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Energetics (Enthalpy Change (Reactions can endothermic or exothermic.…
Energetics
Enthalpy Change
Reactions can endothermic or exothermic. Endothermic reactions require energy to happen (KE->PE), exothermic reactions release energy (PE->KE).
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The lower the chemical potential energy of a given species, the more stable it is.
The standard enthalpy of combustion of a substance is the enthalpy change when one mole of that substance is burned in excess oxygen and under standard conditions. It is given the symbol ∆Hc.
The standard enthalpy of formation of a substance is the enthalpy change when one mole of that substance is formed from the most stable allotropes of its elements in their standard states under standard conditions. It is given the symbol ∆Hf.
A negative enthalpy value means that the reaction is exothermic and a positive value means that the reaction is endothermic.
Calorimetry
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∆H=q/n
q is the heat change, n is the number of moles
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Bond Enthalpy
The mean bond enthalpy is the average amount of energy needed to break a specific type of bond, measured over a wide variety of different molecules.
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The value for the bond enthalpy is different when using Hess' Law because the bond enthalpy will be specific for that reaction whereas the accepted value is a mean of all the different enthalpies in varying reactions.
