Enthalpy change when one mole of a compound forms from its elements in standard states.
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The standard enthalpy of formation, symbolized delta Hf standard, is the enthalpy change when one mole of a compound forms from its constituent elements, each in its standard state at 1 bar and usually 298 K. By convention, the formation enthalpy of any element in its most stable standard form is zero. These values are tabulated for thousands of compounds, and they allow reaction enthalpies to be computed by subtracting reactant formation values from product values. A negative formation enthalpy indicates that the compound is more stable than its elements, releasing energy when it forms. Because all values share a common baseline, formation enthalpies provide a consistent scale for comparing compounds.
Key ideas: Formation reactions form exactly one mole of the compound from elements in standard states, so the equation may need fractional coefficients, such as one half O2. Use the formation equation to check that you have the right quantity. Elements in standard states are zero, including O2 gas, graphite and liquid bromine.
Common mistakes and connections: Common errors include assigning a nonzero value to an element in its standard state or using a form that is not standard, such as diamond for carbon. Remember that a negative value indicates energy release on formation, not stability in all senses. These values feed straight into reaction enthalpy calculations.
Standard enthalpies of formation from the elements. Negative values mean the compound is more stable than its elements; elements in standard states are zero. The interactive lesson shows this as a live animation or chart.
The standard enthalpy of formation of gaseous carbon dioxide is -393.5 kJ per mole, the energy released when carbon and oxygen combine to form it.
Enthalpy change when one mole of a compound forms from its elements in standard states.
Standard Enthalpy of Formation is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Bomb Calorimeter, Coffee-Cup Calorimeter, Thermochemical Equation.
The next topics in Thermodynamics & Energetics are Standard State and Enthalpy of Combustion.
Yes. The interactive lesson includes a self-check question on Standard Enthalpy of Formation, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Standard Enthalpy of Formation is topic 19 of 91 in Thermodynamics & Energetics.