1 Enthalpy change

Definition: Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.In an exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants.If an enthalpy change occurs then energy is transferred between system and surroundings. The system is the chemicals and the surroundings is everything outside the chemicals. In an endothermic change, energy is transferred from the surroundings to the system (chemicals). They require an input of heat energy e.g. thermal decomposition of calcium carbonate The products have more energy than the reactantsIn an exothermic reaction the ∆H is negative. Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration. In an endothermic reaction the ∆H is positive. Standard Enthalpy Change of Formation When an enthalpy change is measured at standard conditions the symbol is used Eg ∆H The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states. Symbol ∆fH Mg (s) + Cl2 (g)
MgCl2 (s) 2Fe (s) + 1.5 O2 (g)
Fe2O3 (s) The enthalpy of formation of an element = 0 kJ mol-1 Standard Enthalpy Change of Combustion The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states. Symbol ∆cH CH4 (g) + 2O2 (g)
CO2 (g) + 2 H2O (l) Incomplete combustion will lead to soot (carbon), carbon monoxide and water. It will be less exothermic than complete combustion. Enthalpy changes are normally quoted at standard conditions. Standard conditions are: • 100 kPa pressure • 298 K (room temperature or 25oC) • Solutions at 1mol dm-3 • all substances should have their normal state at 298K