Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chapter 17 Honors Chemistry Thermochemistry
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2. Section 17.3 Heat in Changes of State
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Heats of Fusion and Solidification How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies?
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 The molar heat of fusion (∆H fus ) is the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temp.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 The molar heat of solidification (∆H solid ) is the heat lost when one mole of a liquid solidifies at a constant temperature.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 That is, ∆H fus = –∆H solid.
17.4
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 The amount of heat necessary to vaporize one mole of a given liquid is called its molar heat of vaporization (∆H vap ).
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its molar heat of condensation (∆H cond ).
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 That is, ∆H vap = –∆H cond.
Heats of Vaporization and Condensation 17.3
–Observe the phase changes as ice is converted to steam when heat is added.
17.3
17.5
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Section 17.4 Hess’s Law Reactants Products The change in enthalpy is the same whether the reaction takes place in one step or a series of steps. The change in enthalpy, Δ H, is independent of pathway.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Figure 6.7 The Principle of Hess’s Law It does not matter if H for a reaction is calculated in one step or a series of steps.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 The Principle of Hess’s Law
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Strategy for using Hess’s Law Manipulate equations so that they add up to the desired equation.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Calculations via Hess’s Law 1. If a reaction is reversed, H is also reversed. N 2 (g) + O 2 (g) 2NO(g) H = 180 kJ 2NO(g) N 2 (g) + O 2 (g) H = 180 kJ 2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. 6NO(g) 3N 2 (g) + 3O 2 (g) H = 540 kJ
Copyright©2000 by Houghton Mifflin Company. All rights reserved Focus on the reactants and products of the required reaction. Let’s do some Practice Problems
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Section 17.4 Standard Enthalpies of Formation H f Change in enthalpy that accompanies the formation of one mole of compound from its elements with all substances in their standard states.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 A degree symbol on a thermodynamic function example H says that the process was carried out under standard conditions.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Standard States Compound 4 For a gas, pressure is exactly 1 atmosphere. 4 For a solution, concentration is exactly 1 molar. 4 Pure substance (liquid or solid), it is the pure liquid or solid. Element 4 The form [N 2 (g), K(s)] in which it exists at 1 atm and 25°C.
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Change in Enthalpy Important!!!!! Enthalpies of many reactions can be calculated from enthalpies of formation of reactants and products. H rxn ° = n p H f (products) n r H f (reactants)
Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 H f for an element in its standard state is Zero. Elements in their standard states are not included in the H rxn calculations.