Non-spontaneous reaction and Gibbs Free Energy

In summary, at 25 degrees Celsius, quartz (SiO2) does not spontaneously decompose into silicon and oxygen gas, despite having a large and positive standard entropy change of +182.02 J/K. This is due to the fact that the reaction's Gibbs Free Energy, which takes into account both enthalpy and entropy, is not negative at this temperature. This means that the reaction is not thermodynamically favorable and will not occur spontaneously. Kinetics, which refers to the rate of a reaction, does not play a role in determining spontaneity.
  • #1
jmm5872
43
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Quartz, SiO2(s), does not spontaneously decompose to silicon and oxygen at 25 degrees Celsius even though the standard entropy change of the reaction is large and positive (deltaS = +182.02 J/K). Explain why.

I am confused by this one fundamentally because it seems like common sense that quartz doesn't spontaneously turn to solid silicon and oxygen gas at this temp.

Looking at Gibbs Free Energy and the effects of temperature on this it says that if [tex]\Delta[/tex]H is negative and [tex]\Delta[/tex]S is positive then the reaction is spontaneous at all temperatures.

I don't understand what concept I am missing.
 
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  • #2
jmm5872 said:
I don't understand what concept I am missing.

Kinetics
 
  • #3
SiO2 --> Si + O2
delta H= + 910 KJ/mole ( note kilo joules)
delta S= + 182 J/K mole
T= 298 K

delta G= delta H -T delta S

What you are missing is putting in the correct values and getting the answer.

Kinetics
 
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What is a non-spontaneous reaction?

A non-spontaneous reaction is a chemical reaction that does not occur on its own under normal conditions. It requires an external source of energy in order to proceed.

What is Gibbs Free Energy?

Gibbs Free Energy is a thermodynamic property that measures the amount of energy available to do useful work in a system. It takes into account both the enthalpy and entropy of a system and is represented by the symbol G.

How is Gibbs Free Energy related to non-spontaneous reactions?

In non-spontaneous reactions, the change in Gibbs Free Energy (ΔG) is positive, indicating that energy must be added to the system in order for the reaction to occur. This is because the reactants have a higher Gibbs Free Energy than the products, and energy is needed to overcome this energy barrier.

Can a non-spontaneous reaction ever occur?

Yes, a non-spontaneous reaction can occur if enough energy is added to the system to overcome the energy barrier. This can be achieved through the addition of heat, electricity, or other forms of energy.

How is the equilibrium constant (K) related to Gibbs Free Energy?

The equilibrium constant is related to Gibbs Free Energy through the equation ΔG = -RTlnK, where R is the gas constant and T is the temperature in kelvin. This equation shows that a higher equilibrium constant (K) corresponds to a more negative ΔG, indicating a more spontaneous reaction.

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