AP Chem Driving Force Power point 3/5/2025

What’s the second law of thermodynamics?

Universe loves disorder; spontaneous processes increase entropy (ΔS).

What’s the third law of thermodynamics?

At absolute zero (0 K), a perfect crystal has zero entropy.

What does entropy (ΔS) measure?

The amount of disorder or randomness in a system.

How do you predict the sign of ΔS for a reaction?

More gas molecules or phase change to gas = +ΔS

; more order or phase change to solid = -ΔS.

More disorder mean ΔS is

+ΔS

More order means ΔS

-ΔS

Example of more disorder

solid to liquid to gas (+ΔS)

Example of more order

gas to liquid to solid (-ΔS)

What is the formula for entropy change in a reaction?

ΔS° = ΣS°(products) - ΣS°(reactants).

What is Gibbs Free Energy (ΔG) used for?

Determines if a reaction is spontaneous (-ΔG) or non-spontaneous (+ΔG).

Reaction is spontaneous when ΔG is

-ΔG

Reaction is not spontaneous when ΔG is

+ΔG

What does it mean for something to be spontaneous?

(-ΔG) It happens naturally without external influence or energy input.

What does it mean for something to be non spontaneous?

(+ΔG) It requires external energy or influence to occur.

What is the Gibbs Free Energy equation?

ΔG = ΔH - TΔS.

What does ΔG < 0 mean?

The reaction is spontaneous.

What does ΔG = 0 mean?

The reaction is at equilibrium.

What does ΔG > 0 mean?

The reaction is not spontaneous; energy input is required.

If the reaction is not spontaneous then the ____ is spontaneous?

Reverse Reaction

If ΔH is negative and ΔS is positive, what happens?

Always spontaneous at all temperatures.

If ΔH is positive and ΔS is positive, what happens?

Spontaneous at high temperatures (TΔS dominates).

If ΔH is negative and ΔS is negative, what happens?

Spontaneous at low temperatures (ΔH dominates).

If ΔH is positive and ΔS is negative, what happens?

Never spontaneous; reverse reaction is spontaneous at all temps.

Why do we set ΔG = 0 when solving for temperature?

It marks equilibrium, where the reaction flips between spontaneous & non-spontaneous.

What is the equation to find temperature at equilibrium?

T = ΔH / ΔS (set ΔG = 0 and solve).

At what temperature is boiling at equilibrium or even possible? What is ΔG = to?

At the boiling point of a substance (ex. water 100 C); ΔG=0

What happens if ΔG is negative for a phase change?

The phase change is spontaneous at that temperature.

How can you quickly determine spontaneity?

Look at ΔH (heat flow) and ΔS (disorder) together.

When does entropy (ΔS) increase?

When a gas is formed, number of molecules increases, or a solid/liquid dissolves.

When does entropy (ΔS) decrease?

When a reaction creates more order (gas → liquid → solid).

What’s a quick trick to remember spontaneity?

“High Temp Helps Entropy, Low Temp Helps Enthalpy.”