Flashcards in General Chemistry-Thermochemistry Deck (190)
What is the 1st law of thermodynamics?
Energy is never created nor destroyed but simply changed from one form to another
What is considered the system?
The matter that is being observed- the total amount of reactants and products in a chemical reaction.
Is the boundary between system and surrounding environment permanetly fixed?
No, it can be moved
What are the three characterizations of a thermodynamic system?
What is an isolated thermodynamic system? Example?
The system cannot exchange energy (heat and work) or matter with the surroundings; for example, an insulated bomb calorimeter.
What is a closed thermodynamic system? Example?
The system can exchange energy (heat and Work) but not matter with the surroundings, for example a steam radiator
What is an open thermodynamic system? Example?
The system can exchange both energy (heat and work) and matter with the surroundings, for example a pot of boiling water.
What is the definition of a process?
When a system experiences a change in one or more of its properties (such as concentrations of reactants or products, temperature, or pressure).
Association with a change of the state of a system
How are some processes unique?
Some processes are uniquely identified by some property that is constant throughout the process. Many of these processes create special conditions that allow the first law of thermodynamics to be simplified.
What is the simplified equation for the 1st law of thermodynamics?
/\U= Q - W
What do the letters represent in the simplified equation for the 1st law of thermodynamics?
/\U: The change in internal energy of the system
Q: is the heat added to the system
W: is the work done by the system
When do isothermal processes occur?
When the system's temperature is constant
What does constant temperature imply in a isothermal process?
The total internal energy of the system (U) is constant throughout the process
Why is the total internal energy of an isothermal system constant throughout the process?
Temperature and internal energy are directly proportional. When U is constant, /\U=0, and the first law simplifies to Q=W (the heat added the the system equals the work done by the system).
How does an isothermal process graph appear?
As a hyperbolic curve
What are the axes used to look at an isothermal process graph?
Pressure and volume (P-V Graph)
How is work represented on an isothermal process graph?
Work is the area under a curve
When does an adiabatic process occur?
When no heat is exchanged between the system and the environment; thus the thermal energy of the system is constant throughout the process.
When Q=0, how does the 1st law of thermodynamics simplify?
/\U = -W (the change in internal energy of the system is equal to the work done on the system [the opposite of work done by the system]).
How does adiabatic process appear on a P-V graph?
When do Isobaric process occur?
When the pressure of the system is constant
Which processes are common? Why
Isothermal and isobaric processes are common because it is usually easy to control temperature and pressure.
How do isobaric processes appear on a P-V graph?
Isobaric processes do not alter the 1st law of thermodynamics, but appear as a flat line.
What are isovolumetric processes?
Processes that experience no change in volume
What is another name for isovolumetric processes?
Why is no work performed in an isovolumetric process?
Because the gas neither expands nor compresses, no work is performed
How does the 1st law of thermodynamics simplify in an isovolumetric process?
/\U =Q (The change in Internal energy is equal to the heat added to the system.)
How does an isovoumetric process appear on a P-V graph?
Vertical Line. The area under the graph which represents work done is zero
How can processe by further classified?
Spontaneous or nonspontaneous
What is a spontaneous process?
One that can occur by itself without having to be driven by energy from an outside source
How do you determine if a process will be spontaneous or nonspontaneous?
Calculating the change in the Gibbs free energy (/\G) for a process, such as a chemical reaction.
What are temperature dependent processes?
The process will be temperature dependent, that is spontaneous at some temperatures and nonspontaneous at others.
What is a common method for supplying energy for nonspontaneous reactions?
Coupling nonspontaneous reactions to spontaneous ones.
What are macroscopic properties?
Properties that describe the system in an equilibrium state.
What is another name for macroscopic properties?
What can macroscopic properties not describe?
The process of the system; that is, how the system got to its current equilibrium.
Macroscopic properties can compare what?
One equilibrium state to another
What are process functions?
The pathway taken from one equilibrium state to another
What are the most important process functions?
State function include what?
Internal energy (U)
Gibbs Free Energy (G)
When do state functions change?
When the state of a system changes from one equilibrium to another
Are state function independent of one another?
Not necessarilly, For example Gibbs free energy is related to enthalpy, temperature, and entropy
What are the standard conditions of systems with various temperatures, pressures, and equilibrium states?
25 degrees C. (298 K)
1 atm Pressure
1 M concentration
Is standard conditions the same as standard temperature and pressure?
What is standard temperature and pressure (STP)?
The temperature is 0 Degrees C (273 K)
1 atm Pressure
When are standard conditions used and when are STP conditions used?
Standard conditions are used for kinetics, equilibrium, and thermodynamics
STP are used for ideal gas calculations
What is standard state of a substance?
Under standard conditions, it is the most stable form a substance.
What is the standard state of important elements at standard conditions?
C (s, graphite)
When is it important to know the standard state of an element?
For thermochemical calculations, such as heats of reactions and heats of formation.
What are the names of the changes in enthalpy, entropy, and free energy that occur when a reaction take place under standard conditions? Symbols?
The Standard enthalpy (/\H degree)
Standard Entropy (/\S degree)
Standard Free energy changes (/\G degree)
What does the degree sign represent in the symbols of standard enthalpy, entropy and free energy changes?
Zero, as the standard state is used as the "zero point" for all thermodyamic calculations.
What are phase diagrams?
Graphs that show the standard and nonstandard states of matter for a given substance in an isolated system, as determined by temperature and pressure.
What are phase changes?
(Solid Liquid Gas)
What is important to know about phase changes?
They are reversible and an equilibrium of phases will eventually be reached at any given combination of temperature and pressure
What happens to H2O at 0 Degrees C and 1 atm?
Ice and water exist in equilibrium
How much water vapor is present when H2O reaches equilibrium in liquid and gas state?
3% water vapor by mass
Phase equilibrium is analogous to what?
The dynamic equilibria of reversible chemical reaction: the concentrations of reactants and products are constant because the rate of the forward and reverse reactions are equal.
Temerpature of a substance in any phase is related to what?
The average kinetic energy of the molecules that make up a substance
What is evaporation?
Some of the molecules near the surface of the liquid may have enough kinetic energy to leave the liquid phase and escape into the gaseous phase.
What is another name for evaporation?
What happens each time the liquid loses a high-energy particle?
The temperature of the remaining liquid decreases.
Evaporation is what type of process?
Endothermic process for which the heat source is the liquid water.
What is boiling?
A specific type of vaporization that occurs only under certain conditions.
Boiling is the rapid bubbling of the entire liquid with rapid release of the liquid as gas particles.
What is the difference between normal evaporation and boiling?
Evaporation can happen in all liquids at all temperatures,
Boiling can only occur above the boiling point of a liquid and involves vaporization through the entire volume of the liquid.
What is condensation?
In a covered or closed container, the escaping molecules are trapped above the solution. These molecules exert a countering pressure, which forces some of the gas back into the liquid phase.
When is condensation facilitated?
By lower temperature or higher pressure
What is vapor pressure?
The pressure the gas exerts over the liquid at equilibrium
When does vapor pressure increase? Why?
As temperature increase because more moleucles have sufficient kinetic energy to escape into the gas phase.
What is boilig point?
The temperature at which the vapor pressure of the liquid equals the ambient (also known as external, applied, or incident) pressure.
Will solids have molecule movement?
Each atom or molecule can undergo motions about some equilibrium position
When do the vibrational motions of molecules in a solid state increase?
When heat is applied
What happens to molecules when temperature is increased?
The molecules have greater freedom of movement and energy disperses
What is the transition from the solid state to the liquid state called?
Fusion or Melting
What is the transition from the liquid state to the solid state called?
Solidification, Crystallization or Freezing
What is the temperature of a solid liquid change or a liquid solid change called?
Melting point or freezing point
What has distinct melting points and what has ranges of melting points?
Pure crystalline solids have distinct, very precise melting points, amorphous solids such as glass, plastic, chocolate and candle wax tend to melt or solidify over a larger range of temperatures, due to their less-ordered molecular structure
What is sublimation?
When a solid goes directly into the gas phase
What is an example of a substance that sublimates?
Dry ice (solid CO2) sublimates at room temperature and atmospheric pressure
What is deposition?
Transition from the gaseous state to the solid state
What is a cold finger device in organic chemistry?
A device that may be used to purify a product that is heated under reduced pressure causing it to sublime. The desired produt is usually more volatile than the impurities, so the gas is purer than the original product and the impurities are left in the solid state. The gas then deposits onto the cold finger, which has a cold water flowing through it, yeilding a purified soid product than can be collected.
What are phase diagrams again?
Graphs that show the temperature and pressure at which a substance will be thermodynamically stable in a particular phase. They also show the temperatures and pressures at which phases will be in equilibrium.
What are the lines on a phase diagram called? What do they indicate?
The lines of equilibrium or the phase boundaries
They indicate the temperature and pressure values for the euilibria between phases. The lines of equilibirum divide the diagram into three regions corresponding to the three phases and they themselves represent the phase transformations.
In general, where is the gas phase found?
At high temperatures and low pressures.
In general, where is the solid phase found?
At low temperatures and high pressures.
In general, where is the liquid phase found?
At moderate temperatures and pressures.
What is the triple point?
The point at which the three phase boundaries meet. This is the temperature and pressure at which the three phases exist in equilibrium.
In a phase diagram, the phase boundary seperating the solid and liquid does what?
Extends indefinitely from the triple point
What is the critical point in a phase diagram?
The phase boundary between the liquid and gas phases, terminates at this point. This is the temperature and pressure above which there is no distinction between the phases.
All temperatures and pressures above the critical point are what?
Are the terms heat and temperature interchangeable?
What is the definition of temperature?
(T) Related to the average kinetic energy of the particles of a substance. Temperature is the way that we scale how hot or cold something is.
What are the different scaes used to measure temperature?
Fahrenheit, Celsius, Kelvin
What is thermal energy?
Or Enthalpy is the average kinetic energy of the particle in a substance
How do you calculate total thermal energy content?
Must know how much substance is present
When a substance's thermal energy increases, what happens?
It's temperature also increases
Does something that is hot always have greater thermal energy? Why?
No, it depends on the amount of substance
How was the Kelvin scale determined?
Via the third law of thermodynamics, which elucidated that there is a finite limit to temperature below which nothing exist. There can be no temperature below 0 K because, by definition the system is said to be unable to lose any more heat energy.
What is the definition of heat?
(Q) is the transfer of energy from one substance to another as a result of their differences in temperature.
What does the zeroth law of thermodynamics imply?
Objects are in thermal equilibirum only when their temperatures are equal.
Heat is a process function or a state function?
What does the 1st law of thermodynamics state?
The change in the total internal energy (/\U) of a system is equal to the amount of heat (Q) transfered to the system minus the amount of work (W) done by the system: /\U= Q-W
Why can you assess the transfer of heat through any process regardless of work done?
Because the heat and work are measured independently
What units are used to measure heat?
The unit of energy: Joule (J) or calorie (cal)
What is the conversion of 1 cal to joules?
1 cal= 4.184 joules
What is equivalent to heat that is what the MCAT assumes on most thermodynamic problems?
Enthalpy (/\H) is equivalent to heat under constant pressure
When a substance goes through a endo or exothermic reaction, what also happns?
Heat energy will be exchanged between the system and the environment
What is calorimetry?
The process of measuring transferred heat
What are the two basic types of calorimetry?
What equation is used to calculate the heat absorbed or released in a given process?
q = mc(/\T)
What do the letters represent in the heat absorption equation?
c: the specific heat of the substance
/\T: Change in temperature (in celsius or kelvins)
What is specific heat?
The amount of energy required to raise the temperature of one gram of a substance by one degree celsius (or one Kelvin)
What is the specific heat of H2o (l)?
What is heat capacities?
The product mc (mass times specific heat)
What is an example of a constant pressure calorimeter?
An insulated container covered with a lid and filled with a solution in which a reaction or physical process, such as dissolution, is occurring. The incident pressure, which is atmospheric pressure, remains constant throughout the process, and the temperature can be measured as the reaction progresses. There should be sufficient thermal insulation (such as styrofoam) to ensure that the heat being measured is an accurate representation of the reaction, without gain or loss of heat to the environment.
What is another term for bomb calorimeter?
Bomb calorimeter is used in what type of calorimetry?
How are constant-volume calorimetry reactions done?
A sample of matter is placed in the steel decomposition vessel, which is then filled with almost pure oxygen gas. The decomposition vessel is then placed in an insulated container holding a known mass of water. the contents of the decomposition vessel are ignited by an electric ignition mechanism, the materal combusts in the presence of the oxygen, and the heat that evolves is the heat of the combustion reaction.
Why is no work done in constant-volume calorimetry?
Because W=P(/\V), no work is done in an isovolumetric process.
Why can constant-volume be considered isolated from the rest of the world?
Because of the insulation.
What would be identified as the whole system in a constant-volume calorimetry?
The system is the sample plus the oxygen and steel vessel, and the surroundings is the water.
How can you calculate the heat of combustion?
Since no heat is exchanged between the calorimeter and the rest of the universe, Q calorimeter is 0. So,
/\U system + /\U surroundings = /\U calorimeter = Q calorimeter- W calorimeter = 0
/\U system = -/\U surroundings
And since no work is done:
q system= -q surroundings
m steel c steel /\T + m oxygen c oxygen /\T = -m water c water /\T
Heat exchange between the system and the surroundings makes it possible for use to calculate the heat combustion.
What is an adiabotic process?
No heat is exchanged between the calorimeter and the rest of the universe, but it is exchanged between the steel decomposition vessel and the surrounding water.
What happens when a compound is heated?
The temperature rises until the melting or boiling point is reached. Then the temperature remains constant as the compound is converted to the next phase. Once the entire sample is converted then the temperature begins to rise again.
What do heat curves show?
Heating curves show that phase change reactions do not undergo changes in temperature.
Why can't the equation q=mc/\T be used for phase change reactions?
Phase changes do not undergo changes in temperature, so /\T would be 0.
Where does all the heat/energy go if the temperature is not going up but heat is still being added during a phase change reaction?
The solid absorbs energy, which allows particles to overcome the attractive forces that hold them in a rigid , three-dimensional arrangement. When melting an ice cube, all of the heat added during the process is used to overcome the intermolecular forces between water molecules in ice, forming liquid water.
What must be used during the transition between a solid-liquid boundry?
The enthalpy (or heat) of fusion (/\H fus) must be used to determine the heat transferred during the phase change.
When transitioning from a solid to liquid, change in enthalpy will be ____________. What about from a liquid to a solid.
positive because heat must be added.
From liquid to solid change in enthalpy will be negative because heat must be removed.
What must be used during the transition between the liquid-gas boundry?
The enthalpy (or heat) of vaporization (/\H vap)
What equation is used for enthalpy of vaporization?
What do the letters represent in the equation q=mL?
L: latent heat, a general term for enthalpy of an isothermal process given in the units cal/g.
How do you calculate the total amount of heat needed to cross multiple phase boundaries?
A summation of the heats for changing the temperature of each of the respective phases and the heats associated with phase changes.
How do scientists express heat change at constant pressure?
How do you find the enthalpy change of a reaction?
/\H rxn, one must subtract the enthalpy of the reactants from the enthalpy of the products.
/\H rxn= H products - H reactants.
What corresponds to a endothermic process? What about exothermic process?
A positive /\H rxn is endothermic
Negative /\H rxn is exothermic
Is it possible to measure enthalpy directly?
No, only /\H can be measured and only for certain fast and spontaneous processes.
What is standard enthalpy of formation?
/\H degree f, the enthalpy required to produce one mole of a compound from its elements in their standard states.
What is standard state again?
The most stable physical state of an element or compound at 298 K and 1 atm
What is the "standard heat of a reaction?"
The enthalpy change accompanying a reaction being carried out under standard conditions.
How can the standard heat of a reaction be calculated?
Taking the difference between the sum of the standard heats of formation for the products and the sum of the standard heats of formation of the reactants.
/\H degree rxn= Sum /\H degree f products - sum /\H degree f reactants
What does Hess's law state?
Enthalpy changes of reactions are additive. When thermochemical equation (chemical equations for which energy changes are known) are added to give the net equation for a reaction, the corresponding heats of reaction are also added to give the net heat of reaction.
Hess's law is embodied in which equation?
What are a couple ways to write the equation for hess's law?
/\H rxn=(/\H reactants -> elements) + (/\H elements -> reactants)
/\H degree rxn= (sum /\H degree f products) - (sum /\H degree f reactants)
Hess's law applies to what?
Any state function, including entropy and gibbs free energy
How can Hess's law be expressed?
In terms of bond enthalpies
What is another name for bond enthalpies?
bond dissociation energies
What is bond dissociation energy?
The average energy that is required to break a particular type of bond between atoms in the gas phase- remember bond dissociation is an endothermic process.
What units are used for bond dissociation energy?
kJ/mol of bonds broken
What are bond enthalpies?
The averages of the bond energies for the same bond in many different compounds.
What is important to note about bond formation and bond breaking?
Bond formation is opposite of bond breaking and has the same magnitude of energy but with a negative rather than positive sign. That is the energy is released when bonds are formed. Bond formation is exothermic and bond dissociation is endothermic.
How is the enthalpy change associated with a reaction given?
/\H degree rxn= sum /\H bonds broken - sum /\H bonds formed= total energy absorbed - total energy released
What is the standard heat of combustion?
/\H degree comb, is the enthalpy change associated with the combustion of a fuel.
What are oxidents?
Hydrogen gas will combust chlorine gas
What is the second law of thermodynamics?
That energy spontaneously disperses from being localized to becomibg spread out if it is not hindered from doing so.
What is entropy?
The measure of the spontaneous dispersal of energy at a specific temperature: How much energy is spread out, or how widely spread out energy becomes in a process.
What is the equationn for calculating entropy?
/\S= (Q rev)/ T
What do the letters represent in the equation to calculate entropy?
/\S: change in entropy
Q rev: the heat that is gained or lost in a reversible process
T: temperature in kelvins
What are the units of entropy?
What happens when energy is distributed into a system at a given temperature in regards to the entropy?
Its entropy increase. When energy is distributed out of a system, entropy is decreased.
Does concentration of energy happen spontaneously in a closed system? What must be done?
No, Work usually must be done to concentrate energy
Based on the second law of thermodynamics, what happens to energy and entropy in a closed system?
Energy will spontaneously spread out, and entropy will increase if it is not hindered from doing so.
Entropy only is dependent on what?
The difference in entropies of the final and initial states
How can the standard entropy change be calculated?
/\S degree rxn= sum /\S degree f products - sum /\S degree f reactants
Gibbs free energy is a combination of what?
Temperature, enthalpy, and entropy
What is Gibbs free energy a measure of?
A measure of the change in enthalpy and the change in entropy as a system undergoes a process, and it indicates whether a reaction is spontaneous or nonspontaneous. The change in free energy is the maximum amount of energy released by a process occuring at constant temperature and pressure- that is available to perform useful work.
What is the equation for the change in Gibbs free energy?
/\G= /\H - T /\S
What do the letters represent in the /\G equation?
/\G= /\H - T /\S
T: temperature in Kelvins
T/\S: The total amount of energy that is absorbed by a system when its entropy increased reversibly
What mneumonic can be used to remember /\G equation?
Goldfish are (equal sign) horrible without (minus sign) Tarter Sauce
A decrease in Gibbs free energy indicates what?
A reaction is spontaneous
Movement towards the equilibrium position is associated with what?
A decrease in Gibbs free energy and is spontaneous
What is the name for a system that releases energy?
What is the name of a system that abosorbs energy?
What is another name for energy minimum state?
What happens when a system is at equilibrium, in regards to the free energy?
The system will resist any changes to its state
How can you summarize the results of /\G?
If /\G is negative, the reaction is spontaneous
If /\G is positive, the reaction is nonspontaneous
If /\G is zero, the system is in a state of equilibrium
What are the outcomes for the different signs of /\H and /\S in the Gibbs free energy, if the temperature is always positive when using Kelvins?
/\H: + /\S: + Outcome: Spontaneous at high T.
/\H: + /\S: - OUtcome: Nonspontaneous at all T.
/\H: - /\S: + Outcome: Spontaneous at all T.
/\H: - /\S: - Outcome: Spontaneous at low T.
What is important to remember in regards to the rate of a reaction?
The rate of a reaction depends on the activation energy Ea not /\G
What is the standard free energy of formation?
Is the free energy change that occurs when 1 mole of a compound in its standard state is produced from its respective elements in their standard states under standard state conditions. The standard free energy of formation for any element under standard state conditions is by definition zero.
What is standard free energy of a reaction?
Is the free energy change that occurs when that reaction is carried out under standard state conditions; that is , when the reactants are converted to the products at standard conditions of temperature (298K) and pressure 1 atm
How can you calculate free energies of the reaction?
/\G degree rxn= sum /\G degree f products - /\G degree f reactants
How can you derive the standard free energy change using Keq?
/\G degree rxn= -RT ln Keq
What do the letters represent in the equation /\G degree rxn= -RT ln Keq?
R: ideal gas constant
T: Temp in Kelvins
Keq: The equilibrium constant
The greater the value of Keq the ____________ the value of _________. In the /\G degree rxn= -RT ln Keq equation.
More positive the value of its natural logorithm. The more positive the natural logarithm, the more negative the standard free energy change. The more negative the standard free energy change, the more spontaneous the reaction.
What happens once a reaction begins, when calculating /\G degree rxn?
The standard state conditions (specifically 1 M solutions) no longer apply
How do you change the equation when calculating /\G degree rxn, once a reaction is in progress?
Relate the /\G rxn to the reaction quotient Q:
/\G rxn= /\G degree rxn + RT ln Q= RT ln Q/Keq
When Q/Keq is less than 1, what happens when using the /\G rxn equation?
Then the natural logarithm will be negative, and the free energy change will be negative, so the reaction will spontaneously proceed forward until equilibrium is reached.
What happens when the ratio Q/Keq is greater than 1?
Then the natural logarithm will be positive, and the free energy change will be positive. The reaction will spontaneously move in the reverse direction until equilibrium is reached.
What happens if Q/Keq is equal to 1?
The reaction quotient is equal to the equilibrium constant; the reaction is at equilibrium and the free energy change is zero.
How can reaction profiles be altered?
By the presence of a catalyst.