Enthalpy 4.1+4.2

What is enthalpy change ?

When you measure a heat change at constant pressure

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The standard conditions for measuring enthalpy chnages :

100kpa

298k and 25 degrees

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What is an exothermic reaction ?

* In an exothermic reaction, energy is released to the surroundings.
* In an exothermic change, energy is transferred from the system to the surroundings
* The products have less energy than the reactants
* The enthalpy change is negative

What is endothermic reaction ?

* In an endothermic change, energy is transferred from the surroundings to the system .

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* They require an input of heat energy
* Products have more energy than the reactants
* The enthalpy change is positive

What is bond braking ?

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* To break a bond, energy needs to be absorbed.
* Bonds are broken in the reactants and this is an endothermic process, so enthalpy change is positive.


* If more energy is needed to break bonds, >energy needed to form them=endothermic.

Bonds made ?

* When bonds are formed energy is released
* Bonds are made when products are being produced and this is an exothermic reaction so enthalpy change is negative .
* if more energy is released when bonds are formed >than the energy needed to break the initial bonds=exothermic

How to calculate enthalpy change ?

Mean bond enthalpy

Total energy to break bonds - total energy released forming bonds