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February 7 - midterm review.doc

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CHEM 120
Ariel Fenster

Chem 120 Alanna Houston February 7, 2008 CHEMISTRY REVIEW FOR MIDTERM 1 - Molecules can move (translate), rotate, vibrate, and re-arrange it’s electrons (excite) – same as heat - Heat is the net sum of all the motions of molecules - All molecules don’t end up being equal but assume a distribution of these energies. Look at graph - Temperature is not an intrinsic molecular property but an extrinsic one. - Conservation of Mass: in a system, we neither gain nor lose mass (intuitive, and easy) - Conservation of Energy: a strange thing called ‘energy’ is also conserved - Energy has many different forms, and can interconvert and hide in clever places. - Inputs are added together with outputs (isolated systems are easier than open, closed) – earth, body, house examples - This work can also be expressed as an expansion against constant (external) pressure: work = force times distance = PextdeltaV (in Pa m^3) - We can detect the transfer of HEAT simply as an increase in the temperature of the surroundings. This heat variable is q. The heat transfer is proportional to the temperature change: q = CdeltaT the C we call the heat capacity and the units are: J/mol K (or S.H. in: J/g K) - Work out (done by the system) is negative - Heat leaving the system is negative - Work in (done on the system) is positive - Heat entering the system is positive - Knowing that deltaU = w + q, some sign conventions: expansion against constant pressure (w = -PextdeltaV) – negative sign since we have lost energy as work - The first law of thermodynamics again: in an isolated system, U is constant - Delta H = delta Q - Consider H2O undergoing changes from ice to water to vapour: this requires energy at each step, to break apart the attractions. Look at examples for deltaH (melting) and deltaH (boiling) - The enthalpy of freezing is the same magnitude as that of melting, just opposite in sign - There can also be a phase change directly from (ant to) solid to gas (sublimation). In this case, the enthalpies are simply additive (HESS’ LAW) Chem 120 Alanna Houston - To estimate the enthalpy or a reaction deltarHof formation we need only to know the enthalpies of formation of the reactants and products - Chemical Changes: breaking and forming bonds works in exactly the same way: O2 (g)  2 O (g) - DeltaH = 498 kJ - Bonds always require energy to break (delta H = +) - And you always get energy back when you form them (deltaH = -) - Insert example with deltaH of formations. (Combustion reaction) - Look at c
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