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Entropy Changes in Chemical Reactions.

Entropy Changes in Chemical Reactions. Because entropy is a state function, the property is what it is regardless of pathway, the entropy change for a given reaction can be calculated by taking the difference between the standard entropy values of products and those of the reactants.

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Entropy Changes in Chemical Reactions.

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  1. Entropy Changes in Chemical Reactions. • Because entropy is a state function, the property is what it is regardless of pathway, the entropy change for a given reaction can be calculated by taking the difference between the standard entropy values of products and those of the reactants. • DSoreaction= SnpDSoproducts - SnrDSoreactants

  2. Entropy Changes in Chemical Reactions. • Calculating DSo. • Calculate DSo at 25oC for the reaction 2NiS(s) + 3O2(g)  2SO2(g) + 2NiO(s)

  3. Entropy Changes II Calculate DSo for the reaction of aluminum oxide by hydrogen gas: Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)

  4. Free Energy and Chemical Reactions • Standard Free Energy. • Standard free energy (DGo) is the change in the free energy that will occur if the reactants in their standard states are converted to the products in their standard states. • The value of DGo tells nothing about the rate of a reaction, only its eventual equilibrium position.

  5. Free Energy and Chemical Reactions • Calculating DGo as a State Function. • DGo= DHo - TDSo • Consider the reaction 2 SO2(g) + O2(g)  2SO3(g) • carried out at 25o C and 1 atm. Calculate DHo and DSo, then calculate DGo.

  6. Free Energy and Chemical Reactions • Calculating DGo as a State Function. • Solving DGo Using Hess’s Law. • Using the following data (at 25o C) • Cdiamond(s) + O2(g)  CO2(g) DGo = -397 kJ • Cgraphite(s) + O2(g)  CO2(g) DGo = -394 kJ • Calculate DGo for the reaction Cdiamond(s) Cgraphite(s)

  7. Free Energy and Chemical Reactions • Calculating DGo as a State Function. • Standard Free Energy of Formation (DGfo). • DGo= SnpDGfoproducts - SnrDGforeactants • Methanol is a high-octane fuel used in high-performance racing engines. Calculate DGo for the reaction 2CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(g)

  8. Free Energy and Chemical Reactions • Calculating DGo as a State Function. • A chemical engineer wants to determine the feasibility of making ethanol (C2H5OH) by reacting water with the ethylene (C2H4) according to the equation C2H4(g) + H2O(l)  C2H5OH(l) Is the reaction spontaneous under standard conditions?

  9. The Dependence of Free Energy on Pressure • The equilibrium position represents the lowest free energy value available to a particular reaction. • Free energy changes throughout the course of a reaction because it is pressure and concentration dependent. • For any 1 mole of a gas at a given temperature • S large V > S small V or S low P > S high P

  10. The Dependence of Free Energy on Pressure • This leads to the equation DG = DGo + RT ln(Q) where Q is the reaction quotient, T is the Kelvin temperature, R is the ideal gas constant 8.31 J/molK, DGo is the free energy at 1 atm, and DG is the free energy at a specified temperature. This derivation of the equation has been removed from the equations sheet. There is, however, another derivation that is on the sheet.

  11. The Dependence of Free Energy on Pressure • One method for synthesizing methanol (CH3OH) involves reacting carbon monoxide and hydrogen gases: • CO(g) + 2H2(g)  CH3OH(l) • Calculate DGoat 25oC for this reaction where carbon monoxide gas at 5.0 atm and hydrogen gas at 3.0 atm are converted to liquid methanol.

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