Entropy and Gibbs Energy (More Thermodynamics)

ENTROPY AND GIBBS ENERGY (MORE THERMODYNAMICS)

ENTROPY AND GIBBS ENERGY

SECTION 1

A cup of hot tea is made and is left on a table.What do you expect to spontaneously happen to the temperature of the tea?

A cup of hot tea is made and is left on a table.What do you expect to spontaneously happen to the temperature of the tea?Is this endothermic or exothermic?

A cup of hot tea is made and is left on a table.What do you expect to spontaneously happen to the temperature of the tea?Is this endothermic or exothermic?What if the tea was left long enough to evaporate. Is this endothermic or exothermic?

A cup of hot tea is made and is left on a table.What do you expect to spontaneously happen to the temperature of the tea?Is this endothermic or exothermic?What if the tea was left long enough to evaporate. Is this endothermic or exothermic?Does there seem to be a connection between being endothermic or exothermic and spontaneous?

Enthalpy does have a connection with whether or not a change is spontaneous, but obviously it is not the only factor.

Enthalpy does have a connection with whether or not a change is spontaneous, but obviously it is not the only factor.While making a cup of tea, the box of tea bags falls and opens on the floor. What do you expect the tea bags to spontaneously do?

Enthalpy does have a connection with whether or not a change is spontaneous, but obviously it is not the only factor.While making a cup of tea, the box of tea bags falls and opens on the floor. What do you expect the tea bags to spontaneously do?Does it seem increasing disorder is always spontaneous?

The second law of thermodynamics says the disorder of the universe is always increasing. What two things are included in the universe?

The second law of thermodynamics says the disorder of the universe is always increasing. What two things are included in the universe?If the disorder of the universe is always increasing, how is it possible for you make your bedroom less disordered?

The thermodynamic factor we have been calling disorder is actually called entropy (S). So the second law shows:

Suniverse = Ssystem + Ssurroundings and is always increasing (+)

-S is decreasing disorder, +S is increasing disorder

Brainstorm changes that increase entropy:

*Factors of ΔS that are of major importance:

*Factors of ΔS that are of major importance:1. Phase changes

Which phase changes are +ΔS and which are –ΔS?

*Factors of ΔS that are of major importance:1. Phase changes

Which phase changes are +ΔS and which are –ΔS? Which ones make the biggest change?

*Factors of ΔS that are of major importance:2. Changing the number of moles

What are the two possibilities? Let’s show an example of each:

*Factors of ΔS that are of major importance:2. Changing the number of moles

What are the two possibilities? Let’s show an example of each:

Can this be combined with #1? Let’s show an example:

*Factors of ΔS that are of major importance:3. Molecular complexity

consider H2(g) + Cl2(g) → 2HCl(g)Is there a phase change?Is there a change in the number of moles?What effect does the reaction have on entropy?

Chemists of course want to measure entropy, so the third law of thermodynamics says that the absolute entropy of any substance at absolute zero is zero. What is standard temperature?

Chemists of course want to measure entropy, so the third law of thermodynamics says that the absolute entropy of any substance at absolute zero is zero. What is standard temperature? If the substances are never at absolute zero, what does that mean for elements in their natural state at standard temperature and pressure?

Chemists of course want to measure entropy, so the third law of thermodynamics says that the absolute entropy of any substance at absolute zero is zero. What is standard temperature? If the substances are never at absolute zero, what does that mean for elements in their natural state at standard temperature and pressure? Can these standard entropy values be + or -?

Utilizing standard entropy data, we can calculate the change in entropy for a reaction:

ΔS = ΣSproducts - ΣSreactants



Would Hess’s Law and bond entropy data also work for finding ΔS? What does that tell you about S?



Would Hess’s Law and bond entropy data also work for finding ΔS? What does that tell you about S?

Beware, S is usually measured in J mol-1, not kJ mol-1 as we did with enthalpy.

Rockets can use the following reaction as propulsion: N2H4(l) + 2H2O2(l) → N2(g) + 4H2O(g)

What is the ΔS° (appendix B.16)


SECTION 2

What is spontaneous for liquid water at 100°C?What is the ΔH?What is the ΔS?



Both enthalpy and entropy have an effect on the spontaneity of a reaction, but both need to be considered at the same time for us to predict if a reaction will be spontaneous or not. This relationship is called Gibb’s (Free) Energy. Gibbs energy is a measure of the maximum amount of useful (available) energy in a system.

Gibbs energy is calculated as:G = H - TS

as both enthalpy and entropy are state functions, so is Gibbs energy, therefore:

ΔG = ΔH - TΔS

ΔG = ΔH - TΔSif ΔG is -, then spontaneousif ΔG is +, then not spontaneous

T must be in KelvinH is usually in kJ/mol butS is usually in J/mol

ΔG = ΔH - TΔSif ΔG is -, then spontaneousif ΔG is +, then not spontaneous

A spontaneous (thermodynamically favorable) reaction is one where the reactants will naturally form products. The bigger the - value of ∆G, the greater the tendency to form products. The bigger the + value of ∆G, the greater the tendency to form reactants.

ΔG = ΔH - TΔSThe above equation gives us many options.We could solve for ΔG. What information would we need?

ΔG = ΔH - TΔSThe above equation gives us many options.We could solve for ΔG. What information would we need?We could solve for ΔH. What information would we need?

ΔG = ΔH - TΔSThe above equation gives us many options.We could solve for ΔG. What information would we need?We could solve for ΔH. What information would we need?We could solve for ΔS. What information would we need?

ΔG = ΔH - TΔSThe above equation gives us many options.We could solve for T. What information would we need?

ΔG = ΔH - TΔSA very interesting thing happens when you solve for T. This tells you the temperature at which a reaction that was not spontaneous would become spontaneous. Note that at this point where ΔG goes from + to - the value of ∆G is zero.What is this formula?

ΔG = ΔH - TΔSA very interesting thing happens when you solve for T. This tells you the temperature at which a reaction that was not spontaneous would become spontaneous. Note that at this point where ΔG goes from + to - the value of ∆G is zero.What is this formula?Why do reptiles sunbathe?

Consider this reversible reaction: Br2(l) → Br2(g)At what temperature does this change spontaneity?What is significant about this temperature?

One other alternative way to find ΔG isΔG = ΣΔGproducts - ΣΔGreactants

It is VERY important you know how to predict when a reaction will be thermodynamically favorable or not. To be spontaneous, G must be -.

How can you find the sign of G?

ΔH - T ΔS = ΔG- + spont. + T + spont. - T - spont.

+ - not spont. - T - not spont. + T + not spont.

H

S


SECTION 3

Other things that can be done with ∆G:

∆G° = -n F E°What is each variable?


∆G° = -n F E°What is each variable?What would this allow us to do?


∆G° = -n F E°What is each variable?What would this allow us to do?What is spontaneous E and what is spontaneous for G? How does this work out?


∆G° = -R T ln

What is each variable?

[products][reactants]


∆G° = -R T ln

What is each variable?What would this allow us to do?



∆G° = -R T ln

What is each variable?What would this allow us to do?What is spontaneous for a chemical reaction and what is spontaneous for G? How does thi work out?


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Entropy and Gibbs Energy (More Thermodynamics)