In this video, I show Why Some Reactions EXPLODE and Others COOL: Root Cause of Chemical EnergyI I show how the chemistry behind endothermic and exothermic reactions can be traced to the underlying quantum physics. When molecules react, bonds between atoms are broken, and new ones are formed. Breaking bonds requires energy, while forming new bonds generally releases energy. But the balance of energy between these two processes determines whether a reaction is exothermic or endothermic.
So for example, a methane molecule CH4 has 4 carbon-hydrogen bonds and it consumes some energy to break those into a free carbon and 4 free hydrogens. We get energy back when those free carbons and hydrogens bond with oxygen to form H2O and CO2. This is an exothermic reaction.
Conversely, in photosynthesis, when carbon dioxide and water combine to form the sugar glucose and oxygen, this absorbs energy. Breaking the carbon dioxide and water molecular bond require more energy than the formation of the glucose and oxygen molecules. So since there is an overall absorption of energy, this is called an endothermic reaction.
How do chemical bonds store energy?
This is where quantum mechanics comes in, specifically the Schrodinger equation. This equation shows that electrons exists in regions around the nucleus called orbitals. When two atoms interact to form a molecule, their electron clouds overlap, leading to new quantum states for the electron. If these states are lower in energy, they make the combined structure of the two atoms more stable than the atoms on their own. Excess energy is released in this case.
This concept can be applied in principle to any other atoms that combine to form molecules. Why do certain electron configurations have more or less energy than others, is due to the Pauli Exclusion Principle, which states that no two identical electrons can be at the same place at the same time. This leads to the arrangement of electrons in different orbitals, which are at different energy levels.
These different energy levels provide opportunities to optimize energies when two atoms combine. A chemical reaction between two or more atoms, is about arranging the available electrons of the participating atoms in the most energy-efficient way.
Now the question you might have is, what determines whether certain electron configurations are more energy efficient than others?
We can get a good idea of what the most stable electron configurations are, by looking at the noble elements on the periodic table. The Schrodinger equation shows that these are the most stable electron configurations. These elements have a full complement of electrons in their outermost or valence shell. Just about every bond that forms between atoms to make a molecule, is an attempt by the shared electrons to achieve a full complementary shell like one of the noble elements.
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In endothermic reactions since the products are going to a higher energy state, you might ask how can this reaction occur in the first place since overall energy is not lowered? This is where the second law of thermodynamics links up with quantum mechanics. While minimizing energy is a key concept of physics, we also have to consider entropy of the system as a whole. Nature tends to favor processes that increase entropy in the universe, or increasing disorder and scattering. Even if a reaction absorbs energy, if it increases overall entropy, it can still drive a reaction.
Summary: The driving force behind exothermic and endothermic reactions is the balance between the energy state of the electrons in the resulting molecules, and entropy.
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