Chapter 17 thermochemistry sections 17.3 & 17.4

Chapter 17
Honors Chemistry

Thermochemistry

Copyright©2000 by Houghton 1
Mifflin Company. All rights reserved.

. Section 17.3

Heat in Changes of State


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?


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.


The molar heat of solidification
(∆Hsolid) is the heat lost when one
mole of a liquid solidifies at a
constant temperature.


That is,

∆Hfus = –∆Hsolid.


The amount of heat necessary to
vaporize one mole of a given liquid
is called its molar heat of
vaporization (∆Hvap).


The amount of heat released when
1 mol of vapor condenses at the
normal boiling point is called its
molar heat of condensation
(∆Hcond).


That is,

∆Hvap = –∆Hcond.


17 Heats of Vaporization and Condensation

.3

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

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.

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.


The Principle of Hess’s Law


Strategy for using Hess’s Law

• Manipulate equations so that they
add up to the desired equation.


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


3. Focus on the reactants and products
of the required reaction.

Let’s do some Practice Problems


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.


A degree symbol on a thermodynamic
function

example H

says that the process was carried out
under standard conditions.


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.


Change in Enthalpy
Important!!!!!--------

Enthalpies of many reactions can be calculated
from enthalpies of formation of reactants and
products.

Hrxn° = np Hf (products) nr Hf (reactants)


Hf for an element in its standard state
is Zero.

Elements in their standard states are not
included in the Hrxn calculations.


Chapter 17 thermochemistry sections 17.3 & 17.4

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