Question

The heat of formation of co2 and h2o is -395kj/mol and -285 kj/mol. The heat of combustion of glucose is -2900 kj/mo.Calculate the heat of formation of glucoswol

Answer 1

Germain Henri Hess, in 1840, discovered a very useful principle which is named for him:

The enthalpy of a given chemical reaction is constant, regardless of the reaction happening in one step or many steps.

There is a second way to use Hess' Law and it is usually presented like this:

ΔH°rxn = Σ ΔH°f (products) minus Σ ΔH°f (reactants)

Here's another form in which you see this form of Hess' Law:

ΔH°rxn = Σ ΔH°f, products minus Σ ΔH°f, reactants

The "rxn" above is a common way to abbreviate "reaction." All it means is that we are discussing the enthalpy of a generic reaction, not any specific one. I'll explain the above equation using an example problem.