Thermochemistry and Energetics - AP Chemistry
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Which of the following is considered an exothermic reaction?
Which of the following is considered an exothermic reaction?
An exothermic reaction releases energy into the environment. In contrast, endothermic reactions require an input of energy to initiate the reaction.
Forming a bond is always an exothermic reaction because it releases energy. Breaking a bond always requires energy, and is thus an endothermic process. Synthesis, decomposition, and single-replacement reactions can be either exothermic or endothermic, and cannot be determined without more information.
An exothermic reaction releases energy into the environment. In contrast, endothermic reactions require an input of energy to initiate the reaction.
Forming a bond is always an exothermic reaction because it releases energy. Breaking a bond always requires energy, and is thus an endothermic process. Synthesis, decomposition, and single-replacement reactions can be either exothermic or endothermic, and cannot be determined without more information.
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Which type of reaction will happen spontaneously?
Which type of reaction will happen spontaneously?
Spontaneity is determined by Gibbs free energy. When Gibbs free energy is less than zero, the reaction is considered exergonic and will occur spontaneously. When Gibbs free energy is greater than zero, the reaction is considered endergonic and will not occur spontaneously.
Exothermic reactions cause a release of enthalpy (heat) from the system and endothermic reactions require and input of energy to initiate the reaction. Gibbs free energy is determined by enthalpy, entropy, and temperature. A negative enthalpy, high temperature, and high entropy will cause the reaction to be more spontaneous, but must all come together to contribute. Simply because a reaction is exothermic does not meant that is increases entropy enough to be spontaneous.
Spontaneity is determined by Gibbs free energy. When Gibbs free energy is less than zero, the reaction is considered exergonic and will occur spontaneously. When Gibbs free energy is greater than zero, the reaction is considered endergonic and will not occur spontaneously.
Exothermic reactions cause a release of enthalpy (heat) from the system and endothermic reactions require and input of energy to initiate the reaction. Gibbs free energy is determined by enthalpy, entropy, and temperature. A negative enthalpy, high temperature, and high entropy will cause the reaction to be more spontaneous, but must all come together to contribute. Simply because a reaction is exothermic does not meant that is increases entropy enough to be spontaneous.
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What can be said about the rates of exothermic and endothermic reactions?
What can be said about the rates of exothermic and endothermic reactions?
The definition of an endothermic reaction is that the products have higher energy than the reactants, resulting in a positive enthalpy of reaction. For the reaction to run, there must be an input of energy. The opposite is true for exothermic reactions: the products have lower energy than the reactants, enthalpy of reaction is negative, and heat is released.
Nothing is can be definitively stated about the rates of either type of reaction without additional information, as that will depend on the specific reactions and their respective activation energies.
The definition of an endothermic reaction is that the products have higher energy than the reactants, resulting in a positive enthalpy of reaction. For the reaction to run, there must be an input of energy. The opposite is true for exothermic reactions: the products have lower energy than the reactants, enthalpy of reaction is negative, and heat is released.
Nothing is can be definitively stated about the rates of either type of reaction without additional information, as that will depend on the specific reactions and their respective activation energies.
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Which of the following is an example of an exothermic reaction?
Which of the following is an example of an exothermic reaction?
In an exothermic reaction, heat has been released to the surroundings from the system. As a result, the molecules are at a lower final energy state after releasing the energy to the surroundings.
Going from a solid to a gas (as well as liquid in between) is an endothermic reaction. Energy must be absorbed in order to raise the energy of the molecules so that the phase change can take place. Boiling water, melting ice, or sublimating carbon dioxide all require an input of energy.
The opposite is observed when magma cools. The liquid magma releases energy to the surroundings, allowing it to cool and form igneous rock. The magma is essentially "freezing," turning from a liquid to a solid.
In an exothermic reaction, heat has been released to the surroundings from the system. As a result, the molecules are at a lower final energy state after releasing the energy to the surroundings.
Going from a solid to a gas (as well as liquid in between) is an endothermic reaction. Energy must be absorbed in order to raise the energy of the molecules so that the phase change can take place. Boiling water, melting ice, or sublimating carbon dioxide all require an input of energy.
The opposite is observed when magma cools. The liquid magma releases energy to the surroundings, allowing it to cool and form igneous rock. The magma is essentially "freezing," turning from a liquid to a solid.
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Which of the following statements is true?
Which of the following statements is true?
When graphically tracking the energy of a reaction, you can see that energy is always needed to start a reaction, regardless of its enthalpy. This necessary energy is called the activation energy. Exothermic reactions, however, have a lower activation energy compared to the reverse endothermic reaction. This is because there is a net energy release from an exothermic reaction because the products have less energy than the reactants. To reverse this reaction would be to go from the low energy products back to the high energy reactants, resulting in a net increase in energy (an endothermic process).
Condensation of steam is an exothermic process. Heat must be released since the high energy steam is becoming lower energy water.
When graphically tracking the energy of a reaction, you can see that energy is always needed to start a reaction, regardless of its enthalpy. This necessary energy is called the activation energy. Exothermic reactions, however, have a lower activation energy compared to the reverse endothermic reaction. This is because there is a net energy release from an exothermic reaction because the products have less energy than the reactants. To reverse this reaction would be to go from the low energy products back to the high energy reactants, resulting in a net increase in energy (an endothermic process).
Condensation of steam is an exothermic process. Heat must be released since the high energy steam is becoming lower energy water.
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The combustion of propane gas in a camping stove is an example of what type of reaction?
The combustion of propane gas in a camping stove is an example of what type of reaction?
Combustion reactions occur when a compound is oxidized in a highly exothermic reaction. Most commonly, the reactant is a hydrocarbon (such as propane) and the oxidizing agent is oxygen gas. The result is a large release of heat energy, frequently visualized as a flame.
Note that exothermic reactions by definition release heat, while endothermic reactions absorb heat.
Combustion reactions occur when a compound is oxidized in a highly exothermic reaction. Most commonly, the reactant is a hydrocarbon (such as propane) and the oxidizing agent is oxygen gas. The result is a large release of heat energy, frequently visualized as a flame.
Note that exothermic reactions by definition release heat, while endothermic reactions absorb heat.
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Which condition is always true for an exothermic reaction?
Which condition is always true for an exothermic reaction?
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy, 
, is a measure of the change in heat energy during a reaction. is always negative for an exothermic process because the products always have less heat energy than the reactants.
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy,
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Which condition is always true for an exothermic reaction?
Which condition is always true for an exothermic reaction?
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy, , is a measure of the change in heat energy during a reaction. is always negative for an exothermic process because the products always have less heat energy than the reactants.
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy,
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Which condition is always true for an exothermic reaction?
Which condition is always true for an exothermic reaction?
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy, , is a measure of the change in heat energy during a reaction. is always negative for an exothermic process because the products always have less heat energy than the reactants.
When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.
The change in enthalpy,
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Consider the following balanced reaction:
What is the change in enthalpy if 
of solid carbon is used in the above reaction?
Consider the following balanced reaction:
What is the change in enthalpy if
The enthalpy of 
describes the amount of heat when the amount of carbon in the balanced reaction (two moles) is used. Since only 
of carbon are used, we can find how much heat is released.
When two moles of carbon are used, are released. Two moles of carbon is equal to 
of carbon, based on carbon's atomic mass.
Knowing this, we can set up proportions in order to determine how much heat is released by 
of carbon.
So 
of carbon results in 
of heat being released to the surroundings.
The enthalpy of
When two moles of carbon are used,
Knowing this, we can set up proportions in order to determine how much heat is released by
So
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Consider these two half reactions:
Step 1. 
Step 2. 
Based on these half reactions, find the enthalpy for the following reaction:
Consider these two half reactions:
Step 1.
Step 2.
Based on these half reactions, find the enthalpy for the following reaction:
Hess's law states that the enthalpy of the total reaction is equal to the enthalpy of the steps required to get to the total reaction, regardless of the path that is chosen. This means that we can combine the two half steps with known enthalpies in order to solve for the enthalpy of the main reaction.
Step 1.
Step 2.
Total:
Since step 1 results in two moles of liquid water, we need to use the second step twice in order to replace them with two moles of water vapor.
Combined: 
Since the total reaction is created by step 1 occurring once and step 2 occurring twice, we can write the enthalpy as:
Use the given enthalpies of the steps to calculate the total change in enthalpy.
Hess's law states that the enthalpy of the total reaction is equal to the enthalpy of the steps required to get to the total reaction, regardless of the path that is chosen. This means that we can combine the two half steps with known enthalpies in order to solve for the enthalpy of the main reaction.
Step 1.
Step 2.
Total:
Since step 1 results in two moles of liquid water, we need to use the second step twice in order to replace them with two moles of water vapor.
Combined:
Since the total reaction is created by step 1 occurring once and step 2 occurring twice, we can write the enthalpy as:
Use the given enthalpies of the steps to calculate the total change in enthalpy.
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How much energy is required to heat 
of 
from 
to 
?
How much energy is required to heat
Use the following formula:
Plug in values:
Use the following formula:
Plug in values:
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Which of the following phases and states has the highest entropy?
Which of the following phases and states has the highest entropy?
Entropy is defined as the amount of disorder in a system and is favored in biological and chemical systems. Any system will prefer to have higher entropy, and spontaneous reactions will generally increase entropy in the system.
Gas particles move at higher velocity and with greater range than particles in liquids and solids. This contributes to their high level of entropy. Aqueous solutions gain entropy with the number of ions in solution, but do not reach the same level of entropy of gases. Colloids are homogenized mixtures, such as milk, and follow relatively the same principles as aqueous solutions.
Entropy is defined as the amount of disorder in a system and is favored in biological and chemical systems. Any system will prefer to have higher entropy, and spontaneous reactions will generally increase entropy in the system.
Gas particles move at higher velocity and with greater range than particles in liquids and solids. This contributes to their high level of entropy. Aqueous solutions gain entropy with the number of ions in solution, but do not reach the same level of entropy of gases. Colloids are homogenized mixtures, such as milk, and follow relatively the same principles as aqueous solutions.
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Which of the following results in a decrease in entropy?
Which of the following results in a decrease in entropy?
Entropy can be thought of as the tendency for a system to favor disorder. This means that the least ordered scenario in a system is typically favored by probability. Entropy increases when disorder is increased. Examples include an ice cube melting into a puddle, a gas diffusing all throughout a room, and a mirror shattering. Each of these either increases the energy of the system or results in the creation of multiple pieces/particles from a single object.
When building a road, materials are placed in an ordered, specific manner. This gives it a negative entropy.
Entropy can be thought of as the tendency for a system to favor disorder. This means that the least ordered scenario in a system is typically favored by probability. Entropy increases when disorder is increased. Examples include an ice cube melting into a puddle, a gas diffusing all throughout a room, and a mirror shattering. Each of these either increases the energy of the system or results in the creation of multiple pieces/particles from a single object.
When building a road, materials are placed in an ordered, specific manner. This gives it a negative entropy.
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For any chemical reaction __________.
For any chemical reaction __________.
Any reaction can be thought of as taking place in a system, while the surroundings are the rest of the universe. It helps to remember that the entropy of the universe is constantly increasing. This means that the entropy change in the universe increases following every reaction. A system can have a decrease in entropy, as long as the entropy of the surroundings increases by a greater value.
Any reaction can be thought of as taking place in a system, while the surroundings are the rest of the universe. It helps to remember that the entropy of the universe is constantly increasing. This means that the entropy change in the universe increases following every reaction. A system can have a decrease in entropy, as long as the entropy of the surroundings increases by a greater value.
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Is a process more or less likely to occur as temperature increases when 
is positive?
Is a process more or less likely to occur as temperature increases when
Spontaneity, or the likelihood that a reaction will occur, is determined by Gibbs free energy. The equation for Gibbs free energy is:
is the term for entropy and a negative value for Gibbs free energy indiciates a spontaneous reaction. Thus, as temperature increases the effective value of the entropy term increases as well (since 
is positive). Since the enthalpy,
, remains constant, increasing the entropy term will have the total effect of decreasing the Gibbs free energy since entropy is subtracted from enthalpy. Decreasing the Gibbs free energy will result in a more spontaneous reaction.
Spontaneity, or the likelihood that a reaction will occur, is determined by Gibbs free energy. The equation for Gibbs free energy is:
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What set of conditions always results in a spontaneous reaction?
What set of conditions always results in a spontaneous reaction?
The spontaneity of a reaction can be determined using the Gibb's free energy equation:
In this formula, 
is enthalpy, 
is entropy, and 
is the temperature in Kelvin. In order for a reaction to be spontaneous, Gibb's free energy must be negative. Looking at the equation, we can see that the value for 
will ALWAYS be negative if enthalpy is negative and entropy is positive.
These are the requirements for a reaction that is always spontaneous, regardless of temperature,
The spontaneity of a reaction can be determined using the Gibb's free energy equation:
In this formula,
These are the requirements for a reaction that is always spontaneous, regardless of temperature,
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A reaction has an enthalpy change of 
and an entropy change of 
.
At what temperature range will the reaction be spontaneous?
A reaction has an enthalpy change of
At what temperature range will the reaction be spontaneous?
The first thing we need to do in order to solve this problem is determine at what temperature 
is equal to zero. When 
is equal to zero, the reaction is at equilibrium and will not go one way or the other. Using the Gibb's free energy equation, we can determine the temperature where the reaction is at equilibrium.
The question tells us the enthalpy and entropy values, allowing us to solve for the equilibrium temperature. Remember to convert the enthalpy from kilojoules to Joules.
At 
, the reaction is at equilibrium. In order to make the reaction spontaneous, do we need to raise or lower from this temperature? Notice how entropy in this case is positive. By increasing temperature, the negative portion of the equation will become larger, and will result in a negative value. This means that the above reaction is spontaneous at temperatures higher than 
. You can check you work by solving for the Gibb's free energy at a higher temperature.
Since the value is negative, the reaction is spontaneous at this higher temperature.
The first thing we need to do in order to solve this problem is determine at what temperature
The question tells us the enthalpy and entropy values, allowing us to solve for the equilibrium temperature. Remember to convert the enthalpy from kilojoules to Joules.
At
Since the value is negative, the reaction is spontaneous at this higher temperature.
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Predict the spontaneity of a reaction at 
with an enthalpy change of 
and an entropy change of 
.
Predict the spontaneity of a reaction at
Given the above conditions, we can use the Gibb's free energy equation in order to determine the reaction's spontaneity.
Convert the temperature to Kelvin and the enthalpy to Joules.
Use the given values to solve for the Gibb's free energy.
Since 
is positive at these conditions, we can conclude that the reaction is nonspontaneous and will not take place.
Given the above conditions, we can use the Gibb's free energy equation in order to determine the reaction's spontaneity.
Convert the temperature to Kelvin and the enthalpy to Joules.
Use the given values to solve for the Gibb's free energy.
Since
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The change in Gibb's free energy of a spontaneous reaction is __________.
The change in Gibb's free energy of a spontaneous reaction is __________.
The sign of the change in Gibb's free energy, 
tells us whether a reaction is spontaneous or not (whether the reaction requires the net input of energy or not). For a spontaneous reaction, the 
value is always less than zero. This is because the free energy of the products is less than that of the reactants. This indicates that there is a net release of energy, as opposed to a net energy consumption. When the products are at a lower energy than the reactants, the reaction can proceed spontaneously, and is known as exothermic.
The sign of the change in Gibb's free energy,
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