Energetics

6.1 Exothermic and endothermic reactions

If a reaction produces heat (increases the temperature of the surroundings) it is exothermic. If the temperature of the reaction mixture decreases (ie heat is absorbed) then the reaction is endothermic.

• Exothermic -> a reaction which produces heat (ΔH has a negative value by convention, -ve)
• Endothermic -> a reaction which absorbs heat (ΔH has a positive value by convention, +ve)

Enthalpy of reaction -> the change in internal energy (H) through a reaction is ΔH.

The most stable state is where all energy has been released...therefore when going to a more stable state, energy will be released, and when going to a less stable state, energy will be gained. On an enthalpy level diagram, higher positions will be less stable (with more internal energy) therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if it is higher, heat is gained (less stable, ΔH is +ve).

• Formation of bonds causes an energy release (exothermic).
• Breaking of bonds requires energy (endothermic).

6.2 Calculation of enthalpy changes

Change in energy = mass x specific heat capacity x change in temperature --> E = m x c x ΔT

Enthalpy changes (ΔH) are related to the number of mols in the reaction...if all the coefficients are doubled, then the value of ΔH will be doubled.

When a reaction is carried out in aqueous solution, the water will gain or lose heat from (or to) the reactants. Therefore, the change in energy, and so the ΔH value, can be calculated from E = m x c x ΔT where m is the mass of water present (kilograms), and c = 4.18 kJ Kg-1 K-1. The ΔH value can then be calculated back to find the molar enthalpy change for the reaction.

Experimental

A known mass of solution should be placed in a container, as insulated as possible, to prevent as much heat as possible from escaping. The temperature is measured continuously, the value used in the equation is the maximum change in temperature from the initial reading.

The result will be a change in temperature. This can be converted into a change in heat (or energy) by using the above equation E = m x c x ΔT.

Δ-H may then be calculated for the amount of reactants present, and then this can be used to calculate for a given number of mols.

6.3 Hess' Law

Hess' Law states that the total enthalpy change between given reactants and products is that same regardless of any intermediate steps (or the reaction pathway).

Any equations can be mathematically manipulated using the four rules of number to construct other equations.

Examples.

6.4 Bond enthalpy and bond dissociation enthalpy

Bond dissociation enthalpy is the enthalpy change when one mole of specific bonds are broken.

X-Y(g) -> X(g) + Y(g) : ΔH(dissociation).

Bond enthalpy is the average value of a particular tye of bond which has been measured over a range of molecules.

Example:

CH4 has four C-H bonds, and so will have four different bond dissociation enthalpies corresponding to the following bonds breaking:

CH4 --> CH3 + H

CH3 --> CH2 + H

CH2 --> CH + H

CH   --> C + H

the C-H bond enthalpy is the average value of the four bond dissociation enthalpies.

Bond energies (enthalpies) can be used to calculate unknown enthalpy changes in reactions where only a few bonds are being formed/broken.

Bonds broken (left hand side) - bonds formed (right hand side) = enthalpy change for the reaction.
(all bond values positive)