Spontaneous Reactions

​A spontaneous reaction proceeds on its own once the activation energy (and other conditions) is reached.

• A naturally occurring reaction
• Example:  Rusting of iron, this occurs under normal conditions in the environment

Spontaneous does NOT equal fast. Some spontaneous reactions are fast (i.e. fire, explosions) and others are very slow (rusting of metal). Spontaneous reactions GREATLY favor the product side of a chemical equation.

• Example:  H2CO3  → CO2 + H2O
  • Almost ALL of the mass in this reaction will be products

Spontaneous reactions are exothermic. They release energy in the form of heat or light, and the ∆H (enthalpy) will be negative. This released energy is called free energy, and can be used to do work (i.e. run a car engine).

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Entropy

Entropy (ΔS) is a measure of the randomness, or disorder, of the reactants or products. The universe wants to be in disorder, it takes energy to put things (molecules, atoms, particles) into a structured order (think of your bedroom, it takes energy to keep it clean). The universe tends towards chaos , so, spontaneous reactions are those that increase the amount of randomness, or disorder which means you will have a POSITIVE ∆SExample: a car will rust, degrade, and fall apart over time, it will NEVER unrust and become new again

How is entropy increased?

• ​Solid → liquid → gas
• A substance → parts of the substance
• Fewer number of particles → greater number of particles
• Slower movement of molecules → faster movement of molecules
• Fewer substances → more substances
• Diffusion of one substance through another

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Gibbs Free Energy

• Entropy (ΔS):  Entropy is a measure of the randomness, or disorder, of the reactants or products
• Enthalpy (ΔH): Enthalpy is the change in thermal energy associated with a chemical reaction
• Gibbs Free Energy Equation relates enthalpy and entropy:  ΔG = ΔH – TΔS
  • G = Energy given off by a reaction, Gibbs Free Energy
  • H =  Enthalpy of the system
  • S = Entropy of the system
  • T = Temperature the reaction occurs at, in Kelvin

Watch the following video!

https://youtu.be/huKBuShAa1w

Gibbs Free Energy equation: 

•  ΔG = ΔH – TΔS
• This equation can be use to determine if a reaction is spontaneous, or non-spontaneous

Example:  You have a change in enthalpy of -986 kJ/mol, a change in entropy of 0.33, at a temperature of 288 K.  What is the free energy of the reaction? Is the reaction spontaneous, or non-spontaneous?

• ΔG = ΔH – TΔS 
•  ΔG = -986 – 288 x .33 = -1081.04 kJ/mol
• It is negative, so you have a spontaneous reaction.

Watch the following video: https://safeshare.tv/x/ss59072b82d2bf1

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Is It Spontaneous?