chem chapter 6_2

# chem chapter 6_2 - Lets Review what we know Because...

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Lets Review what we know: Because Enthalpy is a state function Δ H depends only on the initial and final states. The path by which you go from start to finish is irrelevant. H values for products and reactants can be found from tabulated Δ H f values - These are based on the zero point assignment on pure elements in their most stable state having enthalpy of zero. Δ H = H products – H reactants To find Δ H for a reaction, simply look up the Δ H f values and use the equation. - Be sure to include the reaction stoichiometry in your calculations!

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The Thermodynamic Tables The Δ f values in the tables are calculated at 25°C. Enthalpy is slightly temperature dependent – but if the temp range is within 50 to 100 °C we can assume temperature independence. Δ f : ° denotes compounds in Standard States Solids, liquids, Gases: Pure form at 1atm pressure Aqueous solutions: concentration of 1M vs carbon diamond Temp is specified in the tables – Δ H can be calculated at ANY temp value!!! The standard state of an element is the state that has a Δ f value of 0. Standard states in Thermo are NOT the same as STP for a gas. Carbon graphite
An example of a thermodynamic table

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Some Enthalpy questions 2 C 6 H 6(l) + 15 O 2(g) 12 CO 2(g) + 6 H 2 O (g) From The tables: C 6 H 6(l) Δ H f o = + 49.0 kJ/mol O 2(g) Δ H f o = 0 CO 2(g) Δ H f o = - 393.5 kJ/mol H 2 O (g) Δ H f o = - 241.8 kJ/mol What is the value of Δ H rx for the following reaction: Δ H Rxn = 6 Δ H f (H 2 O (g) ) + 12 Δ H f (CO 2 (s) ) 2 Δ H f (C 6 H 6 (l) ) 15 Δ H f (O 2 (g) ) = 6 (-241.8) + 12( -393.5) 2 (49) (0) = - 6.271 × 10 3 kJ Always remember to include your balanced chemical reaction coefficients in your enthalpy calculation!
P 4 O 10 (s) + 6H 2 O (l) 4H 3 PO 4 (aq) Δ H Rxn = 4 Δ H f (H 3 PO 4 (aq) ) Δ H f (P 4 O 10 (s) ) 6 Δ H f (H 2 O (l) ) = 4 ( -1288 KJ/mol) – ( -2984 KJ/mol ) 6 ( -286 KJ/mol ) = -452KJ Calculate the heat absorbed/released during the following reaction? Is this reaction endothermic or exothermic? Some Enthalpy questions This is an exothermic reaction. Always remember to include your balanced chemical reaction coefficients in your enthalpy calculation!

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Δ H RXN for the following? 2NO(g) + O 2 (g) 2NO 2 (g) = -114.14 KJ Is this reaction endothermic or exothermic? Exothermic, because the enthalpy value is negative. 2KClO 3 (s) 2KCl(s) + 3O 2 (g) = -90 KJ How much heat is given off by the decomposition of 0.5 moles of KClO 3 ? What about 0.1 moles of KClO
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## This note was uploaded on 09/20/2010 for the course CHEM 102 taught by Professor Peterpastos during the Summer '08 term at CUNY Hunter.

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chem chapter 6_2 - Lets Review what we know Because...

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