Thermochemistry: Reaction Enthalpy, Hess's Law, and Enthalpies of Formation

Vocabulary flashcards covering key terms from Hess's law, enthalpy of reaction, enthalpies of formation, standard states, and temperature dependence of reaction enthalpy.

Reaction enthalpy

ΔrH; the enthalpy change for a chemical reaction, which can be obtained from products minus reactants or via Hess's law by summing step enthalpies.

Hess's law

Enthalpy is a state function, so the total enthalpy change is the same whether the reaction occurs in one step or multiple steps.

Enthalpy of formation (ΔfH)

The enthalpy change when 1 mole of a compound forms from its elements in their standard states; by convention, zero for elements in their standard states.

Standard enthalpy of formation (ΔfH°)

Enthalpy of formation of a substance from its elements in their standard states at 1 bar and 298 K.

Reference state

A chosen baseline state (often the elements in their most stable form) used to define enthalpies of formation and comparison in Hess's law.

Elements in standard state

The most stable form of each element at 1 bar and 298 K (e.g., O2 gas, N2 gas, C graphite); their formation enthalpy is defined as zero.

Formation reaction

The chemical reaction that forms 1 mole of a substance from its elements in their standard states.

Exothermic

A process that releases heat to the surroundings; ΔH < 0.

Endothermic

A process that absorbs heat from the surroundings; ΔH > 0.

Kirchhoff's law

The temperature dependence of reaction enthalpy: ΔrH(T2) = ΔrH(T1) + ∫ from T1 to T2 of ΔCp dT.

Cp (heat capacity)

The amount of heat required to raise the temperature of a substance by 1 K; can be a function of temperature, often modeled as Cp(T) = a + bT + cT^2.

ΔCp

The difference between the heat capacities of products and reactants, ΔCp = Cp(products) − Cp(reactants), used in Kirchhoff's law.

Standard temperature (298 K)

The reference temperature (25°C) used with standard state data for thermodynamic tables.

1 bar

Standard pressure used with standard state definitions for thermochemical data.

Formation example: benzene

An example formation reaction illustrating formation of a compound (e.g., C6H6) from its elements in standard states to apply ΔfH°.

N2 standard state

N2 in its standard state (gas); its ΔfH° is defined as zero because it is an element in its standard form.

State tracking

Recording the phase (gas, liquid, solid) of reactants and products because enthalpies depend on state.