Heats of Reaction: Screencasts

Explains how to determine heats of reaction at 298 K from heats of formation and how to calculate heats of reaction at elevated temperatures.

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Explains Hess’s law and provides an example of how to use it to solve for the heat of reaction for an equation.

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Uses Hess’s law to show how heat of combustion is used to calculate the heat of reaction. 

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Important Equations:

\[\Delta H_{rxn} = \sum \nu_i \Delta H_{f,i} \]

where \(\Delta H_{rxn} =\) heat of reaction at 298 K, \(\nu_i = \) stoichiometric coefficient for component \(i\), and \(\Delta H_{f,i} = \) heat of formation (enthalpy of formation) or species \(i\) at 298 K.

\[ \Delta H_{reaction,T} = \Delta H_{reaction,298} + \int^{T}_{298} \Delta C_P\,dT \]

Heat capacity (J/(mol K)) can be of the form

\[C_{P,i} = A_i + B_iT + C_iT^2 + D_iT^3\]

where \(C_{P,i}\) is the heat capacity of component \(i\), \( A_i, B_i, C_i, \,and\, D_i\) are the constants for the heat capacity equation for component \(i\), and \(T\) is temperature in Kelvin.

\[\Delta C_P = \sum \nu_i C_{P,i} \]

\[\Delta H_{reaction,T} = \Delta H_{reaction,298} + \sum \nu_i A_i (T-298 + \frac{\sum \nu_i B_i}{2}(T^2-298) + \frac{\sum \nu_i C_i}{3}(T^3 -298^3) +\frac{\sum \nu_i D_i}{4}(T^4-298)\]

\[ \Delta H_{reaction,T} = \Delta H_{C,reactant}^{\circ} – \sum \Delta H_{C,product}^{\circ}\]

where \(\Delta H_{C,reactant}^{\circ} \) is the heat of combustion of a reactant at standard states, and \(\Delta H_{C,product}^{\circ} \) is the heat of combustion of a product at standard states.