GRE Subject Test: Chemistry : Classifying Reactions
Study concepts, example questions & explanations for GRE Subject Test: Chemistry
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Example Questions
Example Question #1 : Endothermic And Exothermic Reactions
When the enthalpy of a reaction has a value of 
Exothermic
Endergonic
Exergonic
Endothermic
Exothermic
When the enthalpy of a reaction is negative, it means that heat has been released from the system to the surroundings. This release of energy is referred to as an exothermic reaction.
Endothermic reactions have a net heat flow into the system. Exergonic and endergonic are terms used to describe the change in free energy for a given system.
Example Question #11 : Thermodynamics And Phases
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 negative
When ΔH is negative and ΔS is negative
When ΔH is positive and ΔS is positive
When ΔH is negative and ΔS is positive
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 both ΔH and ΔS are postive and T is high
When ΔH is negative and ΔS is postive
When both ΔH and ΔS are negative and T is low
All of these scenarios would result in spontaneous reactions
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 #1 : Endergonic And Exergonic Reactions
Which of the following scenarios describes a reaction in equilibrium?
When the forward rate is 0M/s
When 
When Gibbs free energy is negative
When the enthalpy is equal to the entropy
When
According to the Gibbs free energy equation, a system is at equilibrium when 
Since 
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