entropy & free energy a guide for a level students knockhardy publishing 2008 specifications

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

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either clicking on the grey arrows at the foot of each page

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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







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








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








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.








This must mean that energy has to be put in for the reaction to take place








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





G < 0 (- ive) Spontaneous reaction

G > 0 (+ ive) Non-spontaneous reaction(will be spontaneous in reverse direction)

G = 0 The system is in equilibrium





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’





Entropy increases when • solids melt

• liquids boil

• ionic solids dissolve in water

• the number of gas molecules increases

• the temperature increases





Entropy increases when SOLIDS MELT

Regular arrangement of particles in solids

Less regular arrangement; more disorder in liquids





Entropy increases when LIQUIDS BOIL

Irregular arrangement; some disorder in liquids

Random nature and disorder of particles in a gas





Entropy increases when IONIC SOLIDS DISSOLVE IN WATER

Regular arrangement in an ionic crystal lattice

Ions dissociate in water; there is less order





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.





Entropy increases when THE TEMPERATURE INCREASES

Lower temperature; less energy

Higher temperature; more energy and more disorder


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, 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°


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


(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


(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


G = H - TS


(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


G = H - TS


(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


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




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



b) NH3(g) + HCl(g) —> NH4Cl(s)

G = H - T 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

+


positiveG = H - T S


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



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


molecules

Endothermic


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

++


positive

+G = H - T S



d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)

G = H - T S



d) C6H12(l) + 9O2(g) —> 6CO2(g) + 6H2O(g)


molecules

COMBUSTIONis exothermic


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

++


positiveG = H - T S



e) C(s) + O2(g) —> CO2(g)

G = H - T S



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+


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.



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.



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

© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

ENTROPY & ENTROPY & FREE ENERGYFREE ENERGY

The EndThe End

entropy & free energy a guide for a level students knockhardy publishing 2008 specifications

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