25 Calorimetry If two reactants at the same temperature are mixed and the

25 calorimetry if two reactants at the same

This preview shows page 25 - 33 out of 33 pages.

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Calorimetry If two reactants at the same temperature are mixed and the resulting solution gets warmer, this means the reaction taking place is exothermic. An endothermic reaction cools the solution. 26
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Calorimetry Energy released (heat) = s × m × Δ T s = specific heat capacity (J/ ° C·g) m = mass (g) Δ T = change in temperature ( ° C) 27
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28 Q.: A 100.0 g sample of water at 90.°C is added to a 500.0 g sample of water at 10.°C. Calculate the final temperature of the water: q lost + q gained = 0 q = s × m × Δ T 4.18(100)(T f - 90) + 4.18(500)(T f - 10)=0 T f = 23 ° C
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Hess’s Law: In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This idea is called Hess’s law. 29
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N 2 ( g ) + 2O 2 ( g ) → 2NO 2 ( g ) Δ H 1 = 68 kJ This reaction also can be carried out in two distinct steps, with enthalpy changes designated by Δ H 2 and Δ H 3 . N 2 ( g ) + O 2 ( g ) → 2NO( g ) Δ H 2 = 180 kJ 2NO( g ) + O 2 ( g ) → 2NO 2 ( g ) Δ H 3 = 112 kJ N 2 ( g ) + 2O 2 ( g ) → 2NO 2 ( g ) Δ H 2 + Δ H 3 = 68 kJ Δ H 1 = Δ H 2 + Δ H 3 = 68 kJ 30
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The Principle of Hess’s Law 31 The same change in enthalpy occurs when nitrogen and oxygen react to form nitrogen dioxide, regardless of whether the reaction occurs in one (red) or two (blue) steps
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Characteristics of Enthalpy Changes If a reaction is reversed, the sign of Δ H is also reversed. The magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of Δ H is multiplied by the same integer. 32
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Standard Enthalpy of Formation ( Δ H f ° ) Change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states. 33
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