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

ENTHALPY CHANGES www.knockhardy.org.uk/sci.htm 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

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

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

Energy is put in to overcome the electrostatic attraction between ions 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

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

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.

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

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 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

DG < 0 (- ive) Spontaneous reaction 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 = 0 The system is in equilibrium

DG < 0 (- ive) Spontaneous reaction 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 = 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 DS is positive • Values tend to be in JOULES - not kJ DS = Sfinal - Sinitial

‘Entropy tends to a maximum’ 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 tends to a maximum’ 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

‘Entropy tends to a maximum’ 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

‘Entropy tends to a maximum’ 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

‘Entropy tends to a maximum’ 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

‘Entropy tends to a maximum’ 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.

‘Entropy tends to a maximum’ 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

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

and DS = DH T 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

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 and DS is +ive then DG must be negative • If DH is +ive and DS is -ive then DG must be positive

IS A REACTION SPONTANEOUS? Specific examples DG = DH - TDS (1) H2(g) + F2(g) —> 2HF(g) DH - ive highly exothermic process DS 0 same number of gas molecules DG - ive must be negative (taking 0 away from a negative number)

IS A REACTION SPONTANEOUS? Specific examples DG = DH - TDS (2) Na+(g) + Cl¯(g) —> NaCl(s) DH - ive highly exothermic (Lattice Enthalpy) DS - ive more order in a solid DG - ive mostly due to the high value of lattice enthalpy

IS A REACTION SPONTANEOUS? Specific examples DG = DH - TDS (3) NH4NO3(s) —> NH4+(aq) + NO3¯(aq) DH +ive endothermic (the solution goes colder) DS +ive more disorder as lattice breaks up DG - ive if T is high or the value of DS is big enough

IS A REACTION SPONTANEOUS? Specific examples - summary (1) H2(g) + F2(g) —> 2HF(g) DH - ive highly exothermic process DS 0 same number of gas molecules DG - ive must be negative (taking 0 away from a negative number) (2) Na+(g) + Cl¯(g) —> NaCl(s) DH - ive highly exothermic (Lattice Enthalpy) DS - ive more order in a solid DG - ive mostly due to the high value of lattice enthalpy (3) NH4NO3(s) —> NH4+(aq) + NO3¯(aq) DH +ive endothermic (the solution goes colder) DS +ive more disorder as lattice breaks up DG - ive if T is high or the value of DS is big enough

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) Temperature is ALWAYS positive 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

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) Temperature is ALWAYS positive 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

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) Temperature is ALWAYS positive 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

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) Temperature is ALWAYS positive 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

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) Temperature is ALWAYS positive 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

Give reasons for your decision. 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)

Give reasons for your decision. 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 ‘similar’ moles of gas

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