Week 9 Thermodynamics

Question 1

In chemical systems, the change in Gibbs free energy (ΔG) at constant T and P is….
What is this equation

Answer 1

The maximum chemical work of the system
ΔG=Sum(average chemical potential x change in the number of moles)

Question 2

What is the equation for ΔG to determine sponteneity
What signs of ΔG determine spontaneity/non/equilbrium

Answer 2

ΔG=ΔH-TΔS (REFERS TO SYSTEM or RXN)
ΔG > 0= NOT spontaneous
ΔG < 0= Spontaneous
ΔG=0 = Equilibrium

Question 3

What type of graph does a spontaneous reaction look like? Which way is spontaneous, nonspontaneous

Answer 3

Parabola or U shaped
Spontaneous is downwards when ΔG < 0
Non-spontaneous is going upwards when G > 0

Question 4

Given ΔH and ΔS, what are the 4 different sign conventions and when will they be spontaneous?

Examples

Answer 4

1. -ΔH, +ΔS= ALWAYS spontaneous (N2O yoelds N2 and O2)
2. +ΔH, -ΔS= NEVER spontaneous (O2 yields O3)
3. -Δ H and S= spontaneous at LOW TEMPS (Liquid to solid)
4. ΔH and S= spontaneous at HIGH TEMPS (Liquid to gas)

Question 5

When are elements in standard states?
What pressure, tempertature, and molarity?

Answer 5

Elements= PURE substances in most stable
1 atm, 25 C or 298 K, 1 M

Question 6

How do you find the molar entropy in their standard states. What unit is it in?

Answer 6

Srxn= S products- S reactants
J/K x mol

Question 7

What is the 3rd Law of Thermodynamics

Answer 7

The entropy of a perfect crystal tends towards ZERO as the temperature tends towards absolute zero (0 K).
(A perfect crystal is a perfectly ordered solid with no defects)

Only one possible way to arrange its components

Question 8

What things affect standard entropies (5)

Answer 8

1. States of matter (Gases have highest because of high freedom of motion)
2. Different allotropes (S Graphite>S diamon because they are not constrained)
3. An increase in molar mass results in increase of entropy (More microstates available due to closer energy states)
4. Molecular complexity (More bonds and angles=higher degrees of freedom)
5. Dissolution (Aqueous form has higher entropy than solid)

Question 9

What are the Properties of Gibbs Free Energy

Answer 9

1. ΔG rxn are extensive which means it depends on the amount of reactants
2. ΔG rxn for backwards reaction changes the sign of ΔGrxn for the forwards reaction. (If forward is spontaneous, backward is non-spontaneous)
3. You can use Hess’s Law because G is a state function

Question 10

What is ΔG of formation

Answer 10

Gibbs Free energy change associayted with the formation of 1 mol of a compoind with constituent elements at standard state