Hess’s Law and Enthalpy Calculations

Flashcards summarizing Hess’s Law definitions, formulas, example calculations, diagram tips, and common exam pitfalls.

What does Hess’s Law state about the total enthalpy change for a reaction?

It is the same regardless of the reaction pathway, provided the initial and final conditions are identical.

Why is Hess’s Law useful in thermochemistry?

It allows you to calculate enthalpy changes for reactions that cannot be measured directly by using known enthalpy changes of related reactions.

What is the standard enthalpy of formation (ΔHf°)?

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

What is the standard enthalpy of combustion (ΔHc°)?

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

What are the standard conditions used in enthalpy calculations?

Temperature 298 K (25 °C), pressure 100 kPa, concentration 1.0 mol dm⁻³ for solutions, with all substances in their standard states.

Name the two most common types of Hess’s Law cycles.

Formation cycles (using ΔHf values) and combustion cycles (using ΔHc values).

What is the Hess’s Law formula that uses enthalpies of formation?

ΔHreaction = ΣΔHf(products) − ΣΔHf(reactants).

Using formation data, what is ΔH for C₂H₆ + 3.5O₂ → 2CO₂ + 3H₂O?

−1559.7 kJ mol⁻¹.

What is the Hess’s Law formula that uses enthalpies of combustion?

ΔHreaction = ΣΔHc(reactants) − ΣΔHc(products).

Using combustion data, what is ΔH for C₂H₄ + H₂ → C₂H₆?

−137 kJ mol⁻¹.

How do you estimate ΔH using average bond enthalpies?

ΔH = Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds formed).

Why are bond-enthalpy calculations only approximate?

Because bond enthalpies are mean values averaged over many different molecules.

When drawing a Hess’s Law cycle, what must you include on every arrow?

A labeled ΔH value with the correct sign and units.

In a Hess diagram, how do you treat an arrow you traverse in the opposite direction to the one drawn?

Reverse the sign of its ΔH value.

What stoichiometric adjustment must you make when using tabulated ΔH values?

Multiply each ΔH value by the number of moles of the species involved.

Which state-symbol error commonly costs marks in Hess’s Law questions?

Using incorrect or missing state symbols (g, l, s, aq).

What do students often forget about elements such as O₂ when using ΔHf values?

Elements in their standard states have ΔHf = 0.

What is a common confusion between ΔHc and ΔHf values in exams?

Using combustion data instead of formation data, or vice versa, when selecting values for a calculation.