Chapter 17 thermochemistry sections 17.3 & 17.4

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Chapter 17Honors Chemistry

Thermochemistry


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. Section 17.3

Heat in Changes of State


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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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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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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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That is,

∆Hfus = –∆Hsolid.


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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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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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That is,

∆Hvap = –∆Hcond.

Heats of Vaporization and Condensation17.3

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


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Section 17.4 Hess’s LawSection 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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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 ofsteps.


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The Principle of Hess’s Law


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Strategy for using Hess’s Law• Manipulate equations so that they add up to the desired equation.


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Calculations via Hess’s LawCalculations 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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3. Focus on the reactants and products of the required reaction.

Let’s do some Practice Problems


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Section 17.4Standard 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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A degree symbol on a thermodynamicfunction

example H

says that the process was carried out under standard conditions.


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Standard StatesStandard StatesCompound

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.

ElementThe form [N2(g), K(s)] in which it exists at

1 atm and 25°C.


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Change in EnthalpyChange in Enthalpy

Important!!!!!--------

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

Hrxn° = npHf(products) nrHf(reactants)


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Hf for an element in its standard state

is Zero.

Elements in their standard states are notincluded in the Hrxn

calculations.

Chapter 17 thermochemistry sections 17.3 & 17.4

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Transcript of Chapter 17 thermochemistry sections 17.3 & 17.4