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”
