All GRE Subject Test: Chemistry Resources
Example Questions
Example Question #1 : Classifying Reactions
The equation for the change in Gibbs free energy is given below.
ΔH = change in enthalpy
ΔS = change in entropy
Which of the following scenarios guarantees a nonspontaneous reaction?
When ΔH is positive and ΔS is positive
When ΔH is negative and ΔS is negative
When ΔH is negative and ΔS is positive
When ΔH is positive and ΔS is negative
When ΔH is positive and ΔS is negative
A positive value for ΔG (Gibbs free energy) will guarantee a nonspontaneous reaction. When ΔH (enthalpy) is postive and ΔS (entropy) is negative, the change in Gibbs free energy must be positive and, therefore, nonspontaneous.
Because T (temperature) will always have a positive value, a negative entropy and positive enthalpy will always result in a positive Gibbs free energy.
Example Question #2 : Endergonic And Exergonic Reactions
The equation for the change in Gibbs free energy is given below.
ΔH = change in enthalpy
ΔS = change in entropy
In which of the scenarios will the reaction be spontaneous?
When ΔH is negative and ΔS is postive
All of these scenarios would result in spontaneous reactions
When both ΔH and ΔS are postive and T is high
When both ΔH and ΔS are negative and T is low
All of these scenarios would result in spontaneous reactions
All of the following scenarios would lead to spontaneous reaction, since each scenario would result in a negative Gibbs free energy (-ΔG).
Negative enthalpy, positive entropy:
Positive enthalpy and entropy with high temperature:
Negative enthalpy and entropy with low temperature:
Example Question #1092 : Mcat Physical Sciences
Which of the following scenarios describes a reaction in equilibrium?
When Gibbs free energy is negative
When the enthalpy is equal to the entropy
When the forward rate is 0M/s
When is equal to
When is equal to
According to the Gibbs free energy equation, a system is at equilibrium when is equal to 0.
Since , a system is in equilibrium when
. A negative Gibbs free energy means that the reaction will be spontaneous. At equilibrium, the forward reaction rate equals the reverse reaction rate, though the net rate is zero.
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All GRE Subject Test: Chemistry Resources
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