Thermochemistry

Molar Enthalpies

use proper scientific terminology to describe molar enthalpies

calculate molar enthalpies Calculate molar enthalpies using the

calorimetry equation

Learning Goals

define the terms: molar enthalpy, calorimetry, calculating molar enthalpies

calculate molar enthalpies calculate molar enthalpies using the

calorimetry equation Prepare for the calorimetry lab

Success Criteria

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

Molar Enthalpies

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

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)

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Learning Checkpoint

Practice p. 308 UC # 1, 2, 3

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׀

Calorimetry of Physical Change

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)

Calorimeter Assumptions

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

Using Calorimetry to Find Molar Enthalpies

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Learning Checkpoint

Practice p. 310 UC # 4, 5

p. 311 UC # 6, 7, 8, 9