Molar Enthalpies
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use proper scientific terminology to describe
molar enthalpies
calculate molar enthalpies
Calculate molar enthalpies using the
calorimetry equation
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define the terms: molar enthalpy, calorimetry,
calculating molar enthalpies
calculate molar enthalpies
calculate molar enthalpies using the
calorimetry equation
Prepare for the calorimetry lab
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Molar enthalpy is the enthalpy change
associated with a physical, chemical, or
nuclear change involving one mole of a
substance
Molar enthalpy is represented by the
symbol ∆Hx
The “x” is a letter or a combination of
letters to indicated the type of change that
is occurring
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Enthalpy changes for exothermic reactions
are given a negative sign
Enthalpy changes for endothermic reactions
are given a positive sign
We can represent the molar enthalpy of a
physical change, such as the vaporization of
water, as follows:
H2O(l) + 40.8 kJ  H2O(g)
The molar enthalpy of vaporization for water is
∆Hvap = 40.8 kJ/mol
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Molar enthalpy values are obtained
empirically (textbook p. 307)
The amount of energy involved in a change
depends on the quantity of matter undergoing
that change (e.g. twice the amount of ice
requires twice the amount of energy to melt)
To calculate an enthalpy change ∆H you must
1) Obtain the molar enthalpy value ∆Hx from a
reference source
2) Then use the formula
∆H = n∆Hx
 enthalpy change(∆H)
 moles (n)
 molar enthalpy (∆Hx)
Practice
p. 308 UC # 1, 2, 3
10
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Studying energy changes requires an isolated
system, one in which no matter nor energy
can move in or out
Remember, the law of conservation of energy:
the total energy change of the chemical
system is equal to the total energy change of
the surroundings
∆H
system
= ± ‫׀‬q
surroundings‫׀‬
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When using a calorimeter, three assumptions
are made:
1) No heat is transferred between the
calorimeter and the outside environment
2) Any heat absorbed or released by the
calorimeter materials, such as the container,
is negligible
3) A dilute aqueous solution is assumed to have
a density and specific heat capacity equal to
that of pure water
(1.00 g/mL and 4.18 J/g∙ºC or 4.18 kJ/kg∙ºC)
Always recognize the law of conservation of energy
∆H = q
(substance dissolving)
(calorimeter water)
You will also need to combine mathematical formulas
∆H = q
n∆Hx = mc∆T
Practice
p. 310 UC # 4, 5
p. 311 UC # 6, 7, 8, 9
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