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Gibbs Free Energy

 

ΔG = ΔH - TΔS

ΔG= Gibbs Free Energy      ΔH = Enthalpy    T = temperature in Kelvin   ΔS = ENTROPY

Physics Definition-Gibbs free energy, the amount of thermodynamic energy in a fluid system which can be converted into non-mechanical work at a constant temperature and pressure.

What you need to know....If ΔG is negative, the a reaction is spontaneous (it can occur). Spontaneous does not imply that it is instantaneous. It could happen over the course of years or decades (I think you get the point).

ΔG < 0 , the reaction is spontaneous
ΔG > 0, the reaction is nonspontaneous
ΔG = 0, the reaction is at equilibrium

ΔG maybe positive or negative under certain conditions.

ΔG = ΔH - TΔS   

Enthalpy change ΔH Entropy change ΔS Gibbs free energy ΔG Spontaneity
positive positive depends on T, may be + or - yes, if the temperature is high enough
negative positive always negative always spontaneous
negative negative depends on T, may be + or - yes, if the temperature is low enough
positive negative always positive never spontaneous

Example of ΔH being + and ΔS +

Regents Questions-highlight to reveal answers.

1. An Endothermic reaction may proceed spontaneously if there is an increase in

A. Potential Energy

B. Order

C. Concentration

D. Entropy

Answer-->

Choice D, ΔG = ΔH - TΔS, ΔS must increase (+) when  ΔH is (-) to makeΔG (-)

 

2. A reaction must be spontaneous if its occurrence is

A.exothermic with an decrease in entropy

B. exothermic with an increase in entropy

C. endothermic with an decrease in entropy

D. endothermic with an increase in entropy

 

Answer-->

Choice B, ΔG = ΔH - TΔS, ΔG is(-) when ΔS (entropy) must increase (+) and  ΔH is (-)exothermic

 

3. Above 0oC, ice changes spontaneously to water according to the following equation:

H2O(s) + heat --> H2O(l)

The change in H2O(s) involve

A. an absorption of heat and a decrease in entropy

B. a release of heat and a decrease in entropy

C. an absorption of heat and a increase in entropy

D. a release of heat and a increase in entropy

 

Answer-->

Choice C, heat is on the left of the arrow, absorbed. solid to a liquid entropy increases

 

4. A reaction must be spontaneous if the results in products that have

A. lower potential energy and less randomness

B. more potential energy and more randomness

C. lower potential energy and more randomness

D. more potential energy and less randomness

Answer-->

 Choice C, lower Product PE --> ExothermicΔH is (-) and more randomnessΔS (entropy) (+)

 

5. The change of the reactants into products will always be spontaneous if the products, compared to the reactants, have

A. lower enthalpy and higher entropy

B. higher enthalpy and higher entropy

C. higher enthalpy and lower entropy

D. lower enthalpy and lower entropy

Answer-->

 Choice A, ExothermicΔH Enthalpy is (-) and more randomnessΔS (entropy) (+)

 

6. As a solid dissolves in water, the temperature of the water decrease. the dissolving of this solid is

A. endothermic and spontaneous

B. endothermic and nonspontaneous

C. exothermic and nonspontaneous

D. exothermic and spontaneous

Answer-->

 Choice A, EndothermicΔH Enthalpy is (+) and it happens so it is spontaneous

 

6. The ΔG of a chemical reaction refers to the change in 

A. entropy

B. free energy

C. state

D. activation energy

Answer-->

 Choice B, Gibbs Free Energy =ΔG

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