Physical Chemistry Lecture 3: Enthalpy of Reaction

Study Notes

Thermochemical measurements can be taken at constant volume or constant pressure.
The observed changes in these conditions differ.
Change in internal energy (ΔE) is the heat change accompanying a chemical reaction at constant volume. This is because no external work is done.
Most chemical reactions are conducted at constant pressure in the lab.
To examine heat changes in reactions under constant pressure and temperature, enthalpy (H) is used.
Enthalpy (H) is defined as the sum of internal energy (E) and the product of pressure (P) and volume (V). (H = E + PV)
Enthalpy is a state function, meaning its value depends only on the current state of the system, not how it reached that state.
Absolute enthalpy values cannot be measured, only changes in enthalpy (ΔH) can be determined.
ΔH is calculated as the difference in enthalpy between products and reactants: ΔH = H_products - H_reactants = H_p - H_r

For reactions involving solids and liquids, the change in volume (ΔV) is very small.
Therefore, the term PΔV is negligible (ΔH ≈ ΔE).

Reactions under constant pressure can either release or absorb heat.
ΔH = 0: No heat absorption or release; products and reactants have the same enthalpy.
ΔH < 0 (Negative): Products have lower enthalpy than reactants; heat is released (exothermic).
ΔH > 0 (Positive): Products have higher enthalpy than reactants; heat is absorbed (endothermic).

The heat of combustion of ethylene at constant volume is -332.19 kcals at 17°C.
Calculate the heat of combustion at constant pressure considering water as a liquid.
Provided data: R = 2 cal degree⁻¹ mol⁻¹

The heat of combustion of carbon monoxide at constant volume and 17°C is -283.3 kJ.
Calculate the heat of combustion at constant pressure.

A thermochemical equation indicates the heat change during a reaction.
It must be balanced.
Provide ΔE or ΔH values.
Include physical states of reactants and products (e.g., (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous).

Is ΔH calculated negative or positive?
Negative ΔH indicates an exothermic reaction (heat released) and positive ΔH represents an endothermic reaction (heat absorbed).

The stoichiometric coefficients in a thermochemical equation represent moles of substances.
Reversing a reaction changes the sign of ΔH.
Changing the equation's factors by a factor 'n' multiplies the ΔH value by 'n'.

Heat of Formation (ΔH_f): Enthalpy change when one mole of a compound forms from constituent elements.
Heat of Combustion (ΔH_c): Change in enthalpy when one mole of a substance undergoes complete combustion in excess air or oxygen.
Heat of Solution (ΔH_solution): Change in enthalpy when one mole of a substance is dissolved in a specific quantity of solvent at a set temperature.