# 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