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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17.4
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17.4
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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.
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Heats of Vaporization and Condensation17.3
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–Observe the phase changes as ice is converted to steam when heat is added.
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17.3
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17.5
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17.5
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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.