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Entropy, T he U niverse and F ree Energy

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Entropy, The Universe and Free Energy

- For any spontaneous process:
∆Suniverse > 0

Because:

∆Suniverse= ∆Ssystem +∆Ssurroundings

- In nature , ∆Suniverse tends to be positive for reaction processes under the following conditions:
- The reaction or process is exothermic, which means ∆Ssystem is a negative value
(the heat released by the system increases Temp of the surroundings, thus increasing the entropy.)

2. The entropy of the system increases, so ∆Ssystem is positive

- The reaction or process is exothermic, which means ∆Ssystem is a negative value

- Gibbs free energy (Gsystem) is commonly called free energy.
- Free energy is energy that is available to do work, thus free energy is useful energy.

- Gibbs Free Energy can be calculated by:
∆Gsystem = ∆Hsystem - T∆Ssystem

(where T is in Kelvin)

If ∆Gsystem is negative the process is spontaneous

If ∆Gsystem is positive then the process is nonspontaneous

Under standard conditions it is written:

∆G⁰system = ∆H⁰system - T∆S⁰system(T=298K and Pressure=1atm)

N2(g) + 3H2(g) 2NH3(g)

∆H⁰system= -91.8kJ

∆S⁰system=-197 J/K

Note that Entropy decreases because four moles of gaseous molecules react to make only two moles of gaseous molecules. Therefore the entropy is negative which tends to make a reaction nonspontaneous.

Yet, The reaction is exothermic(∆H⁰system= -91.8kJ) which tends to make the reaction spontaneous.

So which one is it?

CONFLICT!!!!!!!!

N2(g) + 3H2(g) 2NH3(g)

∆H⁰system= -91.8kJ

∆S⁰system=-197 J/K

∆G⁰system= -91.8 kJ – (298K)(-197J/K)

∆G⁰system= -91800J + 58700 J

∆G⁰system= -33100 J

∆G⁰systemis negative therefore, the reaction is spontaneous under standard conditions.

- If ∆G⁰system is negativethe reaction is spontaneous
- If ∆G⁰system is positive the reaction is nonspontaneous
- If ∆G⁰system= 0 , we say that the products and reactants are present in the state of chemical equilibrium. In other words, the rate of the forward reaction equals the rate of the reverse reaction.

- Sometimes a series of reactions must occur in order to allow for a nonspontaneous reaction to occur. For example, many of the chemical reactions in living organisms are nonspontaneous reactions.(∆G⁰system is positive) Yet, the reason they occur so readily is because they occur together with other spontaneous reactions.
- Free energy released by one or more spontaneous reactions drives the nonspontaneous reactions.

- Why is the sign of ∆Hfusion positive?
- Why is the sign of ∆Sfusion positive?
- Solve for ∆G for the ice melting at the two temperatures.
- Using your results, explain why ice melts at 274K but not at 272K