Hess’s Law and Standard enthalpies

Exam priority: Very high. These are important because they require you to manipulate equations and use enthalpy data accurately.

Q: What is Hess's Law?

A: The total enthalpy change for a reaction is independent of the pathway taken, provided the initial and final states are the same.

Q: Why can Hess's Law be used to calculate unknown enthalpy changes?

A: Enthalpy is a state function, so the overall energy change is the same regardless of the route taken.

Q: What happens to ΔH when an equation is reversed?

A: The sign of ΔH changes.

Q: What happens to ΔH when an equation is multiplied by a number?

A: ΔH is multiplied by the same number.

Q: What is standard enthalpy of formation?

A: The enthalpy change when 1 mol of a compound is formed from its elements in their standard states.

Q: What is standard enthalpy of combustion?

A: The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions.

Q: What is the standard enthalpy of formation of an element in its standard state?

A: Zero.

Q: What is the formula for reaction enthalpy using formation enthalpies?

A: ΔrH° = ΣΔfH°(products) − ΣΔfH°(reactants).

Q: Why are formation enthalpies multiplied by coefficients?

A: The coefficients show the number of moles of each substance involved in the reaction.

Hess's Law method

Write the target equation.

Reverse equations if needed.

Multiply equations if needed.

Add the equations together.

Cancel substances on both sides.

Add the ΔH values.

Formation enthalpy method

Δ

r

​

H

∘

=∑nΔ

f

​

H

∘

(products)−∑nΔ

f

​

H

∘

(reactants)

Memory trick: Products minus reactants.