Examples of 1st Law for Reacting Systems Reference Page
React Hard and React Harder

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Intro to the Examples of the 1st Law for Reacting Systems

Applying the First Law to reacting systems requires combining energy balances, heats of reaction, and heat capacity data to determine the heat transferred or temperature change during a chemical reaction.

These videos work through detailed examples of energy balances for reacting systems using both the heat-of-reaction method and the heat-of-formation method.

React Hard: The First Law Gets Chemical

This video walks through a detailed example applying the First Law of Thermodynamics to a reacting system using tabulated thermodynamic data.

Visuals

React Harder: Formation Heats and Alternate Endings

This video continues the example calculations and shows how the same problem can be solved using alternative thermodynamic methods.

Visuals

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.

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