MCAT Physical : Endergonic and Exergonic Reactions
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Example Questions
Example Question #1 : Endergonic And Exergonic Reactions
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.
Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.
In this law, 
A scientist is studying solution chemistry to better understand vapor pressure. He finds that, for one solution he creates, the beaker is cool to the touch after the solute is fully dissolved. Which of the following is true of this solution? (Note: The beaker, solute, and solvent are the system, the remainder of the universe is the surroundings)
It always forms spontaneously
It forms spontaneously only if dissolution decreases entropy of the system
It forms spontaneously only at high temperatures
It forms spontaneously only at low temperatures
It never forms spontaneously
It forms spontaneously only at high temperatures
The act of dissolving a solute in a solvent is a local increase in entropy, converting a single molecule to multiple ions. The absorption of heat from the surroundings (cool beaker) indicates that this is an endothermic dissolution. We can look at the equation for Gibbs free energy to evaluate the possible answers.
In order to be spontaneous, the reaction must have a negative Gibbs free energy. To accomplish this, a reaction may have a negative enthalpy (exothermic) and positive entropy, however we know that our reaction has a positive enthalpy (endothermic) and positive entropy. A reaction will be spontaneous if it has a positive 
Example Question #1 : Endergonic And Exergonic Reactions
A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:
In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.
Assume that the forward reaction in the passage is exothermic, and has a 
The reaction is never spontaneous
The reaction is spontaneous only at low temperatures
The reaction is always spontaneous
The reaction is spontaneous only at high temperatures
The spontaneity of the reaction cannot be predicted
The reaction is always spontaneous
The question specifies that this reaction is exothermic, and that it has a positive local increase in entropy as it progresses. This means that the change in enthaply is negative and the change in entropy is positive.
For a reaction to be spontaneous, Gibbs free energy must be negative.
If enthalpy is negative and entropy is positive, the temperature is irrelevant and Gibbs free energy will always be negative.
The reaction is spontaneous at any temperature.
Example Question #1 : Endergonic And Exergonic Reactions
For “Reaction A,” 
High temperature
The reaction is always spontaneous
Moderate temperature and high pressure
Low temperature
The reaction cannot be spontaneous
The reaction cannot be spontaneous
The spontaneity of a reaction is determined by the equation for Gibbs free energy.
Here, H is a positive number and S is negative, meaning that G will always be a positive value. T will be given in Kelvin, and cannot be negative. Reactions with positive Gibbs free energy values are never spontaneous.
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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