4.2 a/b/c: Enthalpy and Hess' Law

Syllabus dot points

• Enthalpy = measure of total energy (potential) that is in the bonds of a system

• We calculate in enthalpy changes or ΔH

• endothermic rxn = positive ΔH

• exothermic rxn = negative ΔH

• law of conservation of energy - can't be created or destroyed, only changed in form

• requires energy to break bonds - endothermic

• releases energy when forming bonds - exothermic

• we determine net release or absorption of energy

• Hess' Law brings all of this + 4.1 together

• in a reaction, the enthalpy changes is always constant regardless of the steps within the reaction

• directly relates to law of conservation of energy

• reactions may have several pathways, but the overall enthalpy change will always be the same - because the amount of energy to break and form bonds will always be the same

• these are two pathways (though incomplete and complete combustion)

• Reaction 1 = complete combustion, and Reaction 1 = Reaction 2 + Reaction 3

• The enthalpy change data also supports this

• aka heat of formation/reaction

• ΔH°_f

• the increase in enthalpy when 1 mole of compound in its standard state is formed from elements in their standard state

• standard state = 100kPa, 1mol/L, temp depends on state

• forming an element means that ΔH°_f = 0

• can either be done with just the data/equations, or with an energy cycle diagram

• both can display the same information, and often it depends on the question - or in industry it's personal preference

• both are just displaying ΔH° ₌ ΣΔH°_{f (products)} - ΣΔH°_{f (reactants)}

• must have 1 mole of product! Means you can have fractions on the left hand side, but means that you can use the data.

• negative of the enthalpy change for the combustion process

• because enthalpy change is heat gained, and heat of combustion is heat released when a substance undergoes combustion

• calculating is simple; use ΔH°_{C =} ΣΔH°_{f (products)} - ΣΔH°_{f (reactants)}

• Either asked to calculate it out right, or calculate the heat of formation of a reac/prod

• both are multistep reactions, but are simplified into one step

• they are also the reverse of one another

• except photosynthesis = endothermic, and respiration = exothermic