Topic 9: Thermochemistry : Topic 9: Thermochemistry Zumdahl 7e, Chapter 6
I. The Nature of Energy : I. The Nature of Energy Review of Terms
Law of conservation of energy: matter/energy cannot be created of destroyed (E = mc2 allows for matter to be converted into energy, but this only happens in nuclear reactions; assume constant energy in chemical reactions)
Potential energy: energy due to the material’s position OR composition (contained in the chemical bonds)
Kinetic energy: energy due to motion (mass and speed of the molecules), related to temperature
: Temperature: proportional to the average speed of the molecules (K scale only), unrelated to the number of molecules
Heat: amount of energy contained in a substance (more matter = more heat)
State function/property: any property that depends only on the initial and final state (unrelated to the path taken to get there)
E.g. ?T = Tf –Ti
Also enthalpy (?H), entropy (?S), Gibbs free energy (?G)
b) Chemical Energy : b) Chemical Energy System vs. surroundings
The reaction is the system
Everything else in the universe is the surroundings
An open system allows energy and matter to be transferred between the system and surroundings
A closed system allows only energy to be transferred
An isolated system allows nothing to be transferred
Exothermic vs. endothermic
If Hp > Hr then ?H = (-) ? exothermic
If Hr > Hp then ?H = (+) ? endothermic
Most reactions tend to be exothermic for stability reasons
First law of thermodynamics: The energy of the universe is constant.
: Internal energy: The sum of all energies of all the particles in the system
Thermodynamic quantities must include both sign (+/-) and magnitude
Work done by the expansion of a gas
Force = pressure x area
Work = force x distance (?h)
Work = pressure x area x height
Work = P?V
If ?V is positive, work is negative (work is being done on the surroundings)
If ?V is negative, work is positive (work is being done on the system
II. Enthalpy & Calorimetry : II. Enthalpy & Calorimetry Enthalpy
?H = ?E + P?V @ constant pressure
?E = qp + w
?E = qp – P?V
qp = ?E + P?V = ?H (the “heat of reaction” is the amount of heat transferred)
Calorimetry
Heat capacity, c = heat absorbed/increase in temperature
: At constant pressure: Q = mc?T, so ?H = mc?T
At constant volume: no work is done!
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