Energetics - Enthalpy

What is Enthalpy change?

Enthalpy change AH (delta H), is the heat energy transferred in a reaction at constant pressure. The units of AH are kJ mol^-1.

You write AH° to show that the reactants and products were in their standard states and that the measurements were made under standard contortions. Standard conditions are 100kPa ( about 1 atm) pressure and a stated temperature ( e.g AH°). Usually all Enthalpy changes are measured at 298 K (25 °C). Sometimes the notion will also include a letter to signify whether the Enthalpy change is for a reaction( r), combustion (c ), or the formation of a new compound (f).

Exothermic reactions

Exothermic reactions give out energy to their surroundings, so the temperature in the reaction usually goes up. The products of the reaction end up with less energy than the reactants. This means that the Enthalpy changes for the reaction , AH, will be negative

Endothermic reactions

Endothermic reactions take in energy from their surroundings, so the temperature in the reaction usually falls. This means that the products of the reaction have more energy than the reactants, so the Enthalpy change for the reaction, AH, is positive.

Give the notation

For