All Chemical Reactions Proceed Toward

REACTION DYNAMICS

All chemical reactions proceed toward :

A. Maximum Disorder ( S ) ( + S is favorable )

Entropy – measure of the disorder of a system ( S )  All systems strive toward increased entropy à prefer greater disorder  Nature favors high randomness over order: 1. conversion of solid to liquid or liquid to gas - gases are more disordered than liquids - liquids are more disordered than solids 2. conversion of complex molecules into more simple ones - more smaller molecules/particles from fewer larger ones 3. weakening of ionic forces - breaking of ionic bonds in very structured crystal structures 4. dissolving substances - dissolved substances are more disordered than solids 5. reactions with more moles of product gases than reactant gases - the greater the number of gas molecules the more disorder

B. Minimum Energy ( H ) (- H is favorable)

Enthalpy – heat content of a system ( H )  All systems strive to release energy à prefer to be exothermic - stronger bonds are made than are broken (forming bonds releases energy) - the product will always contain the most powerful interactions

thermochemical equation – chemical equation that includes the amount of energy required or released

heat of reaction- the heat that is released or absorbed during a chemical reaction

If the change in enthalpy is decreasing (giving off heat), the reaction is EXO In exothermic reactions the heat appears on the products side of the equation.

- H = EXO [ H is negative if the reaction is exothermic ]

If the change in enthalpy is increasing (taking in heat), the reaction is ENDO In endothermic reactions the heat appears on the reactants side of the eq.

+ H = ENDO [ H is positive if the reaction is endothermic ] C. Spontaneity – the likelihood of a reaction to proceed without outside intervention -- spontaneous reactions happen by themselves; they can be fast or slow (Not all reactions are spontaneous.)

A reaction will occur spontaneously only if:

1. both enthalpy and entropy are favorable - energy is released (exo) and disorder increases

2. favorable enthalpy overcomes unfavorable entropy - the release of heat overcomes decreased disorder

3. favorable entropy overcomes unfavorable enthalpy - increased disorder overcomes absorption of heat

Example A: Na2CO3 (s)  Na2O (s) + CO2 (g) + 1131.1 kJ

This reaction will occur spontaneously because:

 enthalpy is favorable – reaction is exothermic ( H = -1131.1 kJ)  entropy is favorable – products formed are more disordered than reactants (more simpler molecules formed & more product gases)

Example B: 2 O2 (g) + N2 (g) + 67.7 kJ  2 NO2 (g)

This reaction will not occur spontaneously because:

 enthalpy is unfavorable – reaction is endothermic ( H = + 67.7kJ)  entropy is unfavorable – products formed are less disordered than reactants (fewer complex molecules formed & more reactant gases)

Example C: 3 H2 (g) + N2 (g)  2 NH3 (g) + 46.19 kJ

 Shows favorable enthalpy (reaction is exothermic)  Shows unfavorable entropy (reactants are more disordered than products)

This reaction will occur spontaneously only if favorable enthalpy overcomes unfavorable entropy.

This reaction will not occur spontaneously if unfavorable entropy overcomes favorable enthalpy.