ENTROPY & FREE ENERGYA guide for A level students

KNOCKHARDY PUBLISHING2015 SPECIFICATIONS

ENTHALPY CHANGESINTRODUCTIONThis 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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ENTROPYCONTENTS Spontaneous changes Free enthalpy and entropy Second Law of Thermodynamics Change of state Is a reaction spontaneous? - worked examples

FREE ENERGY & ENTROPYSPONTANEOUS 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 & ENTROPYSPONTANEOUS 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 & ENTROPYSPONTANEOUS 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 ionsEXOTHERMIC

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

FREE ENERGY & ENTROPYSPONTANEOUS 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)

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.ENDOTHERMIC

EXOTHERMIC

FREE ENERGY & ENTROPYSPONTANEOUS 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 & ENTROPYSPONTANEOUS 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)Spontaneous reactionDG > 0 (+ ive)Non-spontaneous reaction(will be spontaneous in reverse direction)DG = 0The 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)Spontaneous reactionDG > 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

THE SECOND LAWThe 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 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 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 solidsLess regular arrangement; more disorder in liquids

THE SECOND LAWThe 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 liquidsRandom nature and disorder of particles in a gas

THE SECOND LAWThe 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 latticeIons dissociate in water; there is less order

THE SECOND LAWThe 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 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 energyHigher temperature; more energy and more disorder

FREE ENERGY & ENTROPYFree energy, enthalpy and entropy are related ... DG = DH - TDS

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

If DG = ZERO then DH = T DS and DS = DH T

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

FREE ENERGY & ENTROPYFree energy, enthalpy and entropy are related ... DG = DH - TDS

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

If DG = ZERO then DH = T DS and DS = DH T

Worked ExampleCalculate the entropy change when water turns to steam at 100C (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 enthalpychange 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 andDS is +ivethenDG must be negative

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

IS A REACTION SPONTANEOUS?(1)H2(g) + F2(g) > 2HF(g)

DH - ivehighly exothermic processDS 0same number of gas moleculesDG - ivemust be negative(taking 0 away from a negative number)

Specific examplesDG = DH - TDS

IS A REACTION SPONTANEOUS?(2)Na+(g) + Cl(g) > NaCl(s)

DH- ivehighly exothermic (Lattice Enthalpy)DS - ivemore order in a solidDG - ive mostly due to the high value of lattice enthalpy

Specific examplesDG = DH - TDS

IS A REACTION SPONTANEOUS?(3)NH4NO3(s) > NH4+(aq) + NO3(aq)

DH +iveendothermic (the solution goes colder)DS +ivemore disorder as lattice breaks upDG - ive if T is high or the value of DS is big enoughSpecific examplesDG = DH - TDS

IS A REACTION SPONTANEOUS?(1)H2(g) + F2(g) > 2HF(g)

DH - ivehighly exothermic processDS 0same number of gas moleculesDG - 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 solidDG - 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 upDG - ive if T is high or the value of DS is big enoughSpecific examples - summary

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

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)increase in the number of gas moleculesEndothermicBecause DS is positive; TDS must be positiveDH is positive (very endothermic reaction)For DG to be NEGATIVE, TDS must be bigger than DHThis is more likely with a higher temperatureThe reaction is more likely to be spontaneous if heated+++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

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)

decrease in the number of gas moleculesExothermicBecause DS is negative; TDS must be negativeDH is negativeFor DG to be negative TDS must be less negative than DHThis is more likely if the value of T is lowerThe higher the temperature the less likely that the reaction will proceed +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?

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?

Na(s) > Na(g)(Equivalent to Enthalpy of Atomisation)increase in the number of gas moleculesEndothermicBecause DS is positive; TDS must be positiveDH is positiveFor DG to be NEGATIVE, TDS must be bigger than DHThis is more likely with a higher temperature so the reactionSolids are more likely to become gases if heated+++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?

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

d) C6H12(l) + 9O2(g) > 6CO2(g) + 6H2O(g)increase in the number of gas moleculesCOMBUSTIONis exothermicBecause DS is positive; TDS must be positiveDH is negativeTaking a +ive value away from a ive value must give a ive valueDG must be NEGATIVE so the reaction is SPONTANEOUS ++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

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)no change in the number of gas moleculesCOMBUSTIONis exothermicDS is very small (could be ive or +ive)TDS will therefore not be a large numberDH will be have a relatively large negative valueTaking a small +ive value away from a ive value must give a ive valueDG must be NEGATIVE so the reaction is SPONTANEOUS very small+DG = DH - T DS

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)

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)+-++0more gas moleculesfewer gas moleculesgoes from solid to gasmore gas moleculessimilar moles of gas

2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHINGENTROPY & FREE ENERGY

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