Chemical Thermodynamics

1. Chemical Thermodynamics the study of Reaction Feasibility

2. Reaction Feasibility • Thermodynamics is concerned with questions such as: why do some reactions take place while others don’t? can we predict whether or not a reaction will occur? under what conditions will a reaction occur? • Increasingly we consider all reactions to be reversible, but, under certain conditions the reaction will be more likely to go in one direction than the other • In one direction the reaction will be spontaneous while the other direction will be non-spontaneous

3. Spontaneous Exothermic Most spontaneous reactions are exothermic But not ALL !

4. (NH4)2CO3 + 2CH3COOH 2NH4CH3COO + CO2 + H2O Spontaneous Endothermic • solid ammonium carbonate reacts with conc. ethanoic acid

5. Spontaneous Processes • H2O(s) ⇋ H2O(l) Ice turning to water is spontaneous at T > 0°C, Water turning to ice is spontaneous at T < 0°C. • Both Exothermic & Endothermic processes can be spontaneous • The direction of a spontaneous process will depend on temperature

6. Energy Rules! The key to understanding Thermodynamics is the appreciation of the ways in which energy interacts with matter. Energy doesn’t just determine the speed at which particles move (Temperature) it is part of everything that affects particles.

7. Energy Rules! Some Processes involve no change in temperature but a major change in the energy of particles is still occurring.

8. Entropy These changes in the energy of particles have an overall affect on the level of disorder shown by a substance. The disorder in a substance is known as its ENTROPY, S . The Third Law of Thermodynamics provides a reference against which Entropies can be measured. “the Entropy of a perfect crystal at 0 K is zero”

9. Entropy - State Molecular Motion Translation Rotation Vibration free to vibrate no freedom to move no freedom to rotate restricted freedom to move some freedom to rotate free to vibrate total freedom to move total freedom to rotate free to vibrate

10. Entropy of Fusion Entropy - Temperature Entropy (S) Entropy of Vaporisation Temperature

11. Entropy - Dissolving Less Randomness More Randomness Less Entropy More Entropy

12. Fewer Vibrations More Vibrations More Vibrations NO2 N2O4 NO Less Entropy More Entropy More Entropy Entropy - Molecules

13. Entropy - Numbers Less Randomness More Randomness Less Entropy More Entropy

14. Entropy - Mixtures Less Randomness More Randomness Less Entropy More Entropy

15. Entropy Values

16. Entropy Calculations Similar to a previous formula: ∆So = ∑ Soproducts - ∑ Soreactants

17. (NH4)2CO3 + 2CH3COOH 2NH4CH3COO + CO2 + H2O The trend solidsliquids gases is associated with an increase in disorder . Entropy Changes, ∆S • spontaneous endothermic reactions tend to have certain characteristics in common • the number of moles of product are greater than the number of moles of reactant • a large proportion of the products are either liquids or gases • reactants are often solids or liquids

18. Entropy - The Answer? Is an Increase In Entropy the driving force behind a spontaneous chemical reaction ? Both ∆S = +ve & ∆S = -ve processes can be spontaneous The direction of a spontaneous process will depend on temperature A spontaneous process will depend on both ∆S and ∆H

19. Entropy - The Answer? The ‘problem’ can be resolved if we take into account changes taking place in the Surroundings. The driving force behind a spontaneous process turns out to be an Overall Increase In Entropy

20. Entropy - The Answer? Water freezing leads to a decrease in entropy within the system. Being Exothermic, however, leads to an increase in entropy in the surroundings Water freezing is a spontaneous process whenever there is an Overall Increase In Entropy

21. Entropy - The Answer? Being Endothermic leads to a decrease in entropy in the surroundings There will have to be an increase in entropy within the system. Water melting is a spontaneous process whenever there is an Overall Increase In Entropy

22. Measuring ∆Sosurr Trying to Calculate the effect on the surroundings would appear, at first, an impossible task. Where do the surroundings start & finish? What is the entropy of air? Glass? etc. How many moles of ‘surroundings’ are there? Fortunately it is much, much simpler than that.

23. Measuring ∆Sosurr Firstly the change in Entropy of the Surroundings is caused by the Enthalpy change of the Surroundings, and……. ∆Hosurr = -∆Hosyst so ∆Sosurr∝ -∆Hosyst Temperature has an inverse effect. For example, energy released into the surroundings has less effect on the entropy of the surroundings, the hotter the surroundings are.

24. ∆Sosurr = -∆Hosyst T ∆Sototal = ∆Sosurr -∆Hosyst T Measuring ∆Sosurr In fact, it turns out that .. It is the Overall Entropy Change that must be considered. ∆Sototal = ∆Sosyst + ∆Sosurr

25. 0 = ∆Sosurr -∆Hosyst T Measuring ∆Sototal We are interested in the point at which the Total Entropy becomes a positive value (ceases being a negative value). We can ‘solve’ for ∆Stotal= 0 Multiplying throughout byT gives us 0 = T∆Sosyst -∆Hosyst

26. Measuring ∆Sototal Remember that this is really the formula for ∆Stotal ∆Sototal = T∆Sosyst -∆Hosyst Armed with ∆S , ∆Hand values forT we can calculate the overall change in Entropy and a positive value would be necessary for a spontaneous reaction. However, for reasons that are beyond this Topic, a term called the Gibbs Free Energy, G, is preferred. A negative value for ∆Gis equivalent to a positive value for ∆S. This requires a slight adjustment in our final formula.

27. Gibbs Free Energy ∆Go ∆Go= ∆Hosyst - T∆Sosyst The convenient thing about this expression is that it allows us to do calculations using only values that can be directly measured or easily calculated. Strictly speaking, the Second Law of Thermodynamics states that Entropy must increase for a Spontaneous Process. In practice, the Second Law of Thermodynamics means that Gibbs Free Energy must decrease for a Spontaneous Process.

28. Gibbs Free Energy ∆Go

29. Calculating ∆Go Fe2O3 + 3 CO ➝ 2 Fe + 3 CO2 ∆Ho = ∑ ∆Hfoproducts - ∑ ∆Hforeactants ∆So = ∑ Soproducts - ∑ Soreactants T in Kelvin ∆Go= ∆Ho - T∆So

30. ∆Go of Formation ∆Go = ∑ ∆Gfoproducts - ∑ ∆Gforeactants The ∆G of a reaction can be calculated from ∆Gf values. By themselves, they give useful information about relative stabilities.

31. Ellingham Diagrams y= c + mx ∆Go= ∆Ho - T∆So

32. Reversible Reactions For a Chemical Reaction ∆Go= ∆Ho - T∆So If ∆G is negative for one direction, it must be positive for the reverse. This implies that only one reaction can proceed (spontaneously) under a given set of conditions. However, ∆S calculations are based on 100% Reactant & 100% Product. In reality, mixtures exist, so larger ∆S values will be obtained than those calculated.

33. Reversible Reactions

34. Equilibrium position ∆G for 100% Reac ➝ 100% Prod is positive but 100% Reac ➝ mixture is negative so forward reaction can take place. ∆G for 100% Prod ➝ 100% Reac and 100% Prod ➝ mixture is more negative so backward reaction is favoured Equilibrium lies over to the left but only slightly since value of ∆Gis positive but relatively small

35. Equilibrium position ∆G for 100% Reac ➝ 100% Prod is very positive but 100% Reac ➝ mixture is still slightlynegative so forward reaction can take place. ∆G for 100% Prod ➝ 100% Reac and 100% Prod ➝ mixture is more negative so backward reaction is favoured Equilibrium lies well over to the left since value of ∆Gis positive and relatively large

36. Equilibrium position ∆G for 100% Reac ➝ 100% Prod is very negative but 100% Prod ➝ mixture is still slightlynegative so backward reaction can take place. ∆G for 100% Reac ➝ 100% Prod and 100% Reac ➝ mixture is more negative so forward reaction is favoured Equilibrium lies well over to the right since value of ∆Gis negative and relatively large

37. Equilibrium position

38. Equilibrium Position There is a mathematical relationship: ∆Go= - RT ln K More simply:

39. Thermodynamic Limits Thermodynamics can predict whether a reaction is feasible or not. Thermodynamics can predict the conditions necessary for a reaction to be feasible. Thermodynamics can predict the position of equilibrium Thermodynamics cannot predict how fast a reaction might be.

40. Kinetics Coming Soon to a Board Near YOU!