The 1st Law for Reacting Systems Reference Page
Chemical Reactions vs. The First Law: Who Wins?

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Intro to The 1st Law for Reacting Systems

When chemical reactions occur, the First Law of Thermodynamics must account not only for temperature changes but also for the energy changes associated with breaking and forming chemical bonds.

These energy changes are commonly expressed using enthalpies of formation and reaction, which allow the heat effects of chemical reactions to be calculated using tabulated thermodynamic data.

This page introduces the form of the First Law used for reacting systems and explains the thermodynamic quantities needed to perform these calculations.

Chemical Reactions vs. The First Law: Who Wins?

This video explains how the First Law of Thermodynamics is applied to reacting systems and introduces the thermodynamic quantities used to calculate energy changes during chemical reactions.

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Examples and Definitions

Definitions

Heat or Enthalpy of Formation, \(\Delta \hat{H}^\circ_f\)
The enthalpy change associated with forming one mole of a chemical species from its constituent elements at their standard temperature, pressure, and state of aggregation.

By definition, the enthalpy of formation of an element in its standard state is zero.

Heat or Enthalpy of Reaction, \(\Delta \hat{H}^\circ_r\)
The enthalpy change associated with one mole of reaction for a chemical reaction as written.

It can be calculated as the sum of the enthalpies of formation of the reactants and products multiplied by their stoichiometric coefficients, with reactant coefficients taken as negative and product coefficients as positive.

Extent of Reaction, \(\xi\) or \(\dot{\xi}\)
A variable describing the progress of a chemical reaction. Changes in the amount of each species are related to the extent of reaction through the stoichiometric coefficients of the reaction.
Heat Capacity at Constant Pressure, \(C_p\)
The change in specific or molar enthalpy with temperature at constant pressure for a substance not undergoing a phase change.

\[ C_p \equiv \left(\frac{\partial \hat{H}}{\partial T}\right)_P \]