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Definition
Definition
The enthalpy change when one mole of that substance is burned in excess oxygen in their standard states under standard conditions
Enthalpy of reaction from enthalpies of combustion
If you burned all the products you should get the same amounts of oxidation products such a CO2 and H2O as if you burned the reactants.
Enthalpy of combustion is an exothermic process
Enthalpy of reaction from enthalpies of combustion
Step 1 Energy is released as reactants
undergo combustion.
Step 2 Energy is released as products
undergo combustion.
DHr = Step 1 - Step 2
Because, in Step 2 the route involves going in the OPPOSITE DIRECTION to the defined change of Enthalpy of Combustion, it’s value is subtracted.
Sample Calculation
Sample Calculation
Calculate the standard enthalpy of formation of methane; the standard enthalpies of combustion of carbon, hydrogen and methane are -394, -286 and -890 kJ mol-1 .
By applying Hess’s Law ... The Standard Enthalpy of Reaction DH°r will be...
REACTANTS PRODUCTS
[ (1 x DHc of C) + (2 x DHc of H2) ] minus [ 1 x DHc of CH4]
DH°r = 1 x (-394) + 2 x (-286) - 1 x (-890)
ANSWER = - 76 kJ mol-1
Activities
Activities
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