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Published byAnis Gregory Modified over 9 years ago
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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. S o reaction = n p S o products - n r S o reactants
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Calculating S o. Calculate S o at 25 o C for the reaction 2NiS(s) + 3O 2 (g) 2SO 2 (g) + 2NiO(s)
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Calculate S o for the reaction of aluminum oxide by hydrogen gas: Al 2 O 3 (s) + 3H 2 (g) 2Al(s) + 3H 2 O(g)
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Standard free energy ( G o ) 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 G o tells nothing about the rate of a reaction, only its eventual equilibrium position.
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Calculating G o as a State Function. G o = H o - T S o Consider the reaction 2 SO 2 (g) + O 2 (g) 2SO 3 (g) carried out at 25 o C and 1 atm. Calculate H o and S o, then calculate G o.
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Calculating G o as a State Function. Solving G o Using Hess’s Law. Using the following data (at 25 o C) C diamond (s) + O 2 (g) CO 2 (g) G o = -397 kJ C graphite (s) + O 2 (g) CO 2 (g) G o = -394 kJ Calculate G o for the reaction C diamond (s) C graphite (s)
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Calculating G o as a State Function. Standard Free Energy of Formation ( G f o ). G o = n p G f o products - n r G f o reactants Methanol is a high-octane fuel used in high- performance racing engines. Calculate G o for the reaction 2CH 3 OH(g) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(g)
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A chemical engineer wants to determine the feasibility of making ethanol (C 2 H 5 OH) by reacting water with the ethylene (C 2 H 4 ) according to the equation C 2 H 4 (g) + H 2 O(l) C 2 H 5 OH(l) Is the reaction spontaneous under standard conditions?
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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
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This leads to the equation G = G o + RT ln(Q) where Q is the reaction quotient, T is the Kelvin temperature, R is the ideal gas constant 8.31 J/mol K, G o is the free energy at 1 atm, and G 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.
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One method for synthesizing methanol (CH 3 OH) involves reacting carbon monoxide and hydrogen gases: CO(g) + 2H 2 (g) CH 3 OH(l) Calculate G at 25 o C 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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