Thermochemistry Flashcards
What is Thermochemistry?
The branch of chemistry which studies energy changes that accompany physical, chemical, or nuclear changes. Also a term used to describe the study of energy and energy transfer.
First Law of Thermodynamics
The energy of the universe is constant: E(universe)=0
Surroundings
The matter surrounding the system which is capable of absorbing or releasing thermal energy.
Exothermic
Sys transfers heat to surroundings and thus the temp of the surrounding inc. (comb, freezing, condensation)
Endothermic
Sys absorbs heat from surroundings thus temp of surroundings decreases (melting, vaporization)
Open system
matter and energy can escape
Closed system
matter can’t escape but energy can
Isolated system
matter and energy can’t escape (ideal)
Calorimetry
process of measuring energy changes in a chem sys. nΔH(x,sys) = -mcΔT(sur)
Calorimetry (Heat measurement)
q=mcΔT
Molar Enthalpy
Enthalpy change associated when 1mol of a substance undergoes a physical/chem/nuclear change. (J/mol)
ΔH(sys) = nΔH(x)
Thermochemical Equation
A balanced chem equation that indicates the amount of heat that is absorbed or released.
What are the 4 different methods for communicating thermochem equations?
Energy term, Enthalpy value, Molar enthalpy, Diagrams
How to write a thermochem equation with energy term?
(In equ.) Energy as reactant is endo, energy as product is exo. Ex. H20(g) + 285.8 kJ -> H(2,g) + 1/2 O(2,g)
How to write a thermochem equation with enthalpy value?
Next to equation. + is endo, - is exo.
Bond Energy
Energy to break is endo, energy released is exo.
ΔH = ΣR BondEnergy - ΣP BondEnergy
Communicate thermochem equation with Diagram?
Exo goes down (down arrow), endo goes up (up arrow)
Hess’s Law
ΔH = ΣΔH of individual steps.
Ex. ΔH = …
ΔH = …
Enthalpy of Formation
ΔH = ΣnΔH(f,p) - ΣnΔH(f,r)
3rd Law of Thermodynamics
Entropy(S) = 0 at T=0 K
Law of Entropy
+entropy considered spontaneous
Order-disorder ΔS>0 (+)
Disorder-order ΔS<0 (-)
Increasing (S) Physical Change
Volume inc, temp inc, state change, dissolving
Increasing (S) Chem Change
more n on prod side than react, complex molecule broken to many smaller comp, solid reactant but prod are gas/liquid
Entropy
ΔS = ΣnS(p) - ΣnS(r)
Gibb’s Free Energy (G)
ΔG = ΔH - TΔS
Meaning of ΔG
ΔG<0 rxn is spont. in forward direction (release energy)
ΔG>0 rxn is non-spont. in forward direction (req energy)
ΔG=0 rxn is @equilibrium (spont. in both directions)
What makes spontaneous?
Endo (ΔH>0): +entropy - spont depends on T
-entropy - non-spont
Exo (ΔH<0): +entropy - spont @ all T
-entropy - spont depends on T
