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T HE U NIVERSITY O F Q UEENSLAND Foundation Year THERMOCHEMISTRY II

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Lesson Overview

Thermochemistry

Heat Capacity

Endothermic and Exothermic Equations

Specific heat

capacity

Calorimetry

Thermochemical

Equations

Heats of Changes

of State

Hess’s Law

Standard Heats of Formation

Thermochemical Equations 1

- A thermochemical equationis one that includes energy changes.
- In exothermic reactions, heat is a product (it\'s being formed), so a reaction of this kind might look like this:

A + B ---> C + D + heat

- And similarly, if a reaction is endo, then it acts like a reactant (goes on the left side):

A + B + heat ---> C + D

Thermochemical Equations 2

- There are two ways to write a thermochemical equation:
- 2 C2H6(g) + 7 O2(g)4 CO2(g) + 6 H2O(g)

H = -2855 kJ

or

- 2 C2H6(g) + 7 O2(g) 4 CO2(g) + 6 H2O(g) + 2855 kJ
- Are these reactions exothermic or endothermic?

Thermochemical Equations 3

- For endothermic reactions, energy must be added to thereactants to make it happen.
- Heat may be considered as areactant.
- 2 NH3(g) + 92 kJ N2(g) + 3 H2(g)

or

- 2 NH3(g) N2(g) + 3 H2(g)H = +92 kJ

• When writing thermochemical equations, the state symbolsmust be included.This is because changing the state of achemical involves energy changes.

Heat and Changes of State(1)

- All solids absorb heat in melting to liquids. The heat absorbed by one mole of a substance in melting from a solid to a liquid at a constant temperature is called the molar heat of fusion (DHfus.).
- The heat lost when one mole of a liquid changes to a solid at a constant temperature is themolar heat of solidification (DHsolid. )

Heat and Changes of State(2)

- The amount of heat absorbed by one mole of a liquid that is undergoing evaporation is called themolar heat of vaporisation.

(D Hvap)

- The condensation of 1 mole of vapour releases heat as the molar heat of condensation (DHcond).

Heat of Solution

- Heat changes can occur when a substance is dissolved in a solvent.The heat change caused by dissolution of one mole of substance is the molar heat of solution, DHsoln.

http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/Hsolv1gif.gif

http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/SolnProc.htm

Thermochemical Equation Calculations

- Thermochemical equations obey several simple rules that make computation of the enthalpy change in a reaction easy.
- The magnitude of DH is directly proportional to the amount of reactants used in the reaction
- DH for a reaction is equal in magnitude but opposite in sign to the reverse reaction.
- Hess\' Law: The value of DH for a reaction is the same no matter what path is used to get from reactants to products.

Hess’s Law of Summation

- Hess\'s Law of Heat Summation states:

If you add two or more thermochemical equations to give a final equation, then you can also add the heat changes to give the final heat changes.

Hess’s Law of Summation(2)

To find the enthalpy for:

- C(s, diamond) C(s, graphite)
- C(s, graphite)+ O2(g) CO2(g) H = -393.5kJ
- C(s, diamond)+ O2(g) CO2(g) H =-395.4kJ

Write the reverse of equation (a) to give

- CO2(g) C(s, graphite)+O2(g) H = +393.5kJ

Hess’s Law of Summation(3)

- Adding the equations should give us our original equation.
- Now do the same thing with the enthalpy values :-

i.e.: DH = -395.4kJ

+ DH = +393.5kJ

DH = -1.9kJ

Heats of Formation

- The standard heat of formation (Hf) of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements with all substances in their standard states at 25oC.
- The H for a reaction is the difference between the standard heats of formation of all the reactants and products.
- ie: H = Hf(products) - Hf(reactants)

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG07_16.JPGhttp://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/FG07_16.JPG

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB07_02.JPGhttp://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/TB07_02.JPG

References

- http://www.chem.vt.edu/RVGS/ACT/notes/Chap_8_Triptik.html
- http://apchem.virtualave.net/concepts/thermochem.html
- http://chemed.chem.purdue.edu/demos/movies/small_movies/5.2small.mov
- http://www.wou.edu/las/physci/ch412/heatsoln.gif
- http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/Hsolv1gif.gif
- http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/

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