MCAT Physical : Thermochemistry and Energetics
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Example Question #61 : Thermochemistry And Energetics
A system experiences the following energetic changes:
Which of the following is true?
The system loses 
The system loses heat
Remember that a negative sign denotes an exothermic process (heat is released into the surroundings), while a positive energy change denotes an endothermic process (heat is absorbed).
Using the given values we can calculate the total energy change of the system:
Since the overall energy change is positive, 
Example Question #1 : Endothermic And Exothermic Reactions
Is the reaction above endothermic or exothermic?
Exothermic because an increase in temperture would cause the reaction to shift to the right
Endothermic because an increase in temperture would cause the reaction to shift to the left
Endothermic because an increase in temperture would cause the reaction to shift to the right
Exothermic because an increase in temperture would cause the reaction to shift to the left
Exothermic because an increase in temperture would cause the reaction to shift to the left
The reaction is an exothermic reaction, since the heat is added on the products side. An endothermic reaction would have the heat on the reactants side. An increase in heat in this reaction would cause the Keq to decrease, and there will be a shift to the left of the reaction (toward the reactants).
Remember that Keq is dependent on temperature, and can be affected by changes in heat. Keep in mind, also, that the reverse reaction that occurs during the leftward shift will be endothermic, using the additional heat as a reactant rather than a product.
Example Question #11 : Endothermic And Exothermic Reactions
A student is performing a reaction with unknown compounds in his chemistry lab. The only information the student knows about the reaction is that it is endothermic and reversible. Using this knowledge alone, how can the student increase the yield of his product?
Add product to the reaction
Remove reactant from the reaction
Increase the temperature
Decrease the temperature
Increase the temperature
To answer this question, we need to have a solid understanding of Le Chatelier's principle.
In an endothermic reaction, heat is needed to facilitate the reaction. To increase the products, we want to shift the reaction to the right.
We should already know that adding product or removing reactant shifts the equilibrium to the left, and yields more starting material rather than product. In an endothermic reaction, we can consider heat as a reactant; thus, adding heat (increasing temperature) would allow us to shift the reaction to the right.
Decreasing the temperature, removing reactant, or adding product would all increase the yield of the starting materials.
Example Question #11 : Endothermic And Exothermic Reactions
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
Growing frustrated by his inability to decipher what chemical reaction the student had started, the scientist decides to measure the reaction vessel's temperature. Based only on the above reaction diagram, what is he most likely to find if the reaction is ongoing?
(Assume that the reaction vessel is defined as the system, and that the entropy of the system decreases)
A cold container from an exothermic reaction
A warm container from an endothermic reaction
A cold container from an endothermic reaction
A warm container from an exothermic reaction
The products of this reaction have an equal energy level to the reactants
A warm container from an exothermic reaction
Point 5 is the energy level of the products of the reaction, while point 1 is the energy level of the reactants. Point 5 is lower than is point 1, indicating that the products of this reaction contain lower overall energy than do the reactants. This energy must be released in some form, likely as heat, characteristic of an exothermic reaction.
The question further specifies that there is a local decrease in entropy of the system, thus, the only way that entropy of the universe can increase is to release heat and increase the entropy of the surroundings.
Example Question #12 : Endothermic And Exothermic Reactions
Why does a chemical ice pack feel cold?
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an exothermic reaction; this pulls heat from the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an exothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
Endothermic reactions by definition, require heat as a reactant. By drawing in heat from the surrounding, the surrounding will have a lower temperature compared to air temperature, which is why the ice pack feels cold. If it were an exothermic reaction, the pack would release heat, making the pack feel warm.
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 at low temperatures
It never 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 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 #2 : 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 always spontaneous
The reaction is spontaneous only at low temperatures
The reaction is never 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,” 
The reaction cannot be spontaneous
Low temperature
The reaction is always spontaneous
Moderate temperature and high pressure
High temperature
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 #81 : Physical Chemistry
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 negative and ΔS is positive
When ΔH is positive 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 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:
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