## AP Chem Unit 17 Study Guide

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Chapter 17: Spontaneity, Entropy, and Free Energy

Important Terms

Important Terms

- Thermodynamics - The study of energy transformations and the relationships among the physical properties of substances which are affected by these transformations

- Can be fast or slow (most are fast)
- Ex. A bike rusting is a spontaneous reaction because Iron rusts when it comes in contact with water. There is not outside force that influences the process from occurring, rather it is naturally occurring. On the flip side, if you have a rusted bike you cannot spontaneously transform it into a clean one because that does not occur naturally and you would need the help of other acidic chemicals to get the job done.

- Reversible process - A process or cycle where the net change at each stage in the combined entropy of the system and its surroundings is zero
- State and energy remains the same

- Irreversible process - A process or cycle where a certain amount of heat energy is lost due to intermolecular friction and collisions
- Some energy cannot be recovered even when the process is reversed

- Entropy (S) - Measure of randomness or disorder
- The Driving force for a spontaneous reaction is an increase in entropy
- Ex. Imagine 100 red solo cups lying all over the floor. There can be a million different arrangements or microstates for the cups on the floor. Thus, there is high entropy. Now imagine a giant tower of these 100 red solo cups. There is only one possible arrangement, meaning that there is low entropy.

- The Driving force for a spontaneous reaction is an increase in entropy
- Free Energy (ΔG) - The ability of a reaction to do work or be spontaneous
- Positional Entropy - The probability of occurrence of a particular state
- Depends on the number of ways (microstates) in which that arrangement can be achieved
- S solid < S liquid << S gas

- Depends on the number of ways (microstates) in which that arrangement can be achieved

Laws of Thermodynamics

- Zeroth Law - If two thermodynamic systems are in equilibrium with a third, they are also in thermal equilibrium with each other
- If A = B and B = C, then A = C

- If A = B and B = C, then A = C
- First Law - Energy can neither be created nor destroyed
- The energy of the universe remains constant

- The energy of the universe remains constant
- Second Law - Entropy is always increasing (the universe becomes more random)
- Every energy transfer or transformation increases the entropy of the universe

- Every energy transfer or transformation increases the entropy of the universe
- Third Law - As temperature approaches 0 Kelvin (absolute zero), entropy also approaches 0

Tendencies for Entropy (Increased disorder)

Positive/Negative Enthalpy, Entropy, and Free Energy

- Moles of gas formed
- Gas > liquid > solid
- Aqueous > solid
- Greater # of particles
- Greater volume formed
- More complex molecule formed
- Examples:
- 2 NH3 ➝ N2 + 3H2
- NaCl(s) ➝ NaCl(g)

Positive/Negative Enthalpy, Entropy, and Free Energy

Value |
Positive |
Negative |

ΔH Enthalpy/Heat Content |
- Not Favorable - Endothermic (feels cold) - Non spontaneous - Heat gained by system |
- Favorable - Exothermic (feels warm) - Spontaneous - Heat lost by system |

ΔS Entropy/Disorder |
- Spontaneous - Less organized - Favorable |
- Non spontaneous - More organized - Not Favorable |

ΔG Free Energy The ability to do work |
- Non spontaneous in the forward reaction - Spontaneous in reverse - Work done on the system |
- Spontaneous in the forward reaction - Not spontaneous in reverse - Work done by the system |

H, S, G and Spontaneity

ΔH |
TΔS |
ΔG |
Spontaneity |

negative |
positive |
negative |
spontaneous |

positive |
negative |
positive |
nonspontaneous |

negative |
negative |
? |
Spontaneous in low temperatures |

positive |
positive |
? |
Spontaneous in high temperatures |

Equations

- ΔS react = ∑S react - ∑S product
- ΔH react = ∑H react - ∑H product
- ΔG react = ∑G react - ∑G product
- ΔG = ΔH - TΔS
- ΔG = -RTln(K)
- R = 8.3145
- T = temp
- K = equilibrium value

- R = 8.3145
- ΔG = ΔG formation + RTln(K)