Unit 9: Applications of Thermodynamics

## Entropy

Entropy, S, is the amount of disorder or chaos in a system. More disorder, greater S value.

Standard entropy is S° and measured at 25 celsius

Standard entropy change ∆S° is measure at the end of a reaction

∆S° = (sum of ∆S° products) - (sum of ∆S° reactants)

If a reaction goes from less moles to more moles (such as 2 moles on the reactant side to 3 moles on the product side) there is more disorder and a positive ∆S

If a reaction goes from a gas to liquid, liquid to solid, or gas to solid, the reaction has a negative ∆S

If bonds are broken and phase change becomes more disordered, the ∆S is positive

## Gibbs Free Energy

∆G is

**Gibbs Free Energy**which determines if a process is thermodynamically favored or unfavored, also known as spontaneous or nonspontaneous

### Free Energy Change

Standard free energy change, ∆G°, is calculated the same as ∆S°

∆G° = (sum of ∆G° products) - (sum of ∆G° reactants)

For a reaction,

If ∆G is negative, it is TFP (thermodynamically favored process)

If ∆G is positive, it is not TFP

If ∆G is 0, it is at equilibrium

### ∆G, ∆H, and ∆S

TFP must result in decreasing enthalpy, increasing entropy, or both

∆G° = ∆H° - T∆S°

T = temperature in Kelvin

∆S° is usually given in j/mol*K and must be converted to kj/mol*K

Gibbs Free Energy is usually kj/mol*K

∆H | ∆S | T | ∆G | Favorability |
---|---|---|---|---|

- | + | LowHigh | -- | Always TFP |

+ | - | LowHigh | ++ | Never TFP |

+ | + | Low High | +- | Not TFPTFP |

- | - | Low High | -+ | TFPNot TFP |

### Standard Free Energy Change and the Equilibrium Constant

Gibbs free energy can be calculated if equilibrium constant is known

∆G° = -RT(ln K)

R = gas constant (8.31 j/mol*k)

T = kelvin temperature

K = equilibrium constant

If ∆G° is negative, K is greater than 1, the products are favored at equilibrium

If ∆G° is positive, K must be less than 1, the reactants are favored at equilibrium

### Reduction Potentials

Every half reaction has electric potential. Potentials are given as reduction half-reactions. If the reaction is reversed, flip the sign to get the oxidation potential

## Galvanic Cells

**Galvanic cells**(voltaic cell) use favored redox reactions to generate currentTwo half-reactions take place in separate chambers and the electrons from the oxidation pass to the reduction reaction which creates the current

Current is defined as the flow of electrons from one place to another

Oxidation takes place at the

**anode**electrode and reduction takes place at the**cathode**electrodeThe salt bridge keeps electrical neutrality. Without the salt bridge the voltage would be zero. The potassium ion flows to the cathode and the chlorine flows to the anode.

The cell voltage is equal to the total redox reaction voltage.

### Non-Standard Conditions

Reduction potentials are give at standard conditions, 25 celsius, 1 atm, and 1 M

Voltaic cells are very favored with equilibrium constant greater than 1. If the Q = K however, the voltage would drop to ero.

If the reaction quotient increased it would become close to the equilibrium constant and the voltage would decrease.

## Electrolytic Cells

Electrolytic cells use outside voltage sources to power unfavored redox reactions and mainly occur in aqueous solutions.

The sign of total cell potential is always negative

### Electroplating

Electrolytic cells are used for electroplating.

I = (q/t)

I = Current (amperes, A)

q = charge (coulombs, C)

t = time (second, s)

Moles of electrons = (coulombs/ 96,500 coulombs per mol)

## Voltage and Favorability

Redox is favored if the potetial has a positive value. reaction potential can be calculate gibb’s free energy

∆G° = -nFE°

n = number of moles of electrons exchanged in the reaction

F = Faraday’s constant. 96,500 coulombs/mol

E° = standard reaction potential (V)

If E° is positive, ∆G° is negative and is TFP

If E° is negative, ∆G° is positive and not TFP

# Unit 9: Applications of Thermodynamics

## Entropy

Entropy, S, is the amount of disorder or chaos in a system. More disorder, greater S value.

Standard entropy is S° and measured at 25 celsius

Standard entropy change ∆S° is measure at the end of a reaction

∆S° = (sum of ∆S° products) - (sum of ∆S° reactants)

If a reaction goes from less moles to more moles (such as 2 moles on the reactant side to 3 moles on the product side) there is more disorder and a positive ∆S

If a reaction goes from a gas to liquid, liquid to solid, or gas to solid, the reaction has a negative ∆S

If bonds are broken and phase change becomes more disordered, the ∆S is positive

## Gibbs Free Energy

∆G is

**Gibbs Free Energy**which determines if a process is thermodynamically favored or unfavored, also known as spontaneous or nonspontaneous

### Free Energy Change

Standard free energy change, ∆G°, is calculated the same as ∆S°

∆G° = (sum of ∆G° products) - (sum of ∆G° reactants)

For a reaction,

If ∆G is negative, it is TFP (thermodynamically favored process)

If ∆G is positive, it is not TFP

If ∆G is 0, it is at equilibrium

### ∆G, ∆H, and ∆S

TFP must result in decreasing enthalpy, increasing entropy, or both

∆G° = ∆H° - T∆S°

T = temperature in Kelvin

∆S° is usually given in j/mol*K and must be converted to kj/mol*K

Gibbs Free Energy is usually kj/mol*K

∆H | ∆S | T | ∆G | Favorability |
---|---|---|---|---|

- | + | LowHigh | -- | Always TFP |

+ | - | LowHigh | ++ | Never TFP |

+ | + | Low High | +- | Not TFPTFP |

- | - | Low High | -+ | TFPNot TFP |

### Standard Free Energy Change and the Equilibrium Constant

Gibbs free energy can be calculated if equilibrium constant is known

∆G° = -RT(ln K)

R = gas constant (8.31 j/mol*k)

T = kelvin temperature

K = equilibrium constant

If ∆G° is negative, K is greater than 1, the products are favored at equilibrium

If ∆G° is positive, K must be less than 1, the reactants are favored at equilibrium

### Reduction Potentials

Every half reaction has electric potential. Potentials are given as reduction half-reactions. If the reaction is reversed, flip the sign to get the oxidation potential

## Galvanic Cells

**Galvanic cells**(voltaic cell) use favored redox reactions to generate currentTwo half-reactions take place in separate chambers and the electrons from the oxidation pass to the reduction reaction which creates the current

Current is defined as the flow of electrons from one place to another

Oxidation takes place at the

**anode**electrode and reduction takes place at the**cathode**electrodeThe salt bridge keeps electrical neutrality. Without the salt bridge the voltage would be zero. The potassium ion flows to the cathode and the chlorine flows to the anode.

The cell voltage is equal to the total redox reaction voltage.

### Non-Standard Conditions

Reduction potentials are give at standard conditions, 25 celsius, 1 atm, and 1 M

Voltaic cells are very favored with equilibrium constant greater than 1. If the Q = K however, the voltage would drop to ero.

If the reaction quotient increased it would become close to the equilibrium constant and the voltage would decrease.

## Electrolytic Cells

Electrolytic cells use outside voltage sources to power unfavored redox reactions and mainly occur in aqueous solutions.

The sign of total cell potential is always negative

### Electroplating

Electrolytic cells are used for electroplating.

I = (q/t)

I = Current (amperes, A)

q = charge (coulombs, C)

t = time (second, s)

Moles of electrons = (coulombs/ 96,500 coulombs per mol)

## Voltage and Favorability

Redox is favored if the potetial has a positive value. reaction potential can be calculate gibb’s free energy

∆G° = -nFE°

n = number of moles of electrons exchanged in the reaction

F = Faraday’s constant. 96,500 coulombs/mol

E° = standard reaction potential (V)

If E° is positive, ∆G° is negative and is TFP

If E° is negative, ∆G° is positive and not TFP