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To consider the heat (enthalpy) of chemical reactions How do we describe the heat generated or absorbed by chemical reac

Objectives. To consider the heat (enthalpy) of chemical reactions How do we describe the heat generated or absorbed by chemical reactions? To understand Hess’s Law Energy as a state function. A. Thermochemistry (Enthalpy) . Enthalpy, H

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To consider the heat (enthalpy) of chemical reactions How do we describe the heat generated or absorbed by chemical reac

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  1. Objectives • To consider the heat (enthalpy) of chemical reactions • How do we describe the heat generated or absorbed by chemical reactions? • To understand Hess’s Law • Energy as a state function

  2. A. Thermochemistry (Enthalpy) • Enthalpy, H • H is equal to the energy that flows as heat (at constant pressure) • H = q (at constant pressure) Enthalpy change for a reaction is usually measured in kJ and is the heat flow associated with the completion of a “whole” reaction: Fe2O3(s) + 2Al (s) → 2Fe (l) + Al2O3(s) H = -852kJ Exothermic reactions

  3. H Calculations For the reaction below, what is H when the following substances take part? 4Fe (s) + 3O2(g) →2Fe2O3(s) H = -1652k 4 moles of Fe 3 moles of O2 1 mole of Fe 28 g of Fe 48 g of O2 117 g of Fe2O3

  4. A. Thermochemistry (Enthalpy) • Heat of Formation Hf is the enthalpy of formation of onemole of a substance from its constituent elements • 2Na + Cl2 2NaCl H =  822kJ/mol • Na + 1/2Cl2 NaCl Hf =  411kJ/mol NaCl Formation

  5. A. Thermochemistry • Calorimetry • Change in Enthalpy, ΔH is measured using a simple calorimeter. • Change in Enthalpy is specific heat times change in temperature times mass • ΔH = S.m.ΔT

  6. Calorimetry Bomb Calorimeter

  7. B. Hess’s Law • For a particular reaction, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps. Would you expect that? Germain Hess 1802 - 1850 Example: N2(g) + 2O2(g) 2NO2(g)H1 = 68 kJ N2(g) + O2(g) 2NO(g) H2 = 180 kJ 2NO(g) + O2(g)  2NO2(g)H3 = -112 kJ ---------------------------------------- ------------------

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