Using Hess's Law to Calculate DHº Values.

Given the heats of combustion for ethanol and ethanoic (acetic) acid, calculate DHº for the oxidation of ethanol into ethanoic acid.

Given:

DHº_(combustion)(C₂H₅OH_(l)) = -1367 kJ/mol

DHº_(combustion)(CH₃COOH_(l)) = -875 kJ/mol

calculate DHº for:

C₂H₅OH(l) + O₂(g) CH₃COOH(l) + H₂O(g).

Method 1: Adding reactions to give the needed reaction:

C₂H₅OH + 3 O₂ 2 CO₂ + 3 H₂O DHº = -1367 kJ

2 CO₂ + 2 H₂O CH₃COOH + 2 O₂ DHº = +875 kJ

Add

C₂H₅OH + O₂ CH₃COOH + H₂O DHº = -442 kJ

Method 2: Recognising a different route for the process.

DHº = DHº₁ + DHº₂

= -1367 + 875

= -442 kJ

Definitions:

Bond Energy: the average energy needed to break a typical bond. (gas phase only.)

Bond Dissociation Energy:

the actual energy needed to break a particular bond. (gas phase only.)

For tabulation purposes, only bond energies are used. Hence any calculation which uses bond energies is approximate.

When using bond energies:

Bonds broken means energy into the system.

Bonds made means energy out of the system.

DH = åB_E(bonds broken) - åB_E(bonds made).

Use bond energies to calculate DH for the reaction:

H₂(g) + Br₂(g) 2 HBr(g)

DH = åB_E(bonds broken) - åB_E(bonds made).

= B_E(H-H) + B_E(Br-Br) - 2 B_E(H-Br)

= 435 kJ/mol + 192 kJ/mol - 2 mol(368 kJ/mol)

= -109 kJ

Example:

Estimate DH for the following reaction from bond energies.

CH₃COCH₃(g) + Br₂(g) CH₃COCH₂Br(g) + HBr(g)

DH = åB_E(bonds broken) - åB_E(bonds made).

= B_E(C-H) + B_E(Br-Br) - B_E(C-Br) - B_E(H-Br)

= (420 + 192 -280 - 368) kJ (mol * kJ/mol = kJ)

= -36 kJ.

Thermochemistry - Summary.

DE = q + w = q - P_extdV

At constant volume: DE = q_v

At constant pressure: DE = q_p - PDV

Definition: DH = DE + PDV = q_p

Measure DE directly in a bomb calorimeter

Measure DH directly in an open calorimeter

Tabulate DHº as DHº_f

Use these tables with the formula: DHº₂₉₈ = åm_iDHº_f(i)

Bond energies may be used to give approximate values of DH for gaseous reactions.

Thermochemistry and the energy problem.

Energy must be used to do useful work. As the next section shows, only a portion of the available energy can be used as work, some energy must always be wasted as heat. Your car, for example, wastes 70% of the energy available in gasoline as heat.

The energy crisis that we face today is not the lack of energy. There is plenty of energy available, if we could find ways of using it.

Currently, the main source of energy for both heating and machines is oil, natural gas or coal in which the chemical potential energy of hydrocarbons is used. These chemicals are not being replaced so we are using them up and will run out of them.

There is lots of energy available from the sun which we are learning to use. However, mainly we need to use the energy at a different time and in a different place to where it is available. We need to be able to store and transmit the available energy. So this is the real problem: How do we store the abundantly available sun's energy and/or move it to where it is needed.