Thermo dynamics

1. Thermodynamics The universe is in a state of constant change, the only invariant is Energy

2. Consider …. Gravity causes molecules of water move turbine blades turbines move coils of wire in magnetic fields moving magnetic fields move electrons moving electrons drive chemical reactions in a battery chemical reactions power your phone creating light and sound

3. Consider …. • In this example gravity is responsible for a working smartphone • Gravity does work on the blades • The turning blades do work on electrons • Electrons do work in chemical reactions • Chemical reactions do work on a speaker and power LEDs in the screen, power radios etc • The ability of something to do work on something else is transferred from gravity to water to electrons to chemical reactions to moving magnetics and moving air and light from the screen

4. Consider …. Gravity causes hydrogen atoms fuse to make Helium and a little bit of mass is converted into light and heat etc Photons are absorbed by chlorophyll and used to power photosynthesis We extract the oil make biofuel ignite the biofuel and excess energy released gives the molecules to power to move pistons

5. Consider …. • In this example Gravity causes fusion reactions in the sun are responsible for a working car • Mass is converted to light • light moves electrons • Electrons do work in chemical reactions to create sugars oils etc • Oils react with oxygen to create fast moving CO2 and H2O • Molecules push pistons and drive the car • The ability of something to do work on something else is transferred from the sun to chlorophyll to electrons to chemical reactions to moving molecules and moving pistons and moving wheels

6. Energy: the capacity to do work • In each step in the previous examples a capacity to do work is transferred from one thing to another, this is called energy • Gravitational energy is transferred into kinetic energy, into electrical energy, chemical energy, sound and light energy etc • All dynamic processes in the universe are due to the flow of energy • Thermodynamics is the study of heat flow and the laws that govern it • Since we want to understand chemical transformation we need to understand energy transformation

7. Energy • Energy is a universal invariant • It can change from one form to another but cannot be created or destroyed • It is measured in Joules (J) • There is potential energy (energy that something has because of where and what it is) and kinetic energy (the energy is has because of how fast it is moving) • The lower the potential energy the more stable something is. Potential energy can be negative • When some process happens, generally it is to lower the potential energy • The study of energy helps us to predict whether a process is spontaneous or not

8. What isThermodynamics? • Thermodynamics is a branch of physics concerned with energy flow. Historically it had an emphasis on heat, temperature and their relation to energy and work. • Study of energy changes accompanying chemical and physical changes to a system • Defines systems using a few macroscopic (measurable) variables, such as internal energy, entropy, temperature and pressure • Statistical treatment of microstates (atom positions and velocities) to obtain macrostates • In chemistry, thermodynamics predicts if reactions occur, how the equilibrium constant changes with temperature

9. First Law of Thermodynamics • you can’t get something for nothing • First Law of Thermodynamics: Energy cannot be Created or Destroyed • the total energy of the universe cannot change • though you can transfer it from one place to another • ΔEuniv = 0 = ΔEsys + ΔEsurr (1)

10. First Law of Thermodynamics Conservation of Energy • For an exothermic reaction, “lost” heat from the system goes into the surroundings • two ways energy “lost” from a system, • converted to heat, q • used to do work, w • Energy conservation requires that the internal energy E change in the system equal the heat released (q) + work done (w) ΔE = q + w (2) ΔE = ΔH + PΔV (3) • E is the total energy of everything in the system (the kinetic and potential energy of the atoms) • ΔE (ΔU)is a state function • internal energy change independent of how this change occurs

11. The first law and time reversal • The first law tells us that only processes where there is no net change in the total energy are allowed (energy is conserved)

12. The first law and spontaneity ✓ ✗ • In all observed phenomena the total energy is always the same • The energy at t, E(t) is equal to the energy at time time t+dt, • E(t) = E(t+dt) • So if that is the case why do we always see some processes only going one way?

13. The first law and spontaneity ✓ ✗ Clearly the first law isn’t the end of the story regarding energy and what happens in processes

14. Factors Affecting Whether a Reaction Is Spontaneous • It turns out that there are two factors that determine the thermodynamic favorability are the enthalpy H and the entropy S. • The enthalpy is a comparison of the bond energy of the reactants to the products. • bond energy = amount needed to break a bond. • statistical model of collective behavior • ΔH • The entropy factors relates to the randomness/orderliness of a system • ΔS • The enthalpy factor is generally more important than the entropy factor • Let’s look at these

15. Enthalpy • related to the internal energy E, the energy change measured at constant P is ΔH = ΔE + pΔV (generally kJ/mol) • ΔHrxnis related to the breaking and forming of chemical bonds. Stronger bonds = more stable molecules • if products more stable than reactants, energy released • exothermic • ΔH = negative • if reactants more stable than products, energy absorbed • endothermic • ΔH = positive • The enthalpy is favorable for exothermic reactions and unfavorable for endothermic reactions. • Hess’ Law

16. Spontaneity: Enthalpy Driven Processes • All transformations have accompanying energy changes. • Can we tell which transformations will occur spontaneously by studying the energy change? • In many cases, the direction of spontaneity can be determined by comparing the potential energy of the system at the start and the end • Cellulose and O2 have a bigger potential energy than the equivalent amount of carbon dioxide and water • The transformation lowers the overall potential energy, C-O and H-O bonds are more stable than C-C and C-H bonds • exothermic reactions are spontaneous • The extra energy leaves as heat

18. Spontaneity: Entropy Driven Processes • But some processes are spontaneous but not exothermic! • These are entropy driven processes

19. Entropy S • Entropy, S, is a thermodynamic function that increases as the number of equivalent ways of arranging the atoms/molecules (positions and velocities) in a system to give the appropriate V, U and T increases • S generally J/(K.mol) S = k lnW = Q/T (6) • k = Boltzmann Constant = 1.38 x 10-23 J/K • W is the number of energetically equivalent ways accessible, unitless (measure of our lack of knowledge about the system) • Entropy is the energy dispersal per unit temperature • Random systems require less energy than ordered systems • Measure of the unavailability of a system to do work

20. W Energetically Equivalent States for the Expansion of a Gas

21. These microstates all have the same macrostate So there are 6 different particle arrangements that result in the same macrostate This macrostate can be achieved through several different arrangements of the particles Macrostates → Microstates

22. Macrostates and Probability There is only one possible arrangement that gives State A and one that gives State B There are 6 possible arrangements that give State C Therefore State C has higher entropy than either State A or State B The macrostate with the highest entropy also has the greatest dispersal of energy

23. Changes in Entropy,ΔS • entropy change is favorable when the result is a more random system. • ΔS is positive • Some changes that increase the entropy are: • reactions whose products are in a more disordered state. • (solid > liquid > gas) • reactions which have larger numbers of product molecules than reactant molecules. • increase in temperature • solids dissociating into ions upon dissolving

24. Increases in Entropy

25. The 2nd Law of Thermodynamics: Spontaneity • "Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.” • The total entropy change of the universe must be positive for a process to be spontaneous • for reversible process ΔSuniv = 0, • for irreversible (spontaneous) process ΔSuniv > 0 ΔSuniv = ΔSsys + ΔSsurr (7) • if the entropy of the system decreases, then the entropy of the surroundings must increase by a larger amount • when ΔSsys is negative, ΔSsurr is positive • the increase in ΔSsurr often comes from the heat released in an exothermic reaction

26. Temperature Dependence ofΔSsurr • when a system process is exothermic, it adds heat to the surroundings, increasing the entropy of the surroundings • when a system process is endothermic, it takes heat from the surroundings, decreasing the entropy of the surroundings • the amount the entropy of the surroundings changes depends on the temperature it is at originally • the higher the original temperature, the less effect addition or removal of heat has

27. Gibbs Free Energy,ΔG • For a spontaneous process ΔSuniv > 0 • maximum amount of energy from the system available to do work on the surroundings at constant temperature T (9) • when ΔG < 0, there is a decrease in free energy of the system that is released into the surroundings; therefore a process will be spontaneous when ΔG is negative

28. Thermodynamics and SpontaneityFree Energy • spontaneity is determined by comparing the free energy G of the system before the reaction with the free energy of the system after reaction, it includes both the enthalpy and entropy change of a process ΔG = ΔH – T∙ΔS (9) • if the system after reaction has less free energy than before the reaction, the reaction is thermodynamically favorable • spontaneity ≠ fast or slow

29. Gibbs Free Energy,ΔG • process will be spontaneous when ΔG is negative • ΔG will be negative when • ΔH is negative and ΔS is positive • exothermic and more random • ΔH is negative and large and ΔS is negative but small • ΔH is positive but small and ΔS is positive and large • or high temperature • ΔG will be positive when ΔH is + and ΔS is − • never spontaneous at any temperature • when ΔG = 0 the reaction is at equilibrium

30. ΔG, ΔH, andΔS

31. Chemical Potential Energy The chemical potential – is a form of free energy used for chemical reactions, in spontaneous reactions the chemical potential decreases

32. Thermodynamics vs. Kinetics • Kinetics describes how fast things change • Thermodynamics is concerned if they will change and if so what changes we will see in internal energy, temperature, pressure etc

33. Example: Diamond → Graphite Graphite is more stable than diamond, so the conversion of diamond into graphite is spontaneous – but don’t worry, it’s so slow that your ring won’t turn into pencil lead in your lifetime (or through many of your generations).

34. Reversibility of Process • any spontaneous process is irreversible • it will proceed in only one direction • a reversible process will proceed back and forth between the two end conditions • equilibrium • results in no change in free energy • if a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction

35. Entropy Change and State Change Phase changes, melting boiling etc these are endothermic changes driven by entropy concerns not enthalpy concerns

36. Entropy Change in State Change • when materials change state, the number of macrostates it can have changes as well • for entropy: solid < liquid < gas • because the degrees of freedom of motion increases solid → liquid → gas

37. Heat Flow, Entropy, and the 2nd Law Heat must flow from water to ice in order for the entropy of the universe to increase But why that way round? The 1st law is not violated if more ice was formed? Flowing hot to cold we increase energy randomization. Heat flowing into the hot concentrated energy so S decreases

38. T, ΔH ΔS The reaction C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) has ΔHrxn = -2044 kJ at 25°C. Calculate the entropy change of the surroundings. Given: Find: ΔHsystem = -2044 kJ, T = 298 K ΔSsurroundings, J/K Concept Plan: Relationships: Solution: combustion is largely exothermic, so the entropy of the surrounding should increase significantly Check:

39. Free Energy Change and Spontaneity

40. T, ΔH, ΔS ΔG The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K at 25°C.Calculate ΔG and determine if it is spontaneous. Given: Find: ΔH = +95.7 kJ, ΔS = 142.2 J/K, T = 298 K ΔG, kJ Concept Plan: Relationships: Solution: Answer: Since ΔG is +, the reaction is not spontaneous at this temperature. To make it spontaneous, we need to increase the temperature.

41. ΔG, ΔH, ΔS T The reaction CCl4(g) C(s, graphite) + 2 Cl2(g) has ΔH = +95.7 kJ and ΔS = +142.2 J/K. Calculate the minimum temperature it will be spontaneous. Given: Find: ΔH = +95.7 kJ, ΔS = 142.2 J/K, ΔG < 0 T, K Concept Plan: Relationships: Solution: Answer: The temperature must be higher than 673K for the reaction to be spontaneous

42. The 3rd Law of ThermodynamicsAbsolute Entropy • the absolute entropy of a substance is the amount of energy it has due to dispersion of energy through its particles • the 3rd Law states that for a perfect crystal at absolute zero, the absolute entropy = 0 J/mol∙K • therefore, every substance that is not a perfect crystal at absolute zero has some energy from entropy • therefore, the absolute entropy of substances is always +

43. Standard Entropies • S° • Extensive (depends on the system size) • entropies for 1 mole at 298 K for a particular state, a particular allotrope, particular molecular complexity, a particular molar mass, and a particular degree of dissolution

45. Relative Standard EntropiesStates • the gas state has a larger entropy than the liquid state at a particular temperature • the liquid state has a larger entropy than the solid state at a particular temperature

46. Relative Standard EntropiesMolar Mass • the larger the molar mass, the larger the entropy • available energy states more closely spaced, allowing more dispersal of energy through the states

47. Relative Standard EntropiesAllotropes • the less constrained the structure of an allotrope is, the larger its entropy

48. Relative Standard EntropiesMolecular Complexity • larger, more complex molecules generally have larger entropy • more available energy states, allowing more dispersal of energy through the states

49. Relative Standard EntropiesDissolution • dissolved solids generally have larger entropy • distributing particles throughout the mixture

50. SoNH3, SoO2, SoNO, SoH2O, ΔS Calculate ΔS for the reaction4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(l) Given: Find: standard entropies look up in appendix to textbook or google ΔS, J/K Concept Plan: Relationships: Solution: Check: ΔS is +, as you would expect for a reaction with more gas product molecules than reactant molecules