Free Energy and Chemical Reactions Notes

Spontaneous Chemical Reactions

• Some chemical reactions occur spontaneously, requiring little energy from their surroundings to proceed.
• Spontaneous reactions occur at varying rates, from slow (e.g., rusting) to fast (e.g., explosions).
  • Example: Rusting, where iron reacts with oxygen to form rust.

Explosive Spontaneous Reactions

• Some spontaneous chemical reactions occur rapidly, even explosively.
  • Example: $2H<em>2 + O</em>2 \rightarrow 2H_2O$ (Hydrogen and oxygen combine to form water and react explosively.)

Energy and Spontaneous Reactions

• Reactants initially possess an energy state, like a rock atop a cliff.
• Activation energy (a small push) initiates the reaction, releasing energy, often as heat.
• Large energy releases can cause explosions.
• Analogy: Hydrogen and oxygen molecules "falling together" to form water, with a flame providing the activation energy.

Stability of Products

• Water molecules are a stable product of the reaction $2H<em>2 + O</em>2 \rightarrow 2H_2O$.
• Energy is released in the form of heat and light during the explosion.
• Energy is required to break the water molecules apart, reversing the reaction.

Nonspontaneous Reactions

• Breaking $H<em>2O$ into $2H</em>2$ and $O_2$ is a nonspontaneous reaction.
• This reverse reaction requires work, like carrying a rock back up a cliff.
• Nonspontaneous reactions need an input of free energy.
• $2H<em>2O \rightarrow 2H</em>2 + O_2$

Splitting Water

• Electrical energy can be used to split water:
• $2H<em>2O \rightarrow 2H</em>2 + O_2$
  Electrical energy splits water, allowing the separated molecules' energy to be stored for later use.

Storing Energy by Splitting Water

• $2H<em>2O \rightarrow 2H</em>2 + O_2$
• The energy stored in the separated gases can be released later, such as by burning hydrogen to power a car.
• Burning hydrogen produces only water vapor as exhaust.

Energy and Entropy in Water Formation and Splitting

• Forming water from hydrogen and oxygen ( $2H<em>2 + O</em>2 \rightarrow 2H_2O$) releases energy through a spontaneous reaction.
• Splitting water ( $2H<em>2O \rightarrow 2H</em>2 + O_2$) requires an input of free energy during the nonspontaneous reaction.
• Total energy is conserved, but entropy increases when water is split.
• $2H<em>2$ and $O</em>2$ have more potential energy than $2H_2O$.

Energy of Life

• Life requires a constant input of free energy.
• Living things are composed of complex molecules.
• Living organisms take in free energy, transform it, use it to do work, and release heat.
• After each reaction, less free energy is available to do work.

Source of Free Energy

• The sun is the primary source of free energy for life on Earth.
• Without the sun, most life on the surface would cease due to insufficient free energy to perform work.