THE UNIVERSITY

OF QUEENSLAND

Foundation Year

THERMOCHEMISTRY II

Lesson Overview Thermochemistry

Specific heat capacity

Endothermic and Exothermic Equations

CalorimetryThermochemical

EquationsHeats of Changes

of State

Heat Capacity

Hess’s Law

Standard Heats of Formation

Thermochemical Equations 1

• A thermochemical equation is 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 kJor

• 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 the reactants to make it happen.

• Heat may be considered as a reactant.• 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 symbols must be included. This is because changing the state of a chemical 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 (Hfus.).

• The heat lost when one mole of a liquid changes to a solid at a constant temperature is the molar heat of solidification (Hsolid. )

Heat and Changes of State (2)

• The amount of heat absorbed by one mole of a liquid that is undergoing evaporation is called the molar heat of vaporisation.

( Hvap)

• The condensation of 1 mole of vapour releases heat as the molar heat of condensation (Hcond).

Hsolid = -ve

Hcond = -ve

Hfus = +ve

Hvap = +ve

Vapour

Liquid

Solid

Enthalpy

           

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, Hsoln.

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

Exothermic Solvation

Endothermic Solvation

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

http://www.wou.edu/las/physci/ch412/heatsoln.gif

Thermochemical Equation Calculations

• Thermochemical equations obey several simple rules that make computation of the enthalpy change in a reaction easy.

1. The magnitude of H is directly proportional to the amount of reactants used in the reaction

2. H for a reaction is equal in magnitude but opposite in sign to the reverse reaction.

3. Hess' Law: The value of H 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.: H = -395.4kJ

+ H = +393.5kJ

H = -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.JPG

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

References

1. http://www.chem.vt.edu/RVGS/ACT/notes/Chap_8_Triptik.html

2. http://apchem.virtualave.net/concepts/thermochem.html

3. http://chemed.chem.purdue.edu/demos/movies/small_movies/5.2small.mov

4. http://www.wou.edu/las/physci/ch412/heatsoln.gif

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

6. http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/

End of Lecture