1 Introduction and definitions

3.2.1. Enthalpy changes 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 exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants In an exothermic reaction the ∆H is negative 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 reactants In an endothermic reaction the ∆H is positive reactants products Activation Energy: EA ∆H Progress of Reaction Energy reactants Activation Energy: EA ∆H Progress of Reaction Energy Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration The Activation Energy is defined as the minimum energy which particles need to collide to start a reaction 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 When an enthalpy change is measured at standard conditions the symbol is used Eg ∆H Definition: Enthalpy change of reaction ∆rH is the enthalpy change when the number of moles of reactants as specified in the balanced equation react together Standard enthalpy change of formation 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. 2 N Goalby chemrevise.org Enthalpy change of Neutralisation The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water. Symbol ∆neutH