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