Ch 7,8- Thermochemistry, Gases Flashcards
“the first law of thermodynamics is
energy is never created nor destroyed but—at most—simply changed from one form to another”
“Simply put, the ______ is the matter that is being observed—the total amount of reactants and products in a chemical reaction. It could be the amount of solute and solvent used to create a solution. It could be the gas inside a balloon. Then, the ______ are everything outside of that system”
System; surroundings, or environment
“When a system experiences a change in one or more of its properties (such as concentrations of reactants or products, temperature, or pressure), it undergoes a
process”
“first law of thermodynamics equation
ΔU = Q – W
“where ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system.”
______ processes occur when the system’s temperature is constant.
Isothermal
“Constant temperature implies that the total internal energy of the system (U) is constant throughout the process. This is because temperature and internal energy are directly proportional. When U is constant, ΔU = 0 and the first law simplifies to Q = W (the heat added to the system equals the work done by the system). An isothermal process appears as a hyperbolic curve on a pressure–volume graph (P–V graph)”
_______ processes occur when no heat is exchanged between the system and the environment”
“Adiabatic
“ thus, the thermal energy of the system is constant throughout the process. When Q = 0, the first law simplifies to ΔU = –W (the change in internal energy of the system is equal to work done on the system [the opposite of work done by the system]). An adiabatic process also appears hyperbolic on a P–V graph”
processes occur when the pressure of the system is constant.
“Isobaric
Isothermal and isobaric processes are common because it is usually easy to control temperature and pressure. Isobaric processes do not alter the first law, but note that an isobaric process appears as a flat line on a P–V graph”
“Pressure is constant in an isobaric process; the slope of the line is therefore zero.”
processes experience no change in volume.
“isovolumetric (isochoric)
Because the gas neither expands nor compresses, no work is performed in such a process. Thus, the first law simplifies to ΔU = Q (the change in internal energy is equal to the heat added to the system). An isovolumetric process is a vertical line on a P–V graph; the area under the curve, which represents the work done by the gas, is zero.”
“A common method for supplying energy for nonspontaneous reactions is by
coupling nonspontaneous reactions to spontaneous ones”
“The state functions include
pressure (P), density (ρ), temperature (T), volume (V), enthalpy (H), internal energy (U), Gibbs free energy (G), and entropy (S).”
while state functions are independent of the path (process) taken, they are not necessarily independent of one another. Gibbs free energy is related to enthalpy, temperature, and entropy.”
“State functions: When I’m under pressure and feeling dense, all I want to do is watch TV and get HUGS.
Pressure (P), density (ρ), temperature (T), volume (V), enthalpy (H), internal energy (U), Gibbs free energy (G), and entropy (S).”
“The standard conditions are defined as
25°C (298 K), 1 atm pressure, and 1 M concentrations. ”
“Don’t confuse standard conditions with standard temperature and pressure (STP), for which the temperature is 0°C (273 K) and pressure is 1 atm. Standard conditions are used for kinetics, equilibrium, and thermodynamics problems; STP is used for ideal gas calculations.”
“Under standard conditions, the most stable form of a substance is called the _______ of that substance.”
standard state
“You should recognize the standard states for some elements and compounds commonly encountered on the MCAT. For example, H2 (g), H2O (l), NaCl (s), O2 (g), and C (s, graphite)
“The changes in enthalpy, entropy, and free energy that occur when a reaction takes place under standard conditions are called the
standard enthalpy, standard entropy, and standard free energy changes, respectively,
are symbolized by ΔH°, ΔS°, and ΔG°. The degree sign in these variables represents zero, as the standard state is used as the “zero point” for all thermodynamic calculations.”
“As with all equilibria, the rates of the forward and reverse processes will be ______ when considering phase changes.”
The Same
“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. This process is known as
evaporation or vaporization”
“Evaporation is an endothermic process for which the heat source is the liquid water. Of course, the liquid water itself may be receiving thermal energy from some other source, as in the case of a puddle of water drying up under the hot summer sun or a pot of water on the stovetop. Given enough energy, the liquid will completely evaporate.”
“is a specific type of vaporization that occurs only under certain conditions.
Boiling
Any liquid will lose some particles to the vapor phase over time; however, boiling is the rapid bubbling of the entire solution with rapid release of the liquid as gas particles. While evaporation happens 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.”
“Remember that heat and temperature are different. Heat is _____ Temperature is ______
Heat is a specific form of energy that can enter or leave a system, while temperature is a measure of the average kinetic energy of the particles in a system.”
“is the transfer of energy from one substance to another as a result of their differences in temperature. In fact, the zeroth law of thermodynamics implies that objects are in thermal equilibrium only when their temperatures are equal.”
Heat
“Heat is therefore a process function, not a state function: we can quantify how much thermal energy is transferred between two or more objects as a result of their difference in temperatures by measuring the heat transferred.”
“the first law of thermodynamics states that
the change in the total internal energy (ΔU) of a system is equal to the amount of heat (Q) transferred to the system minus the amount of work (W) done by the system: ΔU = Q − W.”
“1 cal =
4.184 J”
“Enthalpy (ΔH) is equivalent to heat (Q) under
constant pressure”
“which is an assumption the MCAT usually makes for thermodynamics problems.”
“The process of measuring transferred heat is called
calorimetry.”
“Two basic types of calorimetry include constant-pressure calorimetry and constant-volume calorimetry. The coffee-cup calorimeter, introduced at the beginning of this chapter, is a low-tech example of a constant-pressure calorimeter, while a bomb calorimeter is an example of a constant-volume calorimeter.”
“The heat (q) absorbed or released in a given process is calculated via the equation:”
q = mcΔT
“where m is the mass, c is the specific heat of the substance, and ΔT is the change in temperature (in Celsius or kelvin).”
“It requires less heat to raise the temperature of a glass of water the same amount as a swimming pool. While these two items have the same specific heat, c, they have different __________—the product mc (mass times specific heat).”
Heat Capacities
“Heating curves show that phase change regions do not undergo changes in temperature. For this reason, we cannot use
q = mcΔT during this interval because ΔT = 0.
“so where does this heat go? 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. Once all of the ice has been turned into liquid water, the temperature of the liquid water can then increase again.”
“When transitioning at the solid–liquid boundary, _______ must be used to determine the heat transferred during the phase change”
the enthalpy (or heat) of fusion (ΔHfus)
“When transitioning from solid to liquid, the change in enthalpy will be positive because heat must be added; when transitioning to a liquid from a solid, the change in enthalpy will be negative because heat must be removed. ”
“At the liquid–gas boundary, _________ must be used, and its sign notations also follow a similar pattern.”
the enthalpy (or heat) of vaporization (ΔHvap)
Heat of fusion and heat of vap are utilized in which equaiton
q=mL
“where m is the mass and L is the latent heat, a general term for the enthalpy of an isothermal process, given in the units”
“To express heat changes at constant pressure, chemists use the term
enthalpy (H)”
“Enthalpy is a state function, so we can calculate the change in enthalpy (ΔH) for a system that has undergone a process—for example, a chemical reaction—by comparing the enthalpy of the final state to the enthalpy of the initial state, irrespective of the path taken”
“The change in enthalpy is equal to the heat transferred into or out of the system at constant pressure. To find the enthalpy change of a reaction, ΔHrxn, one must
subtract the enthalpy of the reactants from the enthalpy of the products:”
ΔHrxn = Hproducts – Hreactants
“A positive ΔHrxn corresponds to an endothermic process, and a negative ΔHrxn corresponds to an exothermic process.”
“is the enthalpy required to produce one mole of a compound from its elements in their standard states.”
“The standard enthalpy of formation of a compound, ΔH°f
“Remember that standard state refers to the most stable physical state of an element or compound at 298 K and 1 atm. Note that ΔH°f of an element in its standard state, by definition, is zero. ”
________ is the enthalpy change accompanying a reaction being carried out under standard conditions
“The standard heat of a reaction, ΔH°run,
T
his can be calculated by 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:
ΔH°rxn = Σ ΔH°f,products − Σ ΔH°f,reactants
“Enthalpy is a state function and is a property of the equilibrium state, so the pathway taken for a process is irrelevant to the change in enthalpy from one equilibrium state to another. As a consequence of this, ________ states that enthalpy changes of reactions are additive. ”
Hess’s Law
“When thermochemical equations (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”
“It is important to realize that Hess’s law applies to any state function, including entropy and Gibbs free energy.”
“Remember that atoms generally form bonds to become more stable (often by completing an octet). Thus, it makes sense that bond formation is _______ and bond dissociation is ______. The enthalpy change associated with a reaction is given by
Exothermic, Endothermic
ΔH°comb, is the enthalpy change associated with the combustion of a fuel. ”
“standard heat of combustion,
“freezing is accompanied by a ______ in entropy, as the relatively disordered liquid becomes a well-ordered solid. Meanwhile, boiling is accompanied by a large _______ in entropy, as the liquid becomes a much more disordered gas”
Decrease; Increase
What is the phase transition with the greatest increase in entropy
Sublimation
“states that energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.”
“The second law of thermodynamics”
______ is a state function, so a change from one equilibrium state to another is pathway independent and only depends upon the difference in entropies of the final and initial states.
EntropyFurther, the standard entropy change for a reaction, ΔS°rxn, can be calculated using the standard entropies of the reactants and products—much like enthalpy:
ΔS°rxn = Σ ΔS°products − Σ ΔS°reactants”
“Be careful not to confuse endergonic/exergonic (describing _______) with endothermic/exothermic (describing ______).”
Gibbs free energy; enthalpy
“If ΔG is zero, the system is in
a state of equilibrium; ΔH = TΔS.”
“It is important to remember that the rate of a reaction depends on the _____ not ΔG.
activation energy Ea,
Spontaneous reactions may be fast or slow.”
If ΔH is + and ΔS is + the outcome is
Spontaneous at High T
If ΔH is + and ΔS is - the outcome is
NonSpontaneous at all T
If ΔH is - and ΔS is + the outcome is
Spontaneous at All T
If ΔH is - and ΔS is - the outcome is
Spontaneous at low T
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 of a compound, ΔG°f,
“Like enthalpy and entropy, the free energy of the reaction can be calculated from the free energies of formation of the reactants and products:”
What is the standard free energy change for a reaction?
“ΔG°rxn = –RT ln Keq”
“where R is the ideal gas constant, T is the temperature in kelvin, and Keq is the equilibrium constant. This equation allows us to make not only quantitative evaluations of the free energy change of a reaction, but also qualitative assessments of the spontaneity of the reaction. The greater the value of Keq, the more positive the value of its natural logarithm. 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”
ln 1 =
0
_____processes occur at a constant temperature.
Isothermal
______ processes exchange no heat with the environment.
Adiabatic
______ processes occur at a constant pressure.”
Isobaric
“________ processes occur at a constant volume.”
“Isovolumetric (isochoric)
“describe the physical properties of an equilibrium state; they are pathway independent and include pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy, and entropy.”
State function
“The ____ _____ of an element is its most prevalent form under standard conditions;
standard state
standard enthalpy, standard entropy, and standard free energy are all calculated under standard conditions.”
______ is a scaled measure of the average kinetic energy of a substance.
_____ is the transfer of energy that results from differences of temperature between two substances.”
Temperature; Heat
“is a measure of the potential energy of a system found in intermolecular attractions and chemical bonds.”
Enthalpy
“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
How are atm and mmHg and torr and kPa related?
“1 atm = 760 mmHg ≡ 760 torr = 101.325 kPa
“refers to conditions of 273 K (0°C) and 1 atm.”
Standard Temperature and Pressure (STP)
“The two standards involve different temperatures and are used for different purposes. STP (273 K and 1 atm) is generally used for gas law calculations; standard state conditions (298 K, 1 atm, 1 M concentrations) are used when measuring standard enthalpy, entropy, free energy changes, and electrochemical cell voltage.”
“real gases deviate from ideal behavior at
high pressures (low volumes) and low temperatures, many compressed real gases demonstrate behavior that is close to ideal.”
Anything that will increase intermolecular forces
Formula for density of a gas
Density= PM/RT
M is molarity
“ In 1801, John Dalton derived an expression, now known as Dalton’s law of partial pressures, which states that
the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the individual components. The equation for Dalton’s law is
“PT = PA + PB + PC +
“When more than one gas is in a container, each contributes to the whole as if it were the only gas present. Add up all of the pressures of the individual gases and you get the whole pressure of the system.”
“The partial pressure of a gas is related to its mole fraction and can be determined using the following equation:
Pa=XaP
where xa is the mole fraction
“The solubility of a gas will _____ with increasing partial pressure of the gas.”
Increase
“Understanding concepts will be much more fruitful on Test Day than memorizing all of the facts. The higher the temperature, _______ The larger the molecules ______
the faster the molecules move; the slower they move.”
The root mean square speed is given by what equation for gases
Urms= Rad(3RT/M)
“The kinetic molecular theory of gases predicts that _____ gases diffuse more slowly than _____ ones because of their differing average speeds”
Heavier; Lighter
“Because all gas particles have the same average kinetic energy at the same temperature, it must be true that particles with greater mass travel at a slower average speed.”
“
What is grams law
r1/r2=Square root (M2/M1)
From this equation, we can see that a gas that has a molar mass four times that of another gas will travel half as fast as the lighter gas.
“is the flow of gas particles under pressure from one compartment to another through a small opening”
Effusion
“Graham used the kinetic molecular theory of gases to show that, for two gases at the same temperature, the rates of effusion are proportional to the average speeds”
______—When gases mix with one another.
______—When a gas moves through a small hole under pressure.
Both will be slower for larger molecules.
Both conditions use the same equation.”
Diffusion; Effusion
“states that the amount of gas dissolved in solution is directly proportional to the partial pressure of that gas at the surface of a solution.”
Henrys law
“Real gases deviate from ideal behavior under
high pressure (low volume) and low temperature conditions.”
“At moderately high pressures, low volumes, or low temperatures, real gases will occupy less volume than predicted by the ideal gas law because the particles have intermolecular attractions.”
“describes the behavior of gas diffusion or effusion, stating that gases with lower molar masses will diffuse or effuse faster than gases with higher molar masses at the same temperature.”
Grahams Law
“Diffusion is the spreading out of particles from high to low concentration.
Effusion is the movement of gas from one compartment to another through a small opening under pressure.”
“The van der Waals equation of state is used to correct the ideal gas law for
Excerpt From: Kaplan. “Kaplan MCAT General Chemistry Review: Created for MCAT 2015 (Kaplan Test Prep).” iBooks.
intermolecular attractions (a) and molecular volume (b).”
“At moderately high pressures, low volumes, or low temperatures, real gases will occupy less volume than predicted by the ideal gas law because the particles have intermolecular attractions.”
“At extremely high pressures, low volumes, or low temperatures, real gases will occupy more volume than predicted by the ideal gas law because the particles occupy physical space.”
