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UNIT 5: THERMOCHEMISTRY (HESS'S LAW, FORMATION (Hess's law: a…
UNIT 5: THERMOCHEMISTRY
HESS'S LAW, FORMATION
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Just set up equations w/enthalpies, make sure the products/reactants are on right side and adjust sign accordingly, and make sure moles are right (adjust magnitude accordingly.)
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Formation takes it one step further, basically being the equations for the formation of something out of its constituent elements.
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BASICS: E, w, q
E: Symbol for internal energy, is the total energy found in an object. E = all the energy in something, potential or kinetic. Kind of abstract.
ΔE: Change in internal energy, slightly more quantifiable. Final energy - initial. ΔE = q + w, where q is heat and w is work done.
q is the heat transfer which is negative when exothermic and vice versa; w is work, = f*d (force and distance) which is negative when the system performs on surroundings and vice versa.
Kinetic energy is (1/2)mv**2, Grav. pot. is mgh, electrostatic is (kQ1Q2)/d
H, other equations
Enthalpy, H, is a function to track heat transfer at constant pressure (and when no work is performed other than PV work.) H = E + PV. But ΔH = ΔE + PΔV.
Work in chemistry context is usually when a gas expands/contracts; under constant pressure, w = -PΔV.
SO: ΔH = q(p), or heat at constant pressure.
Conceptual stuff
Always think: relative to system and surroundings. If the system loses something, the H/E value will be negative. First law: energy is conserved. Anything lost by system is gained by surroundings.
Open, Closed, Isolated: Allows both, only allows energy, and allows neither to transfer from surroundings.
Energy can be either kinetic or potential. Kinetic is like temperature/thermal energy and motion, while potential is everything else.
Quantities have 3 parts: number, unit (which make up magnitude) and direction.
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More conceptual:
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State function: doesn't matter how you got there, rather set points. In other words, multiple paths could have gotten you the same result.
E, H, V, P are SFs because it doesn't matter how you got there (refer to SF def.) But w and q aren't because they ARE the journey (perhaps because they're how everything else was derived.
Within an object, the average kinetic energy is temperature. However, at a given temperature, the molecules are all spread out along a distribution curve of thermal energies.
Energy can take many forms, and so when an initial thing has a certain energy, it can release it in heat but the energy it loses can be chemical, or anything like that.
CALORIMETRY:
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"Things" possess thermal energy, which is expressed in temperature (avg. kinetic energy.) But when things w/differing thermal energy (hotter/colder) touch, hot transfers to cold. Actually this is more of a heat idea but whatever.
Different things, though, change temperature differently when heat is transferred to it.
This is called heat capacity (c), and can be measured per gram or per mole.
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STATE CHANGES
When something changes in heat, it can either change temp or state. in other words, heats of vap/fus, and heat capacities.