LearnChemE

#### 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.