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Gen Chem II - Thermo
Definitions and Equations
40
Chemistry
Undergraduate 1
07/28/2012

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Term
Kinetic Energy
Definition
(1/2)mv^2
Term
Heat
Definition
Thermal energy transferred between two objects as the result of a temperature difference.
Term
Temperature
Definition
the measure of the kinetic energy of molecular motion
Term
The First Law of Thermodynamics
Definition
Energy can neither be created nor destroyed
Term
Isolated system
Definition
Neither mass nor energy can be exchanged with the surroundings
Term
Open system
Definition
Both mass and energy can be exchanged with the surroundings
Term
Closed system
Definition
Energy, but not mass can be exchanged with the surroundings
Term
Force
Definition
m * d => m*a*d
Term
Work
Definition
the force that produces the movement of an object times the distance moved

F * d
Term
Most common type of work in chemical systems
Definition
Expansion work aka Pressure-Volume Work or PV work

W = -P * A * d = -P * DeltaV
Term
Total Energy Change of a System
Definition
q + W =
q - P * DeltaV
q is (+) if the system gains heat, (-) if the system loses heat
Term
Transferred Heat at constant volume
Definition
q = DeltaE
Term
Transferred Heat at constant pressure
Definition
q = DeltaE + P * DeltaV
Term
Enthalpy Change
Definition
The "heat of the reaction"
equivalent to transferred heat at constant pressure (this is the most common type of chemical reaction in practice)

DeltaH = DeltaE + P * DeltaV

! A state function -> only enthalpy change is important (same with internal energy)

! values assume rxn is going in the direction as it is written; switch sign for reverse rxn

! it is useful to remember the equation P * DeltaV = change in mol * R * T ..so..
DeltaH = DeltaE + (Change in mol)*R*T
Term
Thermodynamic Standard State
Definition
Most stable form of a substance at 1 atm pressure and at a specified temperature usually 25 degrees celsius, 1 M concentration for all substances in solution
Term
Sublimation and Enthalpy
Definition
the direct conversion of a solid to a vapor without going through a liquid. Equals the sum of the heat of fusion and heat of vaporization.
Term
Endothermic
Definition
Products have more enthalpy than the reactants, DeltaH is positive; heat flows into the system.
Term
Exothermic
Definition
Products have less enthalpy than the reactants, DeltaH is negative; heat flows out of the system.
Term
Calorimeter
Definition
A stirrer, thermometer and loose-fitting lid to keep the contents at atmospheric pressure; thermometer measures change in temperature to calculate enthalpy.
Term
Bomb Calorimeter
Definition
Measures enthalpy for a combustion rxn.
Used to calculate heat capacity:
C = q/DeltaT

!Note that capital C is for heat capacity
Term
Heat capacity
Definition
The amount of heat required to raise the temperature of an object or substance a given amount.
An extensive property so its value depends on both the size of an object and its composition.
Term
Specific Heat
Definition
the amount of heat necessary to raise the temperature of 1 g of a substance by 1 degree celsius; the amount of heat necessary to raise the temperature of a given object, then is the specific heat times the mass of the object times the rise in temperature:

q = (specific heat) * (Mass of substance) * DeltaT = C * m * DeltaT
Term
Hess' Law
Definition
The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.
Term
Standard Heats of Formation
Definition
The enthalpy change for the formation of 1 mol of a substance in its standard state from its constituent elements in their standard states.
Term
Spontaneous Process
Definition
A Process that, once started, proceeds on its own without a continuous external influence.
Favored by a decrease in H (-) and increase in S (+)
Term
Nonspontaneous Process
Definition
By contrast takes place only in the presence of a continuous external influence.
Favored by an increase in H (+) and decrease in S (-)
Term
Entropy
Definition
(S) the amount of molecular randomness in a system
Term
Bond Dissociation Energies
Definition
(D) a standard enthalpy change for the corresponding bond-breaking reaction

! always positive because energy must always be put into bonds to break them


We can calculate an approximate enthalpy change for any reaction by subtracting the sum of the bond dissociation energies in the products from the sum of the bond dissociation energies in the reactions.
Term
Heat of combustion
Definition
Standard enthalpy values, but per gram instead of per mole, to compare efficiency
Term
Free-Energy
Definition
DeltaG = DeltaH - T * DeltaS

If negative => process is spontaneous
If 0 => process is at equilibrium
If positive => process is nonspontaneous
Term
Celsius to Kelvin
Definition
K - 273
Term
Kelvin to Celsius
Definition
C + 273
Term
The Third Law of Thermodynamics
Definition
The entropy of a perfectly ordered crystalline substance at 0 K is zero
Term
The Second Law of Thermodynamics
Definition
In any spontaneous process, the total entropy of a system and its surroundings always increases
Term
The standard entropy of reaction
Definition
For the rxn aA + bB -> cC + dD,
DeltaS = [(c * S value of C) + (d * S value of D)] - [(a * S value of A) + (b * S value of B)]
Term
Total entropy change
Definition
DeltaS = DeltaS for system + DeltaS for surroundings
Term
Total entropy change for surroundings
Definition
-Enthalpy/T in Kelvins
Term
Standard State Conditions
Definition
Solids, liquids, and gases in pure form at 1 atm pressure

Solutes at 1 m concentration

A specified temperature, usually 25 degrees celsius
Term
Free-energy using REaction Quotient
Definition
DeltaG + R * T lnQ

R is the gas constant
T is the absolute temperature in Kelvins and Q is the reaction quotient
Term
Free Energy for a reaction moving towards equilibrium
Definition
At equilibrium DeltaG equals 0, so...

Standard DeltaG = -R * T * lnK

lnK > 0 K>1 mainly products
lnK < 0 K<1 mainly reactants
lnK = 0 K=1 comparably amounts of reactants and products
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