CHEMICAL THERMODYNAMICS (SPONTANEITY AND ENTROPY)
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CHEMICAL THERMODYNAMICS * Spontaneity and Entropy * Second Law of Thermodynamics * Gibbs Free Energy and Chemical Equilibrium
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CHEMICAL THERMODYNAMICS (SPONTANEITY AND ENTROPY)
- 3. Type of process which does not need the application of energy to take place. A spontaneous process is a physical or chemical change that occurs by itself
- 4. Type of process which needs the use of energy to make it happen. It is one that occurs without outside intervention.
- 5. RUSTING SPONTANEOUS EXOTHERMIC PROCESS ICE MELTING SPONTANEOUS ENDOTHERMIC PROCESS REMOVAL OF SUGAR FROM A CUP OF COFFEE THIS IS A NON SPONTANEOUS PROCESS
- 7. TERMS
- 9. Molar entropy of the solid state will be LOWEST Molar entropy of the liquid state will be INTERMIDIATE Molar entropy of the gaseous state will be HIGHEST
- 10. ΔS = S(final state) – S(initial state)
- 11. ΔS = SH O – SH O2 (l)2 (g) Just like ΔH and ΔU, a positive value of ΔS means an increase in entropy while negative value means decrease in entropy
- 12. ΔS = qrev T
- 14. Sample Problem: Predict whether the entropy or disorderliness of the system where the following changes occur is expected to increase or decrease. (p. 136) KClO3 (s) -> KCl (s) + O2 (g) 2 H2 (g) + O2 (g) -> 2H2 O Determine if the entropy change will be positive or negative for the following reactions: 1) (NH4)2Cr2O7(s) → Cr2O3(s) + 4 H2O(l) + CO2(g) 2) 2 H2(g) + O2(g) → 2 H2O(g) 3) PCl5 (g)→ PCl3 (l) + Cl2(g)
- 15. One Mole of water is vaporized at 100°C with the absorption of 40.7 kJ of heat. What is the molar entropy of vaporization? Given: T= 100°C (373 K) ΔH/ qrev = 40.7 kJ of heat (40,700 J/mol) Note (conversions): Formula: 1 J/mol = 0.001 kj/mol 1 kj/mol = 1000 J/mol Solution: ∆𝑺 = 40,700 𝐽/𝑚𝑜𝑙 373𝐾 Answer: ΔS = 109 J/mol.K
- 16. The second law of thermodynamics states that the total entropy of the universe ALWAYS INCREASES FOR A SPONTANEOUS PROCESS. It is mathematically stated as, ΔSuniverse = ΔSsystem+ ΔSsurroundings ≥ 0 Entropy change of the UNIVERSE is determined from the sum of entropy change of the system and surroundings. (IT MUST BE A POSITIVE VALUE – which corresponds to an increase in entropy)
- 17. THIS EXPLAINS WHY EXOTHERMIC AND ENDOTHERMIC REACTIONS MAY BE SPONTANEOUS. In ENDOTHERMIC REACTIONS The disorder within the system increases sufficiently so that the change in disorder of the system is greater than the decrease of the surroundings, then the total change in disorder can be positive and the reaction becomes spontaneous. In EXOTHERMIC REACTIONS The heat released by the reaction increases the disorder of the surroundings of the system is decreased, the large amount of heat released to the surroundings causes large increase in entropy resulting to a net increase in entropy of the universe.
- 18. The exact entropy change for the process can be determined in the same way as the enthalpy change for the reaction using standard absolute entropies instead of enthalpies of formation. For any chemical reaction: ΔS°reaction = ΔS°products - ΔS°reactants Standard Absolute Entropies of Some Substances at 25°C and 1 atm SUBSTANCE S° (J/mol-K) H2(g) 130.68 O2(g) 205.14 N2(g) 191.61 Cl2(g) 223.07 C(s, graphite) 5.740 C (s, diamond) 2.377 H2O(g) 188.83 H2O(l) 69.91 CO2(g) 213.74 CO(g) 197.67 HCl(g) 186.91 NH3(g) 192.45 NO(g) 210.76 NO2(g) 240.06 SO2(g) 248.22 SO3(g) 256.76 CH4(g) 186.26
- 19. Exercise (Sample Problem) Calculate the change in entropy at standard state and 25°C that accompanies the reaction: 1. C(s, graphite) + CO2(g) 2CO(g) 2. 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)
- 20. Exercise (Sample Problem) Calculate the change in entropy at standard state and 25°C that accompanies the reaction: #2 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g Given: S°NH3 = 193 J/K·mol S°O2 = 205 J/K·mol S°NO = 211 J/K·mol S°H2O = 189 J/K·mol Solution: ΔS°reaction = (4 S°NO + 6 S°H2O) - (4 S°NH3 + 5 S°O2) ΔS°reaction = (4(211 J/K·K) + 6(189 J/K·mol)) - (4(193 J/K·mol) + 5(205 J/K·mol)) ΔS°reaction = (844 J/K·K + 1134 J/K·mol) - (772 J/K·mol + 1025 J/K·mol) ΔS°reaction = 1978 J/K·mol - 1797 J/K·mol) ΔS°reaction = 181 J/K·mol (Note, in this type of problem you'll either be given the molar entropy values of the reactants and products or you'll need to look them up in a table.) Formula: ΔS°reaction = ΔS°products - ΔS°reactants
- 22. A criterion for spontaneity that applies only to the system. It combines enthalpy and entropy. G = H – TS is a measure of the spontaneity of a process and of the amount of energy that can be obtained from it. It is determined from difference in values of two different states. ΔG = Gfinal – Ginitial The sign ΔG is used as the basis to determine whether a process is spontaneous.
- 24. GIBBS FREE ENERGY, ENTHALPY, AND ENTROPY The GIBBS FREE ENERGY CHANGE can be determined from the GIBBS EQUATION ΔG = ΔH – TΔS by using change in enthalpy and change in entropy at constant temperature and pressure as shown by the equation. Free energy and Spontaneity of a Reaction - Just like other thermodynamic functions, the free energy change for a reaction can be calculated by using the standard free energy of formation in the equation ∆𝐆° 𝒓𝒆𝒂𝒄𝒕𝒊𝒐𝒏 = 𝚺 ∆𝐆° 𝒇,𝒑𝒓𝒐𝒅𝒖𝒕𝒔 − 𝚺 ∆𝐆° 𝒕,𝒓𝒆𝒂𝒄𝒕𝒂𝒏𝒕𝒔
- 25. Exercise (Sample Problem) SAMPLE PROBLEM: Determine ΔG° of the following reaction using: (A) ΣΔG°, and (B) Gibbs equation. (C) is the reaction spontaneous at 25°C? (D) Is it spontaneous at 2000K? Standard Thermodynamic Values Reactants Products CO H2O CO2 H2 ΔH°f,25° (KJ/mol) -110.5 -241.83 -393.5 ---- S25°C (J/mol K) 197.5 188.72 213.7 130.6 ΔG°f,25° (KJ/mol) -137.2 -228.6 -394.4 ----
- 26. A. ΔG°reaction = (ΔG°f,products) - (ΔG°t,reactants) = ΔG°f, CO2 + ΔG°f, H2 – [ΔG°f, CO + ΔG°f, H2O] = -394.4 + 0 – [-137.2 – 228.6] = -28.6 kJ Standard Thermodynamic Values Reactants Products CO H2O CO2 H2 ΔH°f,25° (KJ/mol) -110.5 -241.83 -393.5 ---- S25°C (J/mol K) 197.5 188.72 213.7 130.6 ΔG°f,25° (KJ/mol) -137.2 -228.6 -394.4 ----
- 27. (B) ΔH°reaction = ΔH°f, CO2 + ΔH°f, H2 – [ΔH°f, CO + ΔH°f, H2O] = -393.5 + 0 – [-110.5 – 241.83] = - 41.17 kJ ΔS°reaction = ΔS°CO2 + ΔS°H2 – [ΔS°CO + ΔS° H2O] = 213.7 + 130.6 – [197.5+188.72] = - 41.92 kJ ΔG°reaction = ΔH°reaction – TΔS° reaction = - 41.17 kJ – [298 K x (-0.04192 kJ/K)] = - 28.68 kJ Standard Thermodynamic Values Reactants Products CO H2O CO2 H2 ΔH°f,25° (KJ/mol) -110.5 -241.83 -393.5 ---- S25°C (J/mol K) 197.5 188.72 213.7 130.6 ΔG°f,25° (KJ/mol) -137.2 -228.6 -394.4 ----
- 28. Exercise (Sample Problem) SAMPLE PROBLEM: Determine ΔG° of the following reaction using: (A) ΣΔG°, and (B) Gibbs equation. (C) is the reaction spontaneous at 25°C? (D) Is it spontaneous at 2000K? (D)ΔG°reaction = ΔH°reaction – TΔS° reaction = - 41.17 kJ – [298 K x (-0.04192 kJ/K)] = - 28.68 kJ (C) Since ΔG°reaction is negative at 298 K. The reaction is spontaneous.
- 29. https://www.thoughtco.com/entropy-change-problem-609481 https://www.thoughtco.com/entropy-of-reaction-example-problem-609483 https://www.youtube.com/watch?v=YM-uykVfq_E&t=4s http://web.mit.edu/10.213/oldpages/f99/supp/readings/qdh/index.html https://www.quora.com/What-does-q-rev-mean-in-the-definition-of-entropy Ilao et al., (2017). General Chemistry 2; First Edition. Rex Book Store
Notes de l'éditeur
- Examples include: A rock at the top of a hill rolls down. Heat flows from a hot object to a cold one. An iron object rusts in moist air. These processes occur without requiring an outside force and continue until equilibrium is reached. What makes a process spontaneous and what drives the reaction in one direction or another? The driving force for an spontaneous process is an increase in its degree of randomness which is called entropy
- endothermic process - describes a process or reaction in which the system absorbs energy from its surroundings; usually, but not always, in the form of heat. exothermic process – opposite of endothermic. It is a process that releases, or "gives out" energy in the form of heat. Why is Exothermic considered as Spontaneous and not Non Spontaneous? Exothermic reactions are spontaneous because the product created by an exothermic reaction is the most stable product. Exothermic reactions release heat or energy. The more energy something has, the more unstable it will be. Therefore kinetics determines that the most stable product will dominate and as a result exothermic reactions are spontaneous to produce a stable product. EXOTHERMIC REACTION, THE ENTHALPY CHANGE HAS A POSITIVE VALUE: ΔH > 0 Other examples of exothermic a burning candle nuclear fission respiration rain forming from water vapor in clouds any combustion reaction a neutralization reaction rusting of iron (rust steel wool with vinegar) reaction between sodium sulfite and bleach (dilute sodium hypochlorite) reaction between sodium and chlorine to make sodium chloride (table salt) reaction between water and any strong acid dissolving laundry detergent in water freezing water into ice cubes snow forming inside clouds burning sugar Why is Endothermic considered as Spontaneous? The reaction is spontaneous because its Gibbs free energy is lower after the reaction than before; the entropy at the end of the process is greater than the enthalpy. The reaction takes heat from the water, so it is endothermic.\ other examples of endothermic melting ice cubes melting solid salts evaporating liquid water converting frost to water vapor (melting, boiling, and evaporation, in general, are endothermic processes) making an anhydrous salt from a hydrate forming a cation from an atom in the gas phase splitting a gas molecule separating ion pairs cooking an egg baking bread
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate.
- standard molar entropy: actual entropy content of one mole of a substance in a standard state.
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- What does q(rev) mean in the definition of entropy? change in entropy may be defined as the amount of heat added/removed from the system reversibly at a temperature T. Thus, dS=dQ(rev)/T Note that this is valid only for internally reversible process. In reality, all processes are irreversible. Also, it can be proved from the state function property of entropy that dS(irr)>dS(rev). What is reversible and irreversible process During the thermodynamic process, one or more of the properties of the system like temperature, pressure, volume, enthalpy or heat, entropy, etc. changes. The second law of thermodynamics enables us to classify all the processes under two main categories: reversible or ideal processes and irreversible or natural processes. What is REVERSIBLE PROCESS The process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamics properties of the universe is called a reversible process. If the system can be restored from state B to state A, and there is no change in the universe, then the process is said to be a reversible process. The reversible process can be reversed completely and there is no trace left to show that the system had undergone thermodynamic change. What is IRREVERSIBLE PROCESS https://www.youtube.com/watch?v=YoekFxOizj4
- Synthesis combination or composition, in particular.(combination, union, amalgam, blend, mixture, compound, fusion, composite, alloy;More) the production of chemical compounds by reaction from simpler materials. Decomposition Decomposition is the process by which organic substances are broken down into simpler matter.
- Positive Value of entropy means an increase in entropy Negative Value of entropy means an decrease in entropy
- Why kelvin? Absolute zero. In 1848, Kelvin used this as a basis for an absolute temperature scale. He defined "absolute" as the temperature at which molecules would stop moving, or "infinite cold." From absolute zero, he used the same unit as Celsius to determine the increments. Why j/mol? Because entropy uses Joules and not Kilojoules in its SI unit so we need to convert it first to get an accurate answer. Why delta H and not qrev/Qrev? Because H = Q derived from the first law of thermodynamics
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- COMBUSTION
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- in thermodynamics, a state function or function of state is afunction defined for a system relating several state variables or state quantities that depends only on the current equilibrium state of the system. State functions do not depend on the path by which the system arrived at its presentstate. This is not yet the formula in determining entropy. It is just a simple way of showing what kind of change occurred in a phase change (whetherthe final entropy increased or decreased)
- COMBUSTION