Chapter 7: Thermochemistry Flashcards
How are systems classified, regarding thermochemistry.
Systems are classified based on what is or is not exchanged with the surroundings: systems may be isolated, closed, or open, based on the exchange or lack of exchange of energy and matter between the system and its environment.
What is an isolated system? What is a closed system? What is an open system?
Isolated systems exchange neither matter nor energy with the environment such as in a bomb calorimeter.
Closed systems can exchange energy but not matter with the environment such as in a steam radiator.
Open systems can exchange both energy and matter with the environment such as in a pot of boiling water.
What is an isothermal process? Adiabatic process? Isobaric process? Isovolumetric (isochoric) process?
An isothermic process occurs at contact temperature (such as a phase change occurring at a FIXED TEMPERATURE: evaporation of water at a constant temperature) While the temperature stays constant, heat can still be transferred into or out of the system to maintain that constant temperature.
An adiabatic process exchanges no heat with the environment (such as free expansion of a gas into a vacuum as the gas expands against no pressure)
An isobaric process occur at constant pressure (such as water boiling in an open container into constant atmospheric pressure)
An isovolumetric (isochoric) processes occur at constant volume (such as heating a gas in a closed container)
What is a state function?
State functions describe the physical properties of an equilibrium state. They are pathway independent and include P, density, T, V, enthalpy, internal energy, Gibbs free energy, and entropy.
State functions quantitatively describe an equilibrium state of a thermodynamic system, regardless of how the system has arrived in that state.
A state function is a property of a system that only depends on its current state, meaning its value is solely determined by the system’s conditions at a given moment
What are standard conditions? How does this relate to standard state?
Standard conditions are defined at 298 K (approx 76 f and 25 C), 1 atm, and 1 M concentrations.
The standard state of an element is its most prevalent form under standard conditions. Standard enthalpy, standard entropy, and standard free energy are all calculated under standard conditions.
Describe the phase changes at the boundaries between the phases.
Fusion (melting) and freezing (crystallization or solidification) occur at the boundary between solid and liquid phases.
Vaporization (evaporation or boiling) and condensation occur at the boundary between liquid and gas phases.
Sublimation and deposition occur at the boundary between solid and gas phases.
What is a phase diagram?
A phase diagram for a system graphs the phases and phase equilibria as a function of T and P.
Are heat and temperature the same thing? Explain.
Heat and temperature are not the same thing. Temperature is a scaled measure of the average kinetic energy of a substance. Heat is the transfer of energy that results from the differences of temperature between two substances.
The heat content of a system undergoing heating, cooling, or phase changes is the sum of all the respective energy changes.
What is enthalpy?
Enthalpy is a measure of the potential energy of a system found in the intermolecular attractions and chemical bonds.
Enthalpy can also be calculate using heats of formation, heats of combustion, or bond dissociation energies.
H=E+PV
H is enthalpy
E is internal energy
P is pressure
V is volume
What is Hess’s law?
Hess’s law, also known as the law of constant heat summation, states that the total change in potential energy of a system is equal to the changes of potential energies of the individual steps of the process.
Hess’s law states that the total enthalpy change for a chemical reaction is independent of the reaction’s path or the number of steps involved. It’s an early expression of the law of conservation of energy, stating that the heat released depends on the starting and ending states, not the process itself. Key points of Hess’s law: Enthalpy change is proportional to the amount of reactants and products.
What is entropy?
Entropy is often thought of as disorder. Entropy is a measure of the degree to which energy has been spread throughout a system or between a system and its surroundings.
Entropy is a ratio of heat transferred per mole per unit kelvin.
Entropy is MAXIMIZED at equilibrium. Entropy is maximized at equilibrium because, according to the Second Law of Thermodynamics, in an isolated system, the natural tendency is for disorder (entropy) to increase until it reaches a maximum value, and once at this maximum, the system is considered to be in equilibrium, meaning there will be no further spontaneous changes as any change would result in a decrease in entropy, which is not allowed by the Second Law.
How is Gibbs free energy derived?
Gibbs free energy is derived from both the enthalpy and entropy values for a given system.
What does the change in Gibbs free energy determine?
The change in Gibbs free energy determines whether a process is spontaneous or nonspontaneous.
Gibbs free energy depends on temperature. Temperature dependent processes change between spontaneous and nonspontaneous, depending on the temperature. A simple example being melting of ice: ice melts spontaneously above 0°C (deltaG<0), it does not melt spontaneously below 0°C (deltaG>0)
What is the first law of thermodynamics? What is it telling us?
The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another; essentially, it is the principle of conservation of energy, meaning the total amount of energy in a closed system remains constant.
What is an isothermal process? What does this imply regarding the first law of thermodynamics? Talk about the PV graph of an isothermal process. Provide an example of an isothermal process.
An isothermal process occurs when the systems temperature is constant. (First law of thermodynamics is deltaU=Q-W) Constant temperature implies that the total internal energy of the system (U) is constant throughout the process because T and U are directly proportional. When U is constant, deltaU=0 and therefor Q=W (the heat added to the system equals the work done by the system).
Any phase change occurring at a fixed temperature is an example of an isothermic process. Melting of ice at 0°C, boiling of water at 100°C. To maintain a constant temperature, heat must flow into or out of the system as needed.
What is an adiabatic process? Describe it in terms of the first law of thermodynamics. Describe the PV graph. Give an example of an adiabatic process.
An adiabatic process occurs when no heat is exchanged between the system and the environment. (First law of thermodynamics is deltaU=Q-W) When Q=0, then deltaU= -W (the change in internal energy of the system is equal to work done ON the system (opposite of work done BY the system)). Adiabatic processes appears hyperbolic on a PV graph.
An example of an adiabatic process is compressing air into a tire. As the tire fills up the air inside it rapidly compresses, which causes a rise in temperature. This rise in temperature occurs without any heat being transferred into or out of the system, thereby making it an example of an adiabatic process.
What is an isobaric process? Describe in terms of the first law equation. Describe the PV graph. Provide example of an isobaric process.
An isobaric process occurs when the pressure of a system is constant. (First law of thermodynamics is deltaU=Q-W) Isobaric processes are common because it is usually easy to control temperature and pressure. Isobaric processes do not alter the first law. The PV graph appears as a flat line for isobaric processes.
An example of a isobaric process would be heating a gas in a moving piston where the pressure is allowed to change the volume; or boiling a pot of water in an open container where the volume of water increases and pressure maintains constant (atm).
What is an isovolumetric process? Describe in terms of the first law equation. Describe the PV graph of an isovolumetric process. Provide an example of an isovolumetric process.
An isovolumetric process experience no change in volume. Because the gas doesn’t expand or compresses, no work is performed in an isovolumetric process. (First law of thermodynamics is deltaU=Q-W) The first law simplifies to deltaU=Q (because W=0). An isovolumetric process PV graph will be a vertical line (area under curve, representing work done by the gas, is zero).
An example of a an isovolumetric process would be heating a gas in a rigid container where the gas cannot expand.
What is the difference between adiabatic and isothermic processes?
The key difference between an adiabatic process and an isothermal process is that in an adiabatic process, no heat is exchanged with the surroundings, while in an isothermal process, the temperature remains constant throughout, meaning heat transfer can occur to maintain that constant temperature; essentially, an adiabatic process happens without any heat flow, whereas an isothermal process allows heat flow to maintain a constant temperature.
What is a process function?
A process function defines the pathway taken from one equilibrium to another and include heat (Q) and work (W).
Put as simply as possible, what is heat? What is it defined as? Is heat a state function?
Heat is a specific form of energy that can enter or leave a system. Heat is the transfer of energy from one substance to another as a result of their differences in temperature.
Heat is not a state function. Heat is a process function. We can quantify how much thermal energy is transferred between two or more objects as a result of their differences in temperatures by measuring the heat transferred.
When something is hot, does that mean that it necessarily has greater thermal energy than something that is cold?
No we cannot necessarily say that when something is hot is has more thermal energy than something that is cold. For example, a large body of lukewarm water has a greater TOTAL heat content than a small body of hot water.
What are the four laws of thermodynamics?
First law: energy cannot be created or destroyed, only change forms. The net change of energy of a system (deltaU) is the sum total of heat added to the system (Q) minus the work done by the system (W).
Second law: in a spontaneous process, the entropy of the universe increases.
Third law: there is a finite limit to temperature below which nothing can exist, measured as 0 kelvin. The value of entropy of a completely pure crystalline substance is zero at absolute zero temperature.
Zeroth law: if two bodies A and B are in thermal equilibrium with a third body C, then A and B are also in thermal equilibrium with each other. Implies that objects are in thermal equilibrium only when their temperatures are equal.
What is the equation of the first law of thermodynamics? Describe it in words.
The first law of thermodynamics states that the change in the total internal energy of a system is equal to the amount of heat transferred to the system minus the work done by the system.
Is enthalpy equivalent to heat?
Yes, enthalpy is equivalent to heat under constant pressure, which is an assumption the MCAT usually makes for thermodynamic problems.
What are processes that absorb heat called? Release heat?
Processes that absorb heat are called endothermic (deltaG>0)
Processes that release heat are called exothermic (deltaG<0). In an exothermic process, where heat is released to the surroundings, a negative delta G value indicates that the reaction is spontaneous and will proceed naturally, as the system is losing energy and becoming more stable; essentially, a negative delta G means the exothermic reaction is thermodynamically favored
What is the process of measuring heat called? What are the two processes of calorimetry? What is the difference and what is determined by each?
The process of measuring heat is called calorimetry. There are two processes:
Constant pressure calorimetry (coffee cup calorimetry) and constant volume calorimetry (bomb calorimetry).
The key difference between constant pressure and constant volume calorimetry is that in constant pressure calorimetry, the pressure of the system remains constant during a reaction, allowing the measurement of enthalpy change (ΔH), while in constant volume calorimetry, the volume is kept constant, enabling the measurement of the change in internal energy (ΔU) of a reaction; essentially, the main distinction lies in which thermodynamic property is directly measured due to the controlled condition.
At constant pressure, the heat of reaction is equal to the enthalpy change. At constant volume, the heat of reaction is equal to the change in internal energy
Heat (q) absorbed or released in a given process is calculated via what equation?
What is specific heat defined as? What is the specific heat of water?
Specific heat is defined as the amount of energy required to raise the T of one gram of a substance by one degree C or kelvin.
One calorie = 4.184 Joules
What is heat capacity? What is a good conceptualization of heat capacity?
Heat capacity is the product of mass (m) and specific heat (c). A good example is a swimming pool v a glass of water. They have the same specific heat, but different masses and therefor different heat capacity, so it takes more energy to raise their respective temperatures.
Why do aluminum and wood feel hotter on a sunny day even thought they are the same temperature?
Those two materials have different specific heats and therefor feel different to us at the same temperature as we detect heat transfers.
Describe a constant pressure calorimeter.
To picture the set up of a constant pressure colorimeter, just think of the coffee cup calorimeter: an insulated container covered with a lid and filled with a solution in which reaction or some physical process as occurring.
This is a constant pressure process as atmospheric pressure remains constant. When properly insulated, the heat being measured is an accurate representation of the reaction.
With pressure being constant, the heat transfer (q) is directly equal to the enthalpy change (deltaH) recalling q=mcdeltaT. Under a constant pressure, q equals deltaH (change in enthalpy) recalling that H=E+PV.
Describe a bomb colorimeter, also known as a decomposition vessel.
Decomposition vessels are used for constant volume calorimetry. These are typically used for combustion reactions.
No work is done in an isovolumetric process because Delta V equals zero. Given that, the heat that evolves is the heat of the combustion reaction.
What is the equation for work?
What is the equation for enthalpy? How does this relate to constant pressure and constant volume calorimetry? Describe this in terms of work done and how you can extrapolate heat absorbed or released (q) and its relationship to change in enthalpy of a system (deltaH).
Describe this and how it directly relates us to how to solve for energy transfer between systems using calorimeters.
Because the heat is exchanged between the calorimeter and the rest of the universe, Qcalorimeter=0.
Therefore…
qsystem = -qsurroundings
Example from book page 243
Describe a heating curve for a single compound. Explain what this shows specifically regarding phase changes and relate this to how we calculate q for a phase change.
Heating curve show that phase change reactions do not undergo changes in temperature and for this reason we cannot use q=mcdeltaT because during this interval deltaT=0 and we will therefore erroneously get q=0 even thought heat is being transferred to the molecular bonds allowing for a phase change.
During phase changes, we must use values based on enthalpy, or in this case heat.
What is the equation for a phase change and briefly describe why this equation needs to be used and why we cannot use q=mcdeltaT.
We cannot use q=mcdeltaT during phase changes as the change in temperature during phase changes is zero, giving us a value of zero for q.
Instead, we must use an equation in terms of enthalpy, or heat, as heat is still being transferred to the intermolecular bonds while temperature remains unchanged. That equation is:
Example from the book page 245:
We need to first recognize that this calculation requires multiple phase changes and we need to use two different equations to solve this problem. To solve this problem, we will need to calculate an energy increase based on change in temperature to the phase transition of the solid, a solid liquid face transition, another change in energy related to an increase in temperature and an ice to liquid face transformation.
What is enthalpy and how can it be calculated?
Enthalpy is a measure of the potential energy of a system found in intermolecular attractions and chemical bonds. In other words, ENTHALPY EXPRESSES HEAT CHANGES AT CONSTANT PRESSURE. The change in entropy is equal to the heat transferred into or out of the system at constant pressure.
Enthalpy can be calculated using heats of formation, heats of combustion, or bond dissociation energies.
What does a positive deltaHrxn and a negative deltaHrxn correspond to regarding thermochemical reactions?
What is standard enthalpy of formation?
Standard heat of formation, or standard enthalpy of formation, is the enthalpy required to produce one mole of a compound from its elements in their standard states. Recall that standard state refers to the most stable physical state of an element or compound at 298 K and 1 atm.
Note that the standard entropy of formation of an element in it standard state by definition is zero.
What is standard enthalpy of a reaction?
Standard heat of reaction, or standard enthalpy of a reaction is the enthalpy change accompanying a reaction being carried out under standard conditions. This is calculated by taking the difference between the sum of standard heat of formation for the products and the sum of standard heat of formation of the reactants:
What is the equation for generalized enthalpy of reaction?
What is the equation for standard enthalpy of a reaction?
Example problem in book page 249.
What is the equation for bond enthalpy of a reaction?
Example from book page 251:
What is a standard heat of combustion?
Standard heat of combustion is the enthalpy change associated with the combustion of fuel. Because measurements of enthalpy change require a reaction to be spontaneous and fast, combustion reactions are the ideal process for such measurements.
Keep in mind that most combustion reactions presented on the MCAT occur in the presence of atmospheric oxygen. However, combustion can occur where oxygen is not the oxidant. For example, diatomic fluorine can be used as an oxidant.
What is the balanced equation for the glycolytic pathway? What kind of reaction is it?
Image showing the overall heat of combustion of glycolysis, showing the numerous reactions and pathways involved. You could be able to use this to solidify your understanding of heat of combustion and Hess’s law.
Page 252
Concept check page 253
Book answer on page 266
Concept check page 253
Book answer on page 266
What is entropy? What is the equation for change in entropy. What are the units of entropy?
Entropy is the measure of spontaneous dispersal of energy at a specific temperature: how much energy is spread out, or how widely spread out energy becomes, in a process.
How do you calculate the standard entropy change for a reaction?
How do you calculate the standard entropy change for a reaction?
What is the second law of thermodynamics state?
The second law of thermodynamic states that energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.
The second law has been described as times arrow because there is a unidirectional limitation on the movement of energy by which we recognize before and after or new and old. For example, you would instantly recognize whether a video recording of an explosion was running forward or backward. Another way of understanding this is to say that energy in a close system will spontaneously spread out, and entropy increases if it is not hindered from doing so.
The second law ultimately claims that the entropy of the universe is increasing:
Concept check on page 255, entropy.
What is Gibbs free energy, G? What is the equation for a change in Gibbs free energy? What is the pneumonic to remember the equation for change in Gibbs free energy?
Gives free energy is a state function that is a combination of temperature, enthalpy, and entropy. The change in Gibbs free energy is a measure of the change in the enthalpy and the change in entropy as a system undergoes a process, and it indicates whether a reaction is spontaneous or non-spontaneous. The change in the free energy is the maximum amount of energy released by a process, occurring at constant temperature and pressure, that is available to perform useful work.
A handy pneumonic to remember the equation for change in Gibbs Free energy is:
Goldfish are horrible without tartar sauce.
Describe the graph of Gibbs, free energy and the reaction quotient regarding spontaneity and equilibrium.
Think of that graph as a valley between two hills. Just as a ball would tend to roll down the hill into the valley and eventually come to rest at the lowest point in the valley, any system (including chemical reactions) move in whichever direction results in a reduction of the free energy of the system. The bottom of the valley represents equilibrium, and the sides of the hill represent the various points in the pathway toward or away from the equilibrium.
Is the following graph of free energy v reaction coordinate exergonic or endogenic?
DeltaG < 0 and therefore the reaction is exergonic.
Is the following graph of free energy v reaction coordinate exergonic or endergonic?
DeltaG > 0 therefore the reaction is endergonic.
If deltaG = 0, what is the reaction doing? Describe this in terms of the equation of deltaG.
If deltaG = 0 then the reaction is in a state of equilibrium.
Because the temperature in Gibbs free energy is in Kelvin, it is always positive. Therefore, the effects of the signs of deltaH and deltaS on the spontaneity of a process can be determined at varying temperatures.
What is the standard Gibbs free energy from equilibrium constant equation? The equation that allows us to make qualitative and quantitative evaluations of the free energy change of reaction, and therefore the spontaneity of a reaction.
What is the Gibbs free energy from reaction quotient equation? Once a reaction begins, the standard state conditions no longer apply. The value of the equilibrium constant must be replaced with another number that is reflective of where the reaction is in its path towards equilibrium.
Will the addition of a catalyst change the free energy of a reaction and therefore the spontaneity of a reaction? How will the addition of a catalyst impact a reaction?
The addition of a catalyst will not change the free energy of a reaction, but will reduce the energy of activation and therefore the expediency of the reaction.
Concept check page 261 regarding Gibbs free energy.
Concept check page 261 regarding Gibbs free energy, question 2.
Concept check page 261 regarding Gibbs free energy, question 3.
If you were to suddenly flood the reaction vessel with significant amounts of ammonia, the reaction would shift to the left, creating reactants until a new equilibrium was reached.
This can be thought of in two different ways :
Le Chatelier’s principal would tell us that adding products would cause stress to the system shifting the equation to the left, creating more products until equilibrium is met.
The value of Q would increase significantly, as concentration of products is the numerator in the equilibrium equation, causing Q>Keq, and deltaG>0, thus forcing the equation to the left.
Page 244 thermochemistry Keq question:
Question page 224 thermochemistry:
Question page 224 thermochemistry:
Question page 225 thermochemistry:
Question page 225 thermochemistry:
Question page 225 thermochemistry:
Question page 225 thermochemistry:
Question page 225 regarding deltaG, Keq, and Q:
The answer is C.
Entropy question page 225:
Potential energy and reaction coordinate graph question page 226:
Calculating deltaS question page 226:
Explosions questions page 226:
Equations to remember for chapter 7:
