Chapter 9 enthalpy

enthalpy

a measure of the heat energy in a chemical system

chemical system

the atoms, molecules, or ions making up the chemicals

enthalpy change ΔH

difference in enthalpies of the products and reactants

conservation of energy

energy cannot be created or destroyed

enthalpy change of formation

the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

average bond enthalpy

the average enthalpy change when one mole of gaseous covalent bonds is broken

enthalpy change of combustion

the enthalpy change when 1 mole of a substance is completely burnt in oxygen with all substances in their standard states in standard conditions

standard conditions ΔH°

100kPa and 298K

enthalpy of reaction Δ_rH°

the enthalpy change for a reaction with the quantities shown in the chemical equation

enthalpy change of neutralisation ΔH_neut

the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water. it is always measured per mole of water formed

equation of energy required (J)

mass (g) x SHC (J) X temp rise

equation of ΔH

q / moles in reaction

exothermic

the energy released from bond making > the energy required to break bonds

endothermic

the energy required to break bonds > energy released from bond making

activation energy E_a

the minimum energy required for colliding products to react

specific heat capacity

the energy required to raise the temperature of 1g of a substance by 1K

Hess’ law

the total enthalpy change of a reaction is independent of the route taken