You will often find enthalpyA thermodynamic state function, symbol H, that equals internal energy plus pressure x volume; the change in enthalpy corresponds to the energy transferred as a result of a temperature difference (heat transfer) when a reaction occurs at constant pressure. changes indicated by ΔH° and called standard enthalpy changes. The superscript is added to indicate that the enthalpy change has occurred at the standard pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container. of 1 bar (100 kPa or 0.9869 atmAbbreviation for atmosphere, a unit of pressure equal to 101.325 kPa or 760 mmHg.). Unless very high pressures are involved, ΔH changes very little with a change in pressure, and so we have ignored superscripts up to this point. However, two other properties of matterAnything that occupies space and has mass; contrasted with energy. called the entropyA thermodynamic state function, symbol S, that equals the reversible heat energy transfer divided by temperature; higher entropy corresponds to greater dispersal of energy on the molecular scale. See also standard entropy., symbol S, and the free energyGibbs energy: a thermodynamic function corresponding to the tendency for spontaneous change in a system; represented by the symbol G., symbol G, are discussed in other sections. These are quite sensitive to pressure, and the inclusion of this superscript is important. For reasons of consistency therefore we will indicate standard enthalpy changes as ΔH° from now on.