Enthalpy, Entropy and Spontaneity : Enthalpy, Entropy and Spontaneity Enthalpy (H) : Enthalpy (H) Amount of energy a sample contains at a certain pressure and temperature.
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 : Spontaneity