1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chapter 17 Honors Chemistry Thermochemistry

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2. Section 17.3 Heat in Changes of State

3. 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?

4. 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.

5. 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.

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 That is, ∆H fus = –∆H solid.

8. 17.4

10. 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 ).

11. 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 ).

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 That is, ∆H vap = –∆H cond.

13. Heats of Vaporization and Condensation 17.3

14. –Observe the phase changes as ice is converted to steam when heat is added.

15. 17.3

16. 17.5

18. 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.

19. 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.

20. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 The Principle of Hess’s Law

21. 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.

22. 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

23. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 3. Focus on the reactants and products of the required reaction. Let’s do some Practice Problems

24. 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.

25. 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.

26. 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.

27. 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)

28. 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.