Energy and Stability Notes

Energy & Stability

Potential Energy and Stability

• A rock at the edge of a cliff has potential energy due to its mass and height.
• When pushed, the rock falls, and its potential energy is converted into heat in the rock and its surroundings.
• "Heat" and "thermal energy" are used interchangeably.
• A rock on the edge of a cliff is unstable.
• It can fall with a small push until it hits the ground, where it becomes stable.
• The rock at the bottom of the cliff is stable and will not spontaneously return to the top.
• The heat energy from its fall cannot launch the rock upwards; energy must be supplied to carry it back up, equal to the energy released when it fell.

Spontaneous vs. Nonspontaneous Actions

• Falling down is spontaneous; no outside energy is needed once started.
• Falling up is nonspontaneous; energy is needed to get the object back up.

Energy in Chemical Reactions

• Energy is stored in chemical bonds.
• Chemical reactions involve the rearrangement of atoms, forming or breaking chemical bonds.
• Energy is released when atoms "fall" together to form a bond.
• Energy is required to break a bond and separate the atoms.
• Bond formation: $+ Energy$
• Bond breakage: $+ Energy$

Covalent Bonds

• A covalent bond forms when atoms share electrons to achieve a stable electron configuration.
• A covalent bond that yields a more stable system will form spontaneously.
• Covalent bonds can be very strong and require a large amount of energy to break.
• The energy needed to break the bond is equal to the energy released when the bond formed, similar to lifting the rock back up the cliff.

Nonspontaneous Reactions

• Chemical reactions can proceed in both directions: products can become reactants.
• However, reactions tend to be spontaneous in one direction and nonspontaneous in the other.

Spontaneous and Nonspontaneous Reactions Example: H + H ⇄ H2

• $H + H \rightarrow H_2$ : Spontaneous; energy is released during bond formation.
• $H_2 \rightarrow H + H$ : Nonspontaneous; energy must be added to separate the H atoms.
• H atoms are more stable when joined as an $H_2$ molecule than when separated.
• The H atoms "fall" together to form $H_2$.

Energy, Stability, and the Formation of H2

• Two separate H atoms are unstable.
• They spontaneously form an $H_2$ molecule, releasing energy.
• It is only a matter of time before they combine, form a molecule, and release energy, similar to the rock falling off the cliff.
• Stability tends to increase over time.

Stability of H2 vs. H

• $H_2$ is more stable than H.
• In spontaneous reactions, the products are more stable than the reactants.
• Breaking apart $H_2$ requires inputting the same amount of energy released during its formation.
• $2H \rightarrow H_2 + Energy Released$

Thermal Energy and Bond Formation

• When a rock falls, potential energy decreases and transforms into kinetic energy, sound, and ultimately heat.
• The same occurs when $H_2$ is formed; heat is released as atoms bond.
• The equation is written as $2H \rightarrow H<em>2 + Heat$ or $2H \rightarrow H</em>2 + Energy Released$ to indicate heat release.