using Hess's Law to calculate enthalpy changes - written method
write equation for enthalpy change you want to find - balanced and with state symbols
use information given in question to complete a cycle that works for Hess's Law
apply Hess's Law - find 2 routes in diagram that don't go against flow of arrows
total enthalpy change on the two routes will be the same so add enthalpy changes on each route & equate them then find unknown
