Standard Enthalpy Changes of a Reaction

IB Chemistry: Standard Enthalpy Changes of a Reaction Standard Enthalpy Changes of a Reaction Define and apply the terms standard state, standard enthalpy change of formation, and standard enthalpy change of combustion. Standard State: The form an element/compound exists under standard conditions. The conditions for “Standard Conditions” are typically: ● Pressure :101 kPA ● Temperature : 25°C (or 298K) Standard Enthalpy change of Formation: This is the enthalpy change that happens when 1 mole of a substance is created from its constituent elements under typical conditions and in the standard state. Standard Enthalpy change of Combustion: Enthalpy change that occurs when one mole of reactants react with oxygen under standard conditions. Determine the enthalpy change of a reaction using standard enthalpy changes of formation and combustion. Let’s consider the combustion of methane. CH 4 + 2O 2 → 2H 2 O + CO 2 We now want to find the (Energy in/kJ – Energy out/kJ) of the reaction to determine the Enthalpy change. We can calculate bond enthalpies using the data from the IB data booklet. If you get a copy of the data booklet, you might be aware of these values: Average bond enthalpies at 298K Energy in C-H = 413 kJmol -1 ● However, there are four C-H bonds in methane, so the energy will be : 413kJmol -1 x 4 = 1652 kJmol -1 2O 2 → O=O → 498kJmol -1 x 2 = 996kJmol -1 Total energy in = 996 + 1652 = 2648 kJmol -1 Energy out CO 2 → 2x C=O → 746kJmol -1 x2 = 1492 kJmol -1 2H 2 O → 4 x H-O → 4 x464 = 1856kJmol -1 Total energy out = 3348 kJmol -1

Therefore: Enthalpy Change = Energy in – Energy out = -700kJmol -1