Chapter 9 Thermochemistry – Key Vocabulary

A set of core vocabulary flashcards covering enthalpy, energy transfer, standard states, and Hess’s law from Chapter 9 Thermochemistry notes.

Thermochemistry

Branch of chemistry that studies the relationships among heat, work, and other forms of energy during chemical and physical processes.

Internal Energy (U or E)

The total of all kinetic and potential energies of the particles in a system; a state function.

Heat (q)

Energy transferred between a system and its surroundings because of a temperature difference; positive when absorbed by the system, negative when released.

Work (w)

Energy transfer resulting from a force acting through a distance; positive when done on the system, negative when done by the system.

First Law of Thermodynamics

Energy cannot be created or destroyed; for a system, ΔU = q + w and ΔEsys = −ΔEsurr.

Pressure–Volume (Expansion) Work

Work associated with volume change against an external pressure (PΔV); w = −PΔV.

State Function

A property that depends only on the current state of the system, not on the path taken (e.g., U, H).

Enthalpy (H)

A thermodynamic quantity defined as H = U + PV; a state function useful for constant-pressure processes.

Enthalpy Change (ΔH)

Difference in enthalpy between final and initial states; equals qp (heat at constant pressure) when only P-V work occurs.

Heat of Reaction at Constant Pressure (qp)

The heat exchanged during a reaction carried out at constant pressure; numerically equal to ΔH.

Exothermic Reaction

Process with ΔH < 0 that releases heat to the surroundings.

Endothermic Reaction

Process with ΔH > 0 that absorbs heat from the surroundings.

Thermochemical Equation

A balanced chemical equation accompanied by a ΔH value specifying the heat change for the stated stoichiometric amounts and physical states.

Extensive Property

Property that depends on the quantity of matter present (e.g., ΔH, mass, volume).

Intensive Property

Property independent of the amount of substance (e.g., temperature, ΔH per mole).

Standard State

Reference condition of 1 bar pressure for gases, 1 M concentration for solutions, and pure substances at 1 bar and 298 K unless otherwise noted.

Standard Enthalpy Change (ΔH°)

Enthalpy change for a process when all reactants and products are in their standard states.

Standard Enthalpy of Formation (ΔHf°)

Enthalpy change when exactly 1 mol of a compound forms from its elements in their most stable standard states.

Standard Enthalpy of Combustion (ΔHc°)

Enthalpy change when 1 mol of a substance burns completely in oxygen under standard-state conditions.

Hess’s Law

If a reaction is carried out in several steps, the overall ΔH equals the sum of the ΔH values for each step because enthalpy is a state function.

Zero ΔHf° of Elements

The standard enthalpy of formation of any element in its most stable form (e.g., O2(g), C(s, graphite)) is defined as zero.

Energy Equation ΔU = q + w

Fundamental relation linking changes in internal energy with heat and work transfers for a system.

Stoichiometric Coefficient (n) in ΔH Calculations

The multiplier used with ΔHf° values when applying Σ nΔHf°(products) − Σ nΔHf°(reactants) to find a reaction’s standard enthalpy change.

Hess’s Law Using Formation Enthalpies

ΔH°reaction = Σ nΔHf°(products) − Σ nΔHf°(reactants), enabling calculation of reaction enthalpy from tabulated ΔHf° values.