Section 10.3 Energy and Chemical Reactions 1.To consider the heat (enthalpy) of chemical reactions –How do we describe the heat generated or absorbed by.

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Section 10.3 Energy and Chemical Reactions 1.To consider the heat (enthalpy) of chemical reactions –How do we describe the heat generated or absorbed by chemical reactions? 2.To understand Hess’s Law –Energy as a state function Objectives

Section 10.3 Energy and Chemical Reactions 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: Fe 2 O 3 (s) + 2Al (s) → 2Fe (l) + Al 2 O 3 (s)  H = -852kJ Exothermic reactions

Section 10.3 Energy and Chemical Reactions  H Calculations For the reaction below, what is  H when the following substances take part? 4Fe (s) + 3O 2 (g) →2Fe 2 O 3 (s)  H = -1652k 4 moles of Fe 3 moles of O 2 1 mole of Fe 28 g of Fe 48 g of O g of Fe 2 O 3

Section 10.3 Energy and Chemical Reactions A. Thermochemistry (Enthalpy) Heat of Formation  H f is the enthalpy of formation of one mole of a substance from its constituent elements 2Na + Cl 2 2NaCl  H =  822kJ/mol Na + 1/2Cl 2 NaCl  H f =  411kJ/mol NaCl Formation

Section 10.3 Energy and Chemical Reactions 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 A. Thermochemistry Calorimetry

Section 10.3 Energy and Chemical Reactions Calorimetry Bomb Calorimeter

Section 10.3 Energy and Chemical Reactions B. Hess’s Law State function –Property of the system that is independent of path –Is distance travelled a state function?

Section 10.3 Energy and Chemical Reactions B. Hess’s Law –Is altitude a state function?

Section 10.3 Energy and Chemical Reactions Is temperature a state function?

Section 10.3 Energy and Chemical Reactions B. Hess’s Law –Are these state functions? Altitude Temperature Energy When a ball rolls down a hill and comes to rest its energy is dependent only on where it is positioned, not how much work it did or heat it gives out Energy has to be a state function – otherwise we could make a perpetual motion machine!!

Section 10.3 Energy and Chemical Reactions 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? B. Hess’s Law Germain Hess Example: N 2 (g) + 2O 2 (g)  2NO 2 (g)  H 1 = 68 kJ N 2 (g) + O 2 (g)  2NO(g)  H 2 = 180 kJ 2NO(g) + O 2 (g)  2NO 2 (g)  H 3 = -112 kJ

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