chpt 6 - thermochemistry energy and framework (definitions) internal energy, heat & work...
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Chpt 6 - Thermochemistry• Energy and framework (definitions)
• Internal energy, heat & work
• Enthalpy & Calorimetry
• Hess’s Law
• Standard Enthalpies
• Applications?• HW: Chpt 6 - pg. 275-283, #s 14, 16, 24, 27, 28, 32, 34, 38,
41, 42, 44, 49, 54, 62, 72, 73, 80, 84, 112 Due Wed Oct. 7
Thermodynamic Framework
• Energy - capacity to do work or produce heat
• Law of conservation of energy
• PE (generally chemical potential energy) and KE
• Heat involves a transfer of energy - Very different from temperature (a measure of motions of particles) Heat is NOT a substance
• Work is force acting over a distance
State Functions
Energy = work + heat– Ball rolling down hill
example - heat and work are different depending on pathway
State Functions - con’t
• State function or state property only depends on current state not pathway– Energy is state function – Heat and work are not state functions– Another example, Elevation vs. Distance
Chemical Energy
• System and surroundings
• Exothermic and endothermic– Chemical PE <--> thermal energy
• Recall energy reaction diagrams– Only concerned with Energy of Reactants
and Products (not pathway) Internal Energy
E = q + w q=heat, w=work
• 1st Law of thermodynamics– Energy of Universe is constant
Universe Energy is Constant
Exothermic or Endothermic?
Chemical Energy - con’t• Quantities have sign and magnitude
– System’s perspective for sign, thus endo is flow into system so q is positive (gaining heat)
E <0 exothermic, E>0 endothermic– Work done on system is positive, w>0– Work done by system is negative, w<0
PV Work
• Common types of work are expansion by a gas and compression on a gas
• PV work,
• w = - PV if volume is expanding
• w = PV if volume is compressingV = Vfinal - Vinitial,
PV Work - derivation
Example that shows Pressure=force/area = F/A so F = P x AWork is force x dist = F x h so W = P x A x h volume of cylinder = A x h
Thus W = PV
The sign is (-) for expanding gas, since work is done by system on surroundings
Enthalpy, H H = E + PV, since E, P and V are state
functions H is also a state function
At constant pressure H = qp (qp is heat at const p) In general, for open laboratory chemical reactions pressure is constant, so the change in enthalpy is used interchangeably with the heat of a reaction.
For a chemical reactionH = Hproducts - Hreactants Exothermic means enthalpy, H < 0Endothermic means enthalpy, H > 0
Calorimetry
The science of measuring heat. Substances absorb heat differently; heat capacity, C, measures this
C = heat absorbed / increase in temp
Specific heat capacity is per gram substance
A calorimeter measures heat change.
q = C x m x T our text uses ‘s’ for C
The AP test will use C, actually Cp
Calorimeter
Simple styrofoam cup calorimeter is easily used for lab measurements and constant pressure measurements.
Bomb Calorimeter
A “Bomb” calorimeter schematic - it only looks like a bomb. It is used when constant volume measurements are needed.
Hess’s Law• Since enthalpy is a state function, heat of
reactions can be calculated from a known set of simple chemical rxns combined together to get the final rxn.
One step:
N2(g) + 2O2(g) --> 2NO2(g) H = 68kJ
Two distinct steps
N2(g) + O2(g) --> 2NO(g) H = 180kJ
2NO(g) + O2(g) --> 2NO2(g) H = -112kJ
Total these reactions
N2(g) + 2O2(g) --> 2NO2(g) H = 68kJ
Hess’s Law schematic
The overall reaction enthalpy is independent of pathway
Hess’s Law rules
Characteristics of H for a rxn
• If a reaction is reversed, the sign of H is also reversed.
• The magnitude of H is directly proportional to the quantities of reactants and products in the rxn– i.e. if the coefficients are multiplied by an
integer, H is multiplied by same integer.
Example 6.8 pg. 254We want to calculate the H for the synthesis of
diborane, B2H6, from its elements.2B(s) + 3H2(g) --> B2H6 (g) H = ?
Use the following data:
2B(s) + 3/2O2(g) --> B2O3(s) H = -1273 kJ
B2H6(g) + 3O2(g) --> B2O3(s) +3H2O(g) H = -2035 kJ
H2(g) + 1/2O2(g) --> H2O(l) H = -286 kJ
H2O(l) --> H2O(g) H = 44 kJHints - 1) work backward from the required/desired reaction, 2) reverse any
reaction as needed to align reactants and products, 3)multiply any reaction as necessary to get correct coefficients.
Recall H is a state function (independent of pathway)
Standard Enthalpies of Formation
Hfo of a substance is the change in enthalpy that
accompanies the formation of 1 mole of the substance from its elements in their standard states.
Standard state: gas is 1 atm, liquid or solid is pure substance, solutions are 1M. Elements are state at 1atm and 25oC.
CalculatingH from Hfo
Since enthalpies are state functions independent of pathway, in chemical reactions, the reactants can be taken apart into their elements and the products can be constructed from their elements.
For a chemical reactionH = npHf
o (products) - nrHfo (reactants)
Data found in Appendix 4 - pg A19 - A22
Example H from Hfo
4 NH3(g) + 7 02(g) --> 4 NO2(g) + 6 H2O(l)
NH3(g) -46 kJ/mol NO2(g) 34kJ/mol
H2O(l) -286kJ/mol O2(g) 0 kJ/mol
H = Products - Reactants…
4x34kJ/mol + 6x(-286kJ/mol) - 0 - 4x(-46kJ/mol)
H = -1396kJ/mol
Greenhouse Effect