Chem Energetics II

What is a spontaneous process?

A process that once started, will continue without any external assistance

What is a non-spontaneous process?

A process that will not occur unless external assistance is continuously applied

What is entropy?

S = A measure of the disorder in the system → The more disordered a system is, the larger the entropy

*Gases have the highest entropy and solids have the lowest

What is the formula for entropy change?

ΔS = S_final - S_initial

What does negative and positive entropy change symbolise?

ΔS > 0 → Final state is more disordered than initial state

ΔS < 0 → Final state is less disordered than initial state

What factors affect the entropy in a chemical system?

• Change in temperature

• Change in phase

• Change in the number of particles

How does change in temperature affect entropy?

Temperature increases → Broadening of the energy distribution of particles → More possible energy states in which particles can adopt at a higher temperature → Entropy increases

*More packets of energy available to spread out within the system → Large number of ways to spread out the large number of packets of energy

What is the order of phases entropy increases in?

Solid < Liquid « Gas

Explain entropy change from solid to liquid

Solid melts into liquid → Order of solid is destroyed → Particles are more randomly arranged and more disordered than those in the solid → Increase in entropy

Explain entropy change from liquid to gas or solid to gas

• Even larger increase in entropy

• Particles in the gaseous state move freely and are the most randomly arranged

• Accompanied by a large volume increase

How does change in number of particles affect entropy change?

More particles → More ways to arrange particles → More ways to distribute energy in the system → Greater disorder in the system, so entropy increases

*Especially for gaseous particles → Increase in no. of moles of gaseous particles

When does dissolution of an ionic solid in water lead to a net increase in disorder?

• Disruption of ionic solid increases disorder → Previously rigidly held ions are now free to move about

• Hydration decreases disorder → Pulls hydrating water molecules into an orderly arrangement about ions

• Increase in entropy due to the disruption of the crystal lattice outweighs the decrease in entropy due to the hydration process

When does dissolution of an ionic solid in water lead to a net decrease in disorder?

• When the salt has highly charged ions, more water molecules are ordered about the ions

• Decrease in entropy in the hydration process outweighs the increase in entropy due to the disruption of the crystal lattice

What is the formula for Gibbs Free Energy Change?

ΔG = ΔH - TΔS

What does a positive or negative ΔG mean?

• ΔG < 0 → Exergonic reaction → Forward reaction is spontaneous / thermodynamically feasible

• ΔG > 0 → Endergonic reaction → Forward reaction is not spontaneous / not thermodynamically feasible

What is ΔG if ΔH < 0 and ΔS > 0?

ΔG < 0 at all temperatures → Reaction is spontaneous at all temperatures

What is ΔG if ΔH > 0 and ΔS < 0?

ΔG > 0 at all temperatures → Reactions is not spontaneous at all temperatures

What is ΔG if ΔH > 0 and ΔS > 0?

• At low temperatures, ΔG > 0 → Not spontaneous

• At high temperatures, ΔG < 0 → Spontaneous

What is ΔG if ΔH < 0 and ΔS < 0?

• At low temperatures, ΔG < 0 → Spontaneous

• At high temperatures, ΔG > 0 → Not spontaneous