 0same number of gas molecules

DG - ivemust be negative

(taking 0 away from a negative number)


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IS A REACTION SPONTANEOUS?

Specific examples

DG = DH - TDS

(2)Na+(g) + Cl¯(g) —> NaCl(s)

DH- ivehighly exothermic (Lattice Enthalpy)

DS - ivemore order in a solid

DG - ive mostly due to the high value of lattice enthalpy


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IS A REACTION SPONTANEOUS?

Specific examples

DG = DH - TDS

(3)NH4NO3(s) —> NH4+(aq) + NO3¯(aq)

DH +iveendothermic (the solution goes colder)

DS +ivemore disorder as lattice breaks up

DG - iveif T is high or the value of DS is big enough


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IS A REACTION SPONTANEOUS?

Specific examples - summary

(1)H2(g) + F2(g) —> 2HF(g)

DH - ivehighly exothermic process

DS 0same number of gas molecules

DG - ivemust be negative (taking 0 away from a negative number)

(2)Na+(g) + Cl¯(g) —> NaCl(s)

DH- ivehighly exothermic (Lattice Enthalpy)

DS - ivemore order in a solid

DG - ive mostly due to the high value of lattice enthalpy

(3)NH4NO3(s) —> NH4+(aq) + NO3¯(aq)

DH +iveendothermic (the solution goes colder)

DS +ivemore disorder as lattice breaks up

DG - iveif T is high or the value of DS is big enough


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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?

  • CaCO3(s) —> CaO(s) + CO2(g)

DG = DH - T DS


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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?

  • CaCO3(s) —> CaO(s) + CO2(g)

DG = DH - T DS

+

+

+

Endothermic

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


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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


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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

+

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


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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?

  • Na(s) —> Na(g)(Equivalent to Enthalpy of Atomisation)

DG = DH - T DS


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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?

  • Na(s) —> Na(g)(Equivalent to Enthalpy of Atomisation)

DG = DH - T DS

+

+

+

Endothermic

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


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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) + 6H2O(g)

DG = DH - T DS


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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) + 6H2O(g)

DG = DH - T DS

+

+

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


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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


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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?

e) C(s) + O2(g) —> CO2(g)

DG = DH - T DS

+

very small

COMBUSTION

is exothermic

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


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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)


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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)

+

-

+

+

‘0’

more gas molecules

fewer gas molecules

goes from solid to gas

more gas molecules

‘similar’ moles of gas


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ENTROPY & FREE ENERGY

The End

© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING


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