Enthalpy & Hess’s Law

A set of flashcards covering key definitions, formulas, and concepts in enthalpy and Hess’s Law.

What is enthalpy (H)?

The heat content of a system at constant pressure; a state function whose absolute value cannot be measured directly.

What is an enthalpy change (ΔH) and how is its sign interpreted?

The heat energy transferred in a reaction at constant pressure. ΔH < 0 indicates an exothermic process (heat released); ΔH > 0 indicates an endothermic process (heat absorbed).

What are the standard conditions used in thermochemistry?

Temperature 298 K (25 °C), pressure 100 kPa, concentration 1 mol dm⁻³ for solutions, and all substances in their standard states.

Define the standard enthalpy change of reaction (ΔHᵣ⦵).

The enthalpy change when a reaction occurs in the molar quantities of a balanced equation under standard conditions with all reactants and products in their standard states.

Define the standard enthalpy change of formation (ΔHf⦵).

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

Define the standard enthalpy change of combustion (ΔHc⦵).

The enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions, with all substances in their standard states.

Define the standard enthalpy change of neutralisation.

The enthalpy change when 1 mol of water is formed from the reaction of an acid with an alkali under standard conditions.

What equation is used to calculate heat energy (q) from experimental data and what do its symbols represent?

q = mcΔT, where m = mass of solution (g), c = specific heat capacity (usually 4.18 J g⁻¹ K⁻¹ for water), ΔT = temperature change (K or °C).

How is experimental ΔH obtained from q?

ΔH = –q / n, where n is the moles of the limiting reactant in the experiment.

Give four reasons why experimental ΔH values may differ from theoretical values.

Heat lost to surroundings, incomplete combustion, evaporation of fuel/solution, inaccurate mass or temperature measurements, or the reaction being incomplete/slow.

State Hess’s Law.

The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.

How is Hess’s Law applied using enthalpies of formation?

ΔH = ΣΔHf(products) – ΣΔHf(reactants); typically shown with elements in their standard states at the bottom of a Hess cycle.

How is Hess’s Law applied using enthalpies of combustion?

ΔH = ΣΔHc(reactants) – ΣΔHc(products); a Hess cycle is drawn with CO₂ and H₂O combustion products at the bottom.

What is mean bond enthalpy and how do bond-breaking and bond-forming relate to ΔH?

Mean bond enthalpy is the average energy required to break 1 mol of a specific bond in gaseous molecules. Breaking bonds is endothermic (+ΔH), whereas forming bonds is exothermic (–ΔH).

What formula uses bond enthalpies to estimate a reaction’s enthalpy change?

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

Why are calculations based on mean bond enthalpies only approximate?

Bond enthalpies are averaged over many compounds and assume all species are gaseous, which may not match actual reaction conditions.