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Semelhante a Chapter 17 thermochemistry sections 17.3 & 17.4 (20)
Chapter 17 thermochemistry sections 17.3 & 17.4
- 2. . Section 17.3
Heat in Changes of State
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- 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?
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- 4. The molar heat of fusion (∆Hfus)
is the heat absorbed by one mole
of a solid substance as it melts to a
liquid at a constant temp.
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- 5. The molar heat of solidification
(∆Hsolid) is the heat lost when one
mole of a liquid solidifies at a
constant temperature.
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- 6. That is,
∆Hfus = –∆Hsolid.
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- 10. The amount of heat necessary to
vaporize one mole of a given liquid
is called its molar heat of
vaporization (∆Hvap).
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- 11. The amount of heat released when
1 mol of vapor condenses at the
normal boiling point is called its
molar heat of condensation
(∆Hcond).
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- 12. That is,
∆Hvap = –∆Hcond.
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- 13. 17 Heats of Vaporization and Condensation
.3
- 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.
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- 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.
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- 20. The Principle of Hess’s Law
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- 21. Strategy for using Hess’s Law
• Manipulate equations so that they
add up to the desired equation.
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- 22. Calculations via Hess’s Law
1. If a reaction is reversed, H is also reversed.
N2(g) + O2(g) 2NO(g) H = 180 kJ
2NO(g) N2(g) + O2(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) 3N2(g) + 3O2(g) H = 540 kJ
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- 23. 3. Focus on the reactants and products
of the required reaction.
Let’s do some Practice Problems
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- 24. Section 17.4
Standard Enthalpies of Formation
• Hf
• Change in enthalpy that accompanies
the formation of one mole of
compound from its elements with all
substances in their standard states.
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- 25. A degree symbol on a thermodynamic
function
example H
says that the process was carried out
under standard conditions.
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- 26. Standard States
Compound
For a gas, pressure is exactly 1 atmosphere.
For a solution, concentration is exactly
1 molar.
Pure substance (liquid or solid), it is the pure
liquid or solid.
Element
The form [N2(g), K(s)] in which it exists at
1 atm and 25°C.
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- 28. Hf for an element in its standard state
is Zero.
Elements in their standard states are not
included in the Hrxn calculations.
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