Reaction Spontaneity

1. Spontaneous Process First Law of Thermodynamics- “Conservation of Energy” – Energy can change form but it cannot be created or destroyed.  Heat can be transformed into work and work can be transformed into heat.  Work is the transfer of energy through mechanical means. It is when a force (push or pull) is done over a distance. Spontaneous Process – a physical or chemical change that occurs with no outside intervention or with a small amount of energy to get the process started.

Examples of spontaneous processes include ice melting, iron rusting, propane burning. Spontaneity depends on:  Change in Enthalpy,  H  Temperature, T (in Kelvin)  Change in Entropy,  S

2. Entropy Entropy, S – the amount of disorder or randomness in a system. The amount of chaos! 2 nd Law of Thermodynamics (Law of Disorder)  A spontaneous process always increases the entropy (disorder) of the universe.  The universe is becoming more chaotic!  The only way to stop this process is to do work – apply mechanical energy to stop this process.

The entropy is low in the first flask. Open the valve, and the particles are moving in straight lines with random motion. Eventually the gas particles are mixed and the disorder is maximized. Nothing was done to make this process happen. Spontaneous process and entropy increased.

Which has more Entropy?  

Change in Entropy,  S system   S system = S products – S reactants  Entropy increases (more disorder) S products > S reactants +  S system  Entropy decreases (less disorder) S products < S reactants -  S system

Changes in Entropy : 1. Entropy changes with changes in state – Solid  Liquid : +  S Liquid  Gas : +  S Gas  Solid : -  S Gas  Liquid : -  S 2. Entropy decreases when a gas dissolves into a solvent. O 2(g)  O 2(aq) : -  S

3. The entropy increases when there is an increase of gaseous products. 2SO 3(g) + CO 2(g)  CS 2(g) + 4O 2(g) : +  S 3 molecules of gas  5 molecules of gas 4. The entropy usually increases when you dissolve a solid or liquid to form a solution. Cu 2 SO 4  Cu 2+ (aq) + SO 4 2- (aq) : +  S

5. The entropy increases when the temperature increases. Temperature  +  S 6. Entropy increases during an exothermic reaction. -  H system : +  S system

3. Gibbs Free Energy Gibbs Free Energy, G, is the energy that is useful for doing work. Transforming chemical energy into heat. This heat can be used to do work. Depends on:  Change in enthalpy,  H system  Temperature, T (in Kelvin)  Change in entropy,  S system  G system =  H system - T  S system

When the reaction occurs at standard conditions (298K and 1 atm)   Gº system =  Hº system - T  Sº system -  G system : the reaction IS spontaneous. +  G system : the reaction IS NOT spontaneous.  G system = 0 : the reaction is in chemical equilibrium. The reaction is reversible. The rate that the products are formed is equal to the rate that the reactants are formed.  N 2(g) + 3H 2(g)  2NH 3(g)

Always Spontaneous :  Exothermic and becoming more disorderly  -  H system and +  S system gives a -  G system  -  H system - (T)(+  S system ) = -  G system Never Spontaneous :  Endothermic and becoming less disorderly  +  H system and -  S system gives a +  G system  +  H system - (T)(-  S system ) = +  G system

Can be spontaneous at low temperatures :  Exothermic and becoming less disorderly  -  H system and -  S system could give a -  G system  -  H system - (Low T)(-  S system ) Can be spontaneous at higher temperatures :  Endothermic and becoming more disorderly  +  H system and +  S system could give a -  G system  +  H system - (High T)(+  S system )

Examples Homework – Reaction Spontaneity