Enthalpy

Some chemical reactions give off tremendous heat and perform work on the surroundings, such as the combustion of rocket fuel causing a space shuttle to lift off from the ground.

The sum of heat “q,” and work “w,” is the change in internal energy, “ΔE,” as given by the first law of thermodynamics.

For chemical reactions involving gases that occur at atmospheric pressure, the work done is the mechanical work associated with volume changes — either expansion or contraction. Work is therefore equal to the negative value of the pressure times the change in volume.

Substituting for w in the first law of thermodynamics and rearranging the terms in the equation shows that q = ΔE + PΔV, giving the expression for heat flow under constant pressure.

In other chemical reactions, such as the burning of wood to cook food, it is more relevant to quantify the heat given off to facilitate cooking, than to measure the amount of expansion work done on the surrounding.

Since internal energy accounts for both heat and work, ΔE is not used for constant pressure conditions. To exclusively discuss the energy flow in the form of heat, a new thermodynamic function – enthalpy – is defined.

Enthalpy, H, equals the sum of internal energy, E, and pressure-volume work, P-V. Because energy, pressure, and volume are state functions, enthalpy is also a state function.

Absolute enthalpy values for specific substances cannot be measured. Only the change in enthalpy can be determined. Enthalpy change, ΔH, equals the change in internal energy, ΔE + PΔV.

Recalling that the change in energy is the sum of heat and pressure-volume work the equations can be combined to show that under constant pressure conditions ΔH equals the heat, q, gained, or lost by the system.

If the system loses energy to the surroundings in the form of heat — as in the burning of wood — the temperature of the surroundings rises. This is described by a negative sign convention for q. Consequently, ΔH becomes negative, and the process is described as exothermic.

On the contrary, if the system gains energy from the surroundings in the form of heat — such as the reaction occurring in a chemical cold pack — the temperature of the surroundings falls. The heat, in this case, is described by a positive sign convention. This makes ΔH positive, and the process is called endothermic.