enthalpy: indirect methods

what is hess’ law?

enthalpy change during a chemical reaction is independent on the path taken, meaning it remains the same regardless of the number of steps in the reaction.

why is it not possible to directly measure enthalpy change for some reactions?

incomplete combustion to produce some over products

what are the combustion products?

h2o, so2, co2, n2

△H direct route = △H indirect route

△H1 = △H2 + △H3

when using enthalpies of combustion

△H(reaction) = Σ△cH (reactants) - Σ△cH (products)

when using enthalpies of formation

△H(reaction) = Σ△fH(products) - Σ△fH(reactants)

what to put in the box at the bottom when using formation enthalpies

elements

what is bond enthalpy?

energy change when one mole of gaseous covalent bonds is broken

what is the box at bottom when using combustion enthalpies

combustion products

why is bond breaking endothermic (+)

reactants require more energy to break as higher bond enthalpy means stronger bond.

why is bond making exothermic (-)

releases more energy when bonds are made in products compared to how much is required to break bonds in reactants, so overall energy change is negative, resulting in a net release of energy to surroundings.