Thermochemistry 2:
Energy & Enthalpy
Reminder
Enthalpy change, ΔH = ΔE+ PΔV
ΔH = Hproducts – Hreactants
W = -PΔV
Thermodynamic standard state
The value of the enthalpy change, ΔH varies with temperature, pressure, and concentration
When we talk about ΔHo, we use a standard set of conditions We say ΔHo for a reaction is 699 kJ/mol, 1 atm pressure, and if in a
solution, at 1 M concentration. The temperature must be given Any change in these conditions would give a different value for ΔH These are standard conditions Std. enthalpy change is ΔHo
Std energy change is ΔEo
Std pressure, Po is 1 atm …
The thermodymanic std. state
The most stable form of a substance at 1 atm pressure (1 bar) and at a specified temperature, usually 25oC; 1 M concentration for all substances in solution
(page 307)
Si (s) + 2 Cl2 (g) SiCl4 (l) ΔH = -687.0 kJ/mol
Determine the value of ΔE, if 1.81 mole of Si react with a
stoichiometric amount of CL2 (g) when the reaction is carried out at
0.50 atm of pressure and with a volume change of -22.4 L.
Step 1: Determine the ΔH for the reaction, taking into consideration that 1.81 moles of Si reacted
Step 2: Determine ΔE from the equation ΔH = ΔE+ PΔV
(watch your units!)
Problem 8.6
The reaction between hydrogen and oxygen to yield water vapour has ΔHo = -484 kJ
2 H2 (g) + O2 (g) 2 H2O (g) ΔHo = -484 kJ
How much PV work is done, and what is the value of ΔE (In kJ) for the reaction of 0.50 mol of H2 with 0.25 mol of O2 at atmospheric pressure if the volume change is -5.6 L?
1) Find w = -PΔV 2) Making sure that w is expressed in kJ, use the eqn. ΔE=ΔH+PΔV
to find ΔE (remember you are only using 0.5 mol H2)
Problem 8.7
The explosion of 2.00 mol of solid trinitrotoluene (TNT) with a volume of approximately 274 mL produces gases with volume 448 L at room temperature.
How much PV work (in kJ) is done during the explosion?
2 C7H5N3O6 (s) 12 CO (g) + 5H2 (g) + 3 N2 (g) + 2 C (s)
Enthalpies of Physical Change
Heat of fusion: The amount of heat energy needed for melting
(6.01 kJ/mol for water) Heat of vaporization: the amount of heat energy needed for
evaporation, breaking down the intermolecular bonds.
40.7 kJ/mol for water at 100oC
Sublimation – the direct conversion of a solid to a vapour without going through a liquid state
Heat of sublimation = heat of fusion + heat of vaporization
Heat of sublimation
The enthalpy change is the same, whichever route is taken.
It doesn’t matter that one route went Via a liquid, and the other sublimed.
What matter is the enthalpy of the vapourMinus the enthalpy of the solid:
ΔH = Hproducts – Hreactants
Enthalpies of Chemical Change
Heats of reaction – enthalpies of chemical change
Endothermic reactions.
Hproducts > Hreactants
Heat flow into the system from the surroundings
ΔH is positive
Exothermic reactions
Hproducts < Hreactants
Heat flow to the surroundings from the system
ΔH is negative ΔHo values for a given equation
Eqn is balanced for the number of moles of reactants and products All substances are in their std states Physical state of each substance is given Refer to the reaction going in the direction given
• Reverse the direction, change the sign of ΔH
Practice, practice, practice, …
Worked example 8.3 and problems 8.8, 8.9 on page 310
Finish for homework
I want to see your working clearly set out