Thermochemistry 2: Energy & Enthalpy. Reminder Enthalpy change, ΔH = ΔE+ PΔV ΔH = H products – H reactants W = -PΔV.

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Presentation transcript:

Thermochemistry 2: Energy & Enthalpy

Reminder Enthalpy change, ΔH = ΔE+ PΔV ΔH = H products – H reactants W = -PΔV

Thermodynamic standard state The value of the enthalpy change, ΔH varies with temperature, pressure, and concentration When we talk about ΔH o, we use a standard set of conditions We say ΔH o 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 ΔH o Std energy change is ΔE o Std pressure, P o 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 25 o C; 1 M concentration for all substances in solution (page 307)

Determine the value of ΔE, if 1.81 mole of Si react with a stoichiometric amount of CL 2 (g) when the reaction is carried out at 0.50 atm of pressure and with a volume change of 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 ΔH o = -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 H 2 with 0.25 mol of O 2 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 H 2 )

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?

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 kJ/mol for water at 100 o C 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 vapour Minus the enthalpy of the solid: ΔH = H products – H reactants

Enthalpies of Chemical Change Heats of reaction – enthalpies of chemical change Endothermic reactions. H products > H reactants Heat flow into the system from the surroundings ΔH is positive Exothermic reactions H products < H reactants Heat flow to the surroundings from the system ΔH is negative ΔH o 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