1.4 Energetics - Yr12

• endothermic: heat energy is taken in and the surrounding temperature decreases
• exothermic: heat energy lost and the surrounding temperature increases
• enthalpy change is heat change at constant pressure
• standard enthalpy of formation: enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states
• standard enthalpy of combustion: enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products in their standard states
• enthalpy of neutralisation: enthalpy change when an acid is neutralised by an alkali and one mole of water is formed under standard conditions
• hess’s law: enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
• mean bond enthalpy: average energy needed to break a certain type of bond over a range of compounds
• the standard enthalpy change conditions are 298K and 100kPa
• calorimeter can be used to measure enthalpy change
• simple calorimetry is not accurate as heat loss from sides and top, it won’t completely burn so forms carbon monoxide and the apparatus is heated
• to prevent heat loss to the surroundings use a polystyrene cup
• flame calorimetry is more accurate as the flame is enclosed, fuel burns in pure oxygen rather than air and there is a copper chimney
• hess’s law formation = products - reactants
• hess’s law combustion = reactants - products
• the value for element in hess’s law is 0
• arrows point upwards for formation
• arrows point downwards for combustion
• if you go against an arrow, then flip the sign
• bonds broken - bonds formed