Enthalpy and Entropy

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Enthalpy, Entropy and Spontaneity : 

Enthalpy, Entropy and Spontaneity

Enthalpy (H) : 

Enthalpy (H) Amount of energy a sample contains at a certain pressure and temperature. (Heat Content) H H is sometimes used in place of q.

Enthalpy Change (DH) : 

Enthalpy Change (DH) Enthalpy cannot be measured. Change in enthalpy is measured during a chemical reaction. H

Heat of Reaction : 

Heat of Reaction Hrxn Heat released or absorbed per one mole of a reactant or product. Release of Heat - Exothermic rxn -H Absorption of Heat - Endothermic rxn +H

Finding Hrxn : 

Finding Hrxn Hrxn = Hproducts – Hreactants Since H is not measured, calorimetry experiments are done.

Example : 

Example 2 H2 + O2  2 H2O Hrxn = -5741.6 kJ Which contains more energy, the products or the reactants? Is this reaction exo- or endo- thermic?

Another Notation : 

Another Notation Sometimes the enthalpy is indicated as part of the chemical equation. C + O2  CO2 -393.5 kJ Energy is a product (exo rxn) H is negative. N2 + O2 + 90 kJ  NO Energy is a reactant (endo rxn) H is positive.

Heat of Combustion : 

Heat of Combustion Amount of heat released in a combustion reaction. Combustion reactions can be recognized by the O2 reactant and the CO2 and H2O products. Hc is always negative.

Entropy (S) : 

Entropy (S) The measure of disorder or randomness of a system. The more disorder there is the greater the entropy. In general changes tend to occur so the highest entropy is reached.

Changes in Entropy : 

Changes in Entropy

Spontaneity : 

Spontaneity Changes that happen on their own. Depends on enthalpy, entropy, and temperature. DG = DH – DST When DG is negative, a reaction will be spontaneous.

Spontaneity : 

Spontaneity An exothermic reaction is more likely to be spontaneous than an endothermic one. A change that results in more entropy is more likely than one that results in less.

Spontaneity : 


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