Thermodynamics - College Chemistry
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How do we know if a reaction is spontaneous?
How do we know if a reaction is spontaneous?
A reaction is spontaneous if and only if 
. The sign of 
only tells us if a reaction is exothermic or endothermic. The rate constant only tells us the rate at which a given chemical reaction proceeds based on the reactants and the products. The first law of thermodynamics only tells us that energy is conserved, and can neither be created nor destroyed. The second law of thermodynamics states that the entropy of the universe is spontaneously increasing, however, under the right conditions, these reactions may proceed.
A reaction is spontaneous if and only if
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Calculate the change in free energy, in kilojoules, of the reaction at 
by using the free energies of formation.
values, in 
, for molecules are as follows:
Calculate the change in free energy, in kilojoules, of the reaction at
Recall how to find the change in free energy for a reaction from using free energies of formation:
Where 
is the number of moles of the molecule.
Plug in the given values to find the change in free energy for the reaction.
Recall how to find the change in free energy for a reaction from using free energies of formation:
Where
Plug in the given values to find the change in free energy for the reaction.
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What is the standard change in free energy for a reaction run at 
with an equilibrium constant of 
?
What is the standard change in free energy for a reaction run at
In this question, we're told of a reaction that is run at a given temperature and with a certain equilibrium constant. We're asked to evaluate the standard change in free energy for this reaction.
First, right off the bat, we can see that the equilibrium constant for the reaction is greater than one. As a result, we know that under standard conditions, equilibrium of this reaction will favor products. Hence, we know right away that 
will be negative, which allows us to rule out any answer choices that are positive.
To actually calculate the value of 
, we'll need to use an equation that relates this quantity with temperature and the equilibrium constant. We can do so by using the following expression.
If we plug in the values that we know, we can solve for the correct answer.
In this question, we're told of a reaction that is run at a given temperature and with a certain equilibrium constant. We're asked to evaluate the standard change in free energy for this reaction.
First, right off the bat, we can see that the equilibrium constant for the reaction is greater than one. As a result, we know that under standard conditions, equilibrium of this reaction will favor products. Hence, we know right away that
To actually calculate the value of
If we plug in the values that we know, we can solve for the correct answer.
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Suppose that a given chemical reaction has an enthalpy change of 
and an entropy change of 
. At what temperature would this chemical reaction be in equilibrium?
Suppose that a given chemical reaction has an enthalpy change of
For this question, we're asked to determine the temperature needed for a reaction to be in a state of equilibrium. We are also provided with the entropy and enthalpy changes for this reaction.
Remember that for a reaction to be in a state of equilibrium, the value for its change in free energy must be equal to zero. Furthermore, we can state the relationship between the change in free energy with the change in entropy, the change in enthalpy, and the temperature as follows.
Because the 
term in the above expression must be equal to zero, we can plug this value in and then isolate the term for temperature.
Now that we have this expression, we just need to plug in the two values given to us in the question stem. Remember to make the units match though!
For this question, we're asked to determine the temperature needed for a reaction to be in a state of equilibrium. We are also provided with the entropy and enthalpy changes for this reaction.
Remember that for a reaction to be in a state of equilibrium, the value for its change in free energy must be equal to zero. Furthermore, we can state the relationship between the change in free energy with the change in entropy, the change in enthalpy, and the temperature as follows.
Because the
Now that we have this expression, we just need to plug in the two values given to us in the question stem. Remember to make the units match though!
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The second law of thermodynamics states which of the following is true regarding an isolated system?
The second law of thermodynamics states which of the following is true regarding an isolated system?
The entropy cannot decrease in an isolated system because the energy can only be degraded. Since the system is isolated, no higher-grade energy—or any energy at all—is being introduced into the system. As a result, the entropy cannot decrease. The other answer choices relate to the other laws of thermodynamics.
The entropy cannot decrease in an isolated system because the energy can only be degraded. Since the system is isolated, no higher-grade energy—or any energy at all—is being introduced into the system. As a result, the entropy cannot decrease. The other answer choices relate to the other laws of thermodynamics.
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Which of the following statements is true of standard states?
Which of the following statements is true of standard states?
Standard states are defined as a specific set of conditions, such as when a gas is at 
, 
concentration, and 
.
Standard enthalpy of formation, the energy required for form 1 mole of a compound from its constituent elements, occurs when elements are in their standard states.
Standard states are defined as a specific set of conditions, such as when a gas is at
Standard enthalpy of formation, the energy required for form 1 mole of a compound from its constituent elements, occurs when elements are in their standard states.
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How much heat is required to raise the temperature of 
of water from 
to 
? (Specific heat capacity of water is 
)
How much heat is required to raise the temperature of
is positive because heat flows into the system to raise the temperature of the water.
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How much heat is required to raise the temperature of 
of water from 
to 
? Specific heat capacity of water is 
How much heat is required to raise the temperature of
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Calculating heat
How much heat is absorbed by a copper penny as it warms from 
to 
assuming the penny is pure copper with a mass of 
? 
of copper is 
.
Calculating heat
How much heat is absorbed by a copper penny as it warms from
Use the equation that relates heat, mass, specific heat, and change in temperature:
Use the equation that relates heat, mass, specific heat, and change in temperature:
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"In a natural thermodynamic process, the sum of the entropies of the interacting systems increases." Which law of thermodynamics does this statement refer to?
"In a natural thermodynamic process, the sum of the entropies of the interacting systems increases." Which law of thermodynamics does this statement refer to?
There are four main laws of thermodynamics, which describe how temperature, energy, and entropy behave under various circumstances. The zeroth law of thermodynamics helps to define temperature; it states that if two systems are each in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. The first law of thermodynamics negates the possibility of perpetual motion; it states that when energy passes into or out of a system, the system's internal energy changes in accord with the law of conservation of energy. The second law of thermodynamics also negates the possibility of perpetual motion; it states that in a natural thermodynamic process, the sum of the entropies of the interacting systems increases. Lastly, the third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature nears absolute zero.
There are four main laws of thermodynamics, which describe how temperature, energy, and entropy behave under various circumstances. The zeroth law of thermodynamics helps to define temperature; it states that if two systems are each in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. The first law of thermodynamics negates the possibility of perpetual motion; it states that when energy passes into or out of a system, the system's internal energy changes in accord with the law of conservation of energy. The second law of thermodynamics also negates the possibility of perpetual motion; it states that in a natural thermodynamic process, the sum of the entropies of the interacting systems increases. Lastly, the third law of thermodynamics states that the entropy of a system approaches a constant value as the temperature nears absolute zero.
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Given the following reaction diagram, which of the following is marked by the red arrow?
Given the following reaction diagram, which of the following is marked by the red arrow?
The fully filled in reaction coordinate diagram is displayed below.
The arrow marked in the question represents the activation energy, which is the energy barrier that must be overcome in order for the reactants to form products.
This reaction is also exothermic because the energy of the products is lower than that of the reactants.
The fully filled in reaction coordinate diagram is displayed below.
The arrow marked in the question represents the activation energy, which is the energy barrier that must be overcome in order for the reactants to form products.
This reaction is also exothermic because the energy of the products is lower than that of the reactants.
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If the 
of a reversible reaction is 
, and the activation energy of the forward reaction is 
, what is the activation energy for the reverse reaction in kilojoules?
If the
Start by drawing and labeling the reaction coordinate diagram.
Since we know that 
is negative, the products must be lower in energy than the reactants.
Now, the question wants you to find the activation energy if we were to reverse the reaction. In other words, we want to start at the products and end up with the reactants. From the diagram, you can see that the reverse activation energy is just the sum of the activation energy for the forward reaction and the 
.
Start by drawing and labeling the reaction coordinate diagram.
Since we know that
Now, the question wants you to find the activation energy if we were to reverse the reaction. In other words, we want to start at the products and end up with the reactants. From the diagram, you can see that the reverse activation energy is just the sum of the activation energy for the forward reaction and the
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Assuming standard conditions, which of the following is a true statement based on the free energy diagram shown?
Assuming standard conditions, which of the following is a true statement based on the free energy diagram shown?
In this question, we're presented with a free energy diagram for a chemical reaction occurring under standard conditions. We're asked to identify a true statement with regards to this diagram.
Right off the bat, we can see that the free energy of the products is greater than that of the reactants. Thus, this reaction has a positive change in free energy and so it is an endergonic reaction. Recall that one expression for the value of free energy change is as follows.
From this equation, we can see that the equilibrium constant is related to the standard change in free energy. Moreover, we can see that if the standard free energy change is positive, then the value of 
must be less than 
. Thus, we know that the reaction is favored towards the reactants.
Additionally, we cannot determine whether the entropy increases or decreases based on the diagram. The reason for this is that entropy is not the only factor that determines the change in free energy of a reaction. The change in enthalpy is also a factor that must be considered.
In this question, we're presented with a free energy diagram for a chemical reaction occurring under standard conditions. We're asked to identify a true statement with regards to this diagram.
Right off the bat, we can see that the free energy of the products is greater than that of the reactants. Thus, this reaction has a positive change in free energy and so it is an endergonic reaction. Recall that one expression for the value of free energy change is as follows.
From this equation, we can see that the equilibrium constant is related to the standard change in free energy. Moreover, we can see that if the standard free energy change is positive, then the value of
Additionally, we cannot determine whether the entropy increases or decreases based on the diagram. The reason for this is that entropy is not the only factor that determines the change in free energy of a reaction. The change in enthalpy is also a factor that must be considered.
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