hess's law and hess cycles

ew

state Hess’s Law:

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

in a Hess cycle, which way do the arrows point for ΔH_f^⦵ data?

up!

why are you never given any ΔH_f^⦵ data for pure elements?

ΔH_f^⦵ does not apply to elements as a compound is not formed

what is the equation for finding the ΔH_f^⦵ ?

ΔH = Σ ΔH_f^⦵ (products) - Σ ΔH_c^⦵ (reactants)

in a Hess cycle, which way do the arrows point for ΔH_c^⦵ data?

down!

what is the formula for finding the ΔH_c^⦵ ?

ΔH = Σ ΔH_c^⦵ (reactants) - Σ ΔH_c^⦵ (products)

what is the significance of Hess’s Law?

can be used to find ΔH of obscure reactions which are difficult to perform in practice

why may a reaction be difficult to perform in practice and ∴ require Hess’s Law?

it may be difficult to prevent one of the reactants reacting w/ excess of another to form a different product (e.g. incomplete combustion, anhydrous copper sulfate)