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Enthalpy

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Aparna Pothukuchi

on 12 April 2014

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Transcript of Enthalpy

Enthalpy
Heat of Formantion
enthalpy of
forming
a compound from its elemental components

When bonds break, energy must be absorbed therefore bond breaking is always endothermic(+). When bonds form, energy is always released therefore bond forming is always exothermic(-).

To calculate enthalpy using heat of formation, imagine that you have to form every bond within the reactants and break every bond within the products.


Reactants(-1) + Products= Reaction

Bond energy
When bonds break, energy must be absorbed therefore bond breaking is always endothermic(+). When bonds form, energy is always released therefore bond forming is always exothermic(-).

To calculate enthalpy from bond energies, imagine that you have to break every bond within the reactants and form every bond within the products.

Reactant bonds+ Product bonds(-1)= Reaction
Conclusion
What is Enthalpy?
Enthalpy is the change in energy for a reaction.It is given the symbol H. The enthalpy change in a physical or chemical transformation is written ΔH, defined as Hfinal - Hinitial. If the ΔH is greater than 0 then it is endothermic which means energy is being absorbed. If ΔH is less than 0 it is exothermic which means energy is lost.
Bond Energy Practice Problem #1
Bond Energy Practice Problem #2
Heat of Formation Practice Problem #2
How can one predict the sign of enthalpy?
For physical processes like melting and freezing, predicting the sign of enthalpy requires just a few steps.

First you have to see if the physical state of the substance is changing from a solid to liquid/gas or changing from a liquid/gas to a solid.
If it is going from a solid to a liquid or gas, then energy is being added to break the bonds. This will always be endothermic( +). However if it is going from a gas to a solid then bonds are being formed which will be exothermic(-)
Heat of Formation Practice Problem #1
2(-32)(-1) + 2(-286)(-1)+ 4(0)+2(-21)+0= 594 kJ
3(0)(-1) + 2(143)= 286 kJ
H--H = 432
Cl--Cl = 239
H--Cl = 427
432+ 239+ 2(427)(-1)= -183 kJ
C--H = 413
O==O = 495
C==O = 745
O--H = 467
4(413)+ 2(495)+ (2)(745)(-1)+ 4(467)(-1)= -716 kJ
Enthalpy is the change in a reaction and you can use heat of formation and bond energies to find it. You can figure out the sign of enthalpy by looking at the states of matter. If it moves from a solid to a liquid its going to be positive but if it moves from a gas to a solid its going to be negative.Breaking bonds is endothermic and forming bonds is exothermic. So when you calculate heat of formation problems, the reactant(s) has to be multiplied by -1 and for bond energy the product bond(s) has to be multiplied by -1.
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