Glossary
Enthalpy Change of a Reaction (ΔHᵣₓₙ)
The total change in enthalpy that occurs during a chemical reaction, representing the heat absorbed or released at constant pressure.
Example:
A negative Enthalpy Change of a Reaction for combustion indicates that heat is released, making it an exothermic process.
Most Stable State
The physical and allotropic form of an element that is naturally most stable at standard conditions (25°C and 1 atm).
Example:
Oxygen exists as diatomic gas, O₂(g), which is its Most Stable State at room temperature and pressure.
Standard Conditions
A specific set of conditions (25°C or 298 K and 1 atm pressure) under which thermodynamic properties like enthalpy of formation are typically measured and reported.
Example:
When you see ΔH° values, it implies that the reaction is occurring under Standard Conditions.
Standard Enthalpy of Formation (ΔHᶠ)
The change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable states under standard conditions (25°C and 1 atm).
Example:
The Standard Enthalpy of Formation of CO₂(g) is the enthalpy change when solid graphite and gaseous O₂ combine to form one mole of CO₂(g).
State Symbols
Letters in parentheses (s, l, g, aq) appended to chemical formulas in an equation to denote the physical state of a substance (solid, liquid, gas, or aqueous solution).
Example:
Always check the State Symbols; the ΔHᶠ for H₂O(g) is different from H₂O(l), which can significantly impact your calculations.
Stoichiometric Coefficients
The numerical values placed in front of chemical formulas in a balanced chemical equation, indicating the relative number of moles of each reactant and product involved.
Example:
In the balanced equation 2H₂(g) + O₂(g) → 2H₂O(l), the Stoichiometric Coefficients are 2 for hydrogen, 1 for oxygen, and 2 for water.
ΔHᶠ for elements in their standard states
The standard enthalpy of formation for any element in its most stable form at standard conditions is defined as zero, serving as a reference point for enthalpy calculations.
Example:
Because iron is a solid at room temperature, the ΔHᶠ for elements in their standard states means ΔHᶠ of Fe(s) is 0 kJ/mol.