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Starter. 1) A -> B D H + B->A D H + or -? 2) What happens to the equilibrium if temperature is increased? 3) What happens to Kc if the temperature is increased?. Free energy. L.O.: Predict which chemical reactions are spontaneous.

jessica-farmer
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  1. Starter 1) A -> B DH + B->A DH + or -? 2) What happens to the equilibrium if temperature is increased? 3) What happens to Kc if the temperature is increased?

  2. Free energy L.O.: Predict which chemical reactions are spontaneous. Calculate the temperature at which some reactions become feasible.

  3. The free energy change DG is the balance between enthalpy, entropy and temperature for a process.

  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 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

  6. 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

  7. 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

  8. 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)

  9. 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

  10. 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)

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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)

  23. 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

  24. 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

  25. Working out the temperature at which a reaction becomes feasible.

  26. Because the free energy change ∆G depends on T∆S, the effect of entropy becomes more important at higher temperatures. If ∆H is –ve and ∆S is positive, the reaction will be spontaneous at all temperatures. If ∆H is +ve and ∆S is negative, the reaction will not be spontaneous at any temperature. However if ∆H and ∆S are both +ve, the reaction will be spontaneous only above a certain temperature. If ∆H and ∆S are both –ve, the reaction will be spontaneous only below a certain temperature. The critical temperature is the temperature at which ∆G = 0, so ∆H = T∆S. T = ∆H/∆S

  27. L.O.: • Explain that the tendency of a process to take place depends on temperature, T, the entropy change in the system, ∆S, and the enthalpy change, ∆H, with the surroundings. • Explain that the balance between entropy and enthalpy change is the free energy change, ∆G. • State and use the relationship ∆G = ∆H – T∆S. • Explain how endothermic reactions are able to take place spontaneously.

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