Thermochemistry (Chang) Flashcards
Energy
Capacity to do work
Work
Directed energy change resulting from a process
Radiant energy/ solar energy
Comes from the sun; Earth’s primary energy source
Thermal energy
Energy associated with random motion of atoms and molecules
Kinetic energy
Energy produced by a moving object
Chemical energy
Stored within the structural units of chemical substances
Potential energy
Energy available by virtue of an object’s position
*Chemical energy can be considered as form of potential energy because it is associated with relative positions and arrangements of atoms within a given substance
Law of conservation of energy
Total quantity of energy in the universe is assumed constant
Heat
Transfer of thermal energy between two bodies that are at different temperatures (hot to cold)
Thermochemistry
-study of heat change in chemical reactions
System
Specific part of the universe that is of interest to us
Surroundings
-the rest of the universe outside the system
Open system
Can exchange mass and energy with its surroundings
Closed system
Allows the transfer of energy (heat) but not mass
Isolated system
-totally insulated container
-does not allow the transfer of either mass or energy
Exothermic process
-any process that gives off heat; transfers thermal energy to surroundings
energy products < energy reactants
Endothermic process
-heat has to be supplied to the system by the surroundings
energy reactants < energy products
Enthalpy, H
H = E + PV
Enthalpy of reaction ∆H
∆H = Hprod - Hreact
Thermochemical equations
Show the enthalpy changes as well as the mass relationships
Calorimetry
Measurement of heat changes
(1) Specific heat, s
and
(2) Heat capacity, C
- Amount of heat required to raise the temperature of one gram of substance by one degree C
- Amount of heat required to raise the temperature of a given quantity of the substance by one degree C
C = ms
Heat absorbed/ release
Q = ms∆T
Q = C∆T
Constant volume bomb calorimeter
-constant volume of container
-Combustion reaction: heat of combustion
-“bomb”: refers to explosive nature of the reaction in the presence of oxygen gas
Qsys = Qwater + Qbomb + Qrxn = 0
Qwater = ms∆T
Qbomb = Cbomb.∆T
Constant-pressure calorimeter
-heat changes for non-combustion reactions
-acid base neutralizations; heat of solution and heat of dilution
Qrxn = ∆H
Qsys = Qsoln + Qcal + Qrxn = 0 (assuming no heat loss to surroundings)
Standard enthalpy of formation, ∆H°f
-Heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm
-standard enthalpy of formation of any element in its most stable form is zero
-elements: standard state at 1 atm
Standard enthalpy of reaction, ∆H°rxn
Enthalpy of a reaction carried out at 1 atm
∆H°rxn = SUM(n. ∆H°f,products) - SUM(n. ∆H°f,reactants)
Methods for determining ∆H°f
1. DIRECT METHOD
-for compounds that can be readily synthesized from their elements
i.e.
C(graphite) + O2(g) ➡️ CO2(g)
1. Measure ∆H°rxn
2. Use
∆H°rxn = SUM(n. ∆H°f,products) - SUM(n. ∆H°f,reactants)
Methods for determining ∆H°f
2. INDIRECT METHOD
-used for compounds that cannot be directly synthesized from their elements
-Use HESS’S LAW:
when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step of into a series of step
Germain Henri Hess
Father of thermochemistry
Heat of solution/
Enthalpy of solution, ∆Hsoln
-Heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent
-may either be - (exothermic) or + (endothermic) depending on the nature of cation/ anion
∆Hsoln = Hsoln - Hcomponents
(∆Hsoln: measured using constant pressure calorimeter)
Dissolution of NaCl
NaCl in solid state ➡️
(1) Latice energy (+): Na+ Cl- ions in gaseous state ➡️
(2) Heat of hydration (-): water molecules shield Na+ and Cl- from each other
Heat of soln: ∆Hsoln = U + ∆Hhydra
Lattice energy, U
Energy required to completely separate one mole of solid ionic compound into gaseous ions
Heat of hydration, ∆Hhydration
-enthalpy change associated with hydration process
-negative quantity for cations and anions
Heat of dilution
-heat change associated with the dilution process
-(endothermic) more heat absorbed when diluted
-(exothermic) more heat released when diluted
Thermodynamics
-study of the interconversion of heat and other kinds of energy
State of a system
-the values of all relevant macroscopic properties (i.e. composition, energy, temperature, pressure volume)
State functions
-properties that are determined by the state of the system, regardless of how that condition was achieved (energy, temperature, pressure and volume)
First Law of Thermodynamics
-based on law of conservation of energy
-energy can be converted from one form to another but cannot be created nor destroyed
∆Esys = -∆Esurr
∆Esys = Q + W
(+) Q: endothermic
(-) Q: exothermic
(+) W: done on the system
(-) W: done by the system
Internal energy, E
•Kinetic energy: various types of molecular motion and the movement of electrons within molecules
•Potential energy: attractive attractions between electrons and nuclei and by repulsive interactions between electrons and between nuclei in individual molecules, as well as interactions between molecules
Work
W = Fd
W = -P∆V
(if gas expanding on vacuum, P=0; W=0)
1 L.atm = 101.3 J
Heat and work
-path functions
-not properties of a system; manifest only during a process/ change
Enthalpy and the first law of thermodynamics
•Constant-volume
∆E = Q
•Constant-pressure
∆E = Q - P∆V
∆H = ∆E + P∆V
Q = ∆H
*kapag may gas sa product, ipupush ung piston pataas (expansion)
(Another way for ∆E of gaseous rxn: assume ideal gas behavior and constant temperature)
∆E = ∆H - RT∆n
∆n = moles product gases - moles reactant gases
Walther Nernst
Third law of thermodynamics
Fugacity of real gas is equal to
The pressure of an ideal gas which has the same chemical potential as the real gas
Clausius Clapeyron does NOT apply to
S-L equilibrium
What is Clapeyron Equation
It postulates that if the temperature and pressure are both change in such a way as to keep the chemical potentials of two phases equal to each other, the rates of change are given by the equation
The coexistence curve where the plot of pressure vs temperature along which the two phases coexist was shown by Clausius-Clapeyron. What did Clausius postulate?
The clapeyron equation was simplified by assuming that the vapor oneys the ideal gas law and by neglecting the molar volume of the gas in comparison with molar volume of the liquid
