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

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Presentation on theme: "ENTROPY & FREE ENERGY A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS."— Presentation transcript:

1 ENTROPY & FREE ENERGY A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

2 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... 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

3 ENTROPY CONTENTS Spontaneous changes Free enthalpy and entropy Second Law of Thermodynamics Change of state Is a reaction spontaneous? - worked examples

4 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

5 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  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

6 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  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

7 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  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.

8 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  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

9 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  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  H  does not give the full story. Free energy changes,  G, give a better picture.

10 FREE ENERGY & ENTROPY 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

11 FREE ENERGY & ENTROPY 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

12 FREE ENERGY & ENTROPY 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 (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 = S final - S initial

13 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’

14 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

15 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

16 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

17 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

18 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.

19 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

20 FREE ENERGY & ENTROPY Free energy, enthalpy and entropy are related...  G =  H - T  S Special case For a reversible reaction at equilibrium the value of  G is zero If  G = ZERO then  H =  T  S and  S =  H T This occurs during changes of state (melting, boiling etc)

21 FREE ENERGY & ENTROPY Free energy, enthalpy and entropy are related...  G =  H - T  S Special reversible reaction at equilibrium case For a reversible reaction at equilibrium the value of  G is zero If  G = ZERO then  H =  T  S and  S =  H 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  S =  H = + 44 kJ mol -1 = 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

22 IS A REACTION SPONTANEOUS? A reaction should be spontaneous if  G is negative, A reaction should be spontaneous if  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 T  S°

23 IS A REACTION SPONTANEOUS? A reaction should be spontaneous if  G is negative, A reaction should be spontaneous if  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 T  S° General examples If  H is –ive and  S is +ivethen  G must be negative If  H is +ive and  S is -ive then  G must be positive

24 IS A REACTION SPONTANEOUS? (1) (1)H 2 (g) + F 2 (g) —> 2HF(g)  H - ivehighly exothermic process  S 0same number of gas molecules  G - ivemust be negative (taking 0 away from a negative number) Specific examples  G =  H - T  S

25 IS A REACTION SPONTANEOUS? (2) (2)Na + (g) + C l ¯(g) —> NaC l (s)  H- ivehighly exothermic (Lattice Enthalpy)  S - ivemore order in a solid  G - ive mostly due to the high value of lattice enthalpy Specific examples  G =  H - T  S

26 IS A REACTION SPONTANEOUS? (3) (3)NH 4 NO 3 (s) —> NH 4 + (aq) + NO 3 ¯(aq)  H +iveendothermic (the solution goes colder)  S +ivemore disorder as lattice breaks up  G - ive if T is high or the value of  S is big enough Specific examples  G =  H - T  S

27 IS A REACTION SPONTANEOUS? (1) (1)H 2 (g) + F 2 (g) —> 2HF(g)  H - ivehighly exothermic process  S 0same number of gas molecules  G - ivemust be negative (taking 0 away from a negative number) (2)Na + (g) + C l ¯(g) —> NaC l (s)  H- ivehighly exothermic (Lattice Enthalpy)  S - ivemore order in a solid  G - ive mostly due to the high value of lattice enthalpy (3) (3)NH 4 NO 3 (s) —> NH 4 + (aq) + NO 3 ¯(aq)  H +iveendothermic (the solution goes colder)  S +ivemore disorder as lattice breaks up  G - ive if T is high or the value of  S is big enough Specific examples - summary

28 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)CaCO 3 (s) —> CaO(s) + CO 2 (g)  G =  H - T  S

29 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)CaCO 3 (s) —> CaO(s) + CO 2 (g) increase in the number of gas molecules Endothermic Because  S is positive; T  S must be positive  H is positive (very endothermic reaction) For  G to be NEGATIVE, T  S 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

30 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)NH 3 (g) + HC l (g) —> NH 4 C l (s)  G =  H - T  S

31 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)NH 3 (g) + HC l (g) —> NH 4 C l (s) decrease in the number of gas molecules Exothermic Because  S is negative; T  S must be negative  H is negative For  G to be negative T  S 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 positive  G =  H - T  S

32 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

33 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; T  S must be positive  H is positive For  G to be NEGATIVE, T  S 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

34 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) C 6 H 12 (l) + 9O 2 (g) —> 6CO 2 (g) + 6H 2 O(g)  G =  H - T  S

35 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) C 6 H 12 (l) + 9O 2 (g) —> 6CO 2 (g) + 6H 2 O(g) increase in the number of gas molecules COMBUSTION is exothermic Because  S is positive; T  S 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 positive  G =  H - T  S

36 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) + O 2 (g) —> CO 2 (g)  G =  H - T  S

37 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) + O 2 (g) —> CO 2 (g) no change in the number of gas molecules COMBUSTION is exothermic  S is very small (could be –ive or +ive) T  S 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 positive  G =  H - T  S

38 ANSWERS What is the sign of the entropy change (  S) in the following reactions ? What is the sign of the entropy change (  S) in the following reactions ? Give reasons for your decision. a) CaCO 3 (s) —> CaO(s) + CO 2 (g) b) NH 3 (g) + HC l (g) —> NH 4 C l (s) c) Na(s) —> Na(g) d) C 6 H 12 (l) + 9O 2 (g) —> 6CO 2 (g) + 6H 2 O(g) e) C(s) + O 2 (g) —> CO 2 (g)

39 ANSWERS What is the sign of the entropy change (  S) in the following reactions ? What is the sign of the entropy change (  S) in the following reactions ? Give reasons for your decision. a) CaCO 3 (s) —> CaO(s) + CO 2 (g) b) NH 3 (g) + HC l (g) —> NH 4 C l (s) c) Na(s) —> Na(g) d) C 6 H 12 (l) + 9O 2 (g) —> 6CO 2 (g) + 6H 2 O(g) e) C(s) + O 2 (g) —> CO 2 (g) ‘0’ more gas molecules fewer gas molecules goes from solid to gas more gas molecules ‘similar’ moles of gas

40 © 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING ENTROPY & FREE ENERGY The End


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