# Standard Enthalpy of Reaction

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Equation / Last modified by mike on 2015/07/30 06:48
Delta H_r^o =
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The theory behind the Standard Enthalpy of Reaction, DeltaH_r^0, asserts that the associated energy of a reaction is equal to the sum of the standard enthalpies of formation of the products subtracted by the sum of the standard enthalpies of formation of the reactants.

The Standard Enthalpy of Reaction is the enthalpy change (change in chemical potential energy of the system) that occurs when one mole of matter undergoes a chemical reaction under standard conditions.  The Standard Enthalpy of Reaction is typically measured in units of kJ/mole.

The symbol of the standard enthalpy of formation is DeltaH_f^0.

&Delta; = Denotes a change in enthalpy
0 = A degree signature that denotes it's a standard enthalpy change.
f = it is a reaction from a substance that's formed from its elements

The inputs to this equation are two summations of enthalpies of formation:

Sigma_"products" = sum_"products" v_i * (DeltaH_f^0)_i

Sigma_"reactants" = sum_"reactants" v_i * (DeltaH_f^0)_i

# Notes

Enthalpy, H, of the system is equal to the internal energy of the system, U,  plus the product of pressure and volume of the system, PV.  The enthalpy of a homogeneous systems is proportional to the size of the system. For various molecular substances the specific enthalpy can be specified as H/m, where M is mass.

The Standard Enthalpy of a Reaction, DeltaH_r^0, is computed under standard conditions that include:

• standard temperature - 25 degrees C
• standard pressure (for gases) - 1 atmosphere
• standard Molar concentration (for solute in an ideal solution) - 1 M