The First Law of Thermodynamics: This is the law of conservation of energy. The total amount of energy in the universe is constant. Energy cannot be created nor destroyed.
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We have seen that energy can be transferred from a system to its surroundings and vice versa. During these processes no energy is created nor destroyed
Enthalpy Changes: Energy is used to break bonds, and energy is then released when new bonds are formed. The difference between these energies is the enthalpy change.
Bond Energy: The energy required to break a bond. High energies correlate to very stable bonds.
|Bond Type||Bond Energy
(kJ / mol)
|H – H||436|
|C – H||413|
|N – H||391|
|C – C||346|
|C = C||615|
|C – N||305|
|O – H||436|
|C – O||358|
|C = O||749|
|N – O||222|
Spontaneous Reactions: Reactions that occur naturally and do not have to be forced to occur (by using heat or electricity).
Non-spontaneous reactions: reactions that do not occur unless energy is put in to force them to occur.
Exothermic reactions are often spontaneous. Eg: combustion reactions.
Endothermic reactions are often non-spontaneous Eg: electrolysis and decomposition
However, many endothermic processes are spontaneous
Eg: dissolving ammonium nitrate in water absorbs heat.
Why do reactions yielding less stable products occur spontaneously?
Entropy (S): The measure of disorder or randomness. It can apply to a system, surroundings, or the universe as a whole.
Eg: A deck of cards laid out with all the suits grouped up and in order from ace to king is very ordered. This would have a low entropy. A deck of cards thrown in the air would land randomly on the floor with all the cards mixed up. This would have high entropy.
Second Law of Thermodynamics: The entropy of the universe is always increasing.
- This means that all processes tend to be spontaneous if they increase in entropy
- Processes that decrease entropy are possible only if they are accompanied by an equal or greater increase in the entropy of the universe.
Eg: People building things must break down glucose and fats for energy.
- Spontaneous processes tend to have increases in entropy (meaning more disorder)
Reaction Entropy: ΔS > 0 if;
- There are more moles of product made than moles of reactant used
- Complex molecules are broken into simpler subunits
- A substance changes state from solid to liquid/gas or from liquid to gas
Gibbs Free Energy
Gibbs FreeEnergy (G): The amount of energy that is available to do work. This can be found by combining the enthalpy and entropy of a system.
Spontaneity: The spontaneity of a process depends on whether or not the change in Gibbs free energy is negative (ΔG<0).
ΔG < 0 reaction is spontaneous
ΔG > 0 reaction is not spontaneous