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Lecture 3

Chemistry 1050 lecture notes summerized week 3 and 4.pdf

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University of Guelph
CHEM 1050
Harvey Marmurek

Chemistry 1050 Lectures summarized Week 3 and 4 Hess’ Law:  We need access to tables of delta H values  Pure and standard states are required  Standard conditions are required  Standard enthalpy of formation (delta H knot) values are the enthalpy changes seen when we form one mole of substance from its elements in their standard states at 1.0 x 10 ^ 5 Pa (1 atm) of pressure Delta H formation for pure elements in the standard state at a chosen temperature is equal to 0. Vap (vaporization reaction): liquid goes to a gas Ions in aqueous solution: To deduce the delta H formation values for all ions in an aqueous solution, we need a method to separate the contribution of the ions Our solution, we define the following o Delta H formation (H+, aq) = 0 Bond Enthalpies (BE):  The energy required to break one mole of bond in the gas phase only – it requires energy to break the bonds  BE is always LARGE AND POSITIVE  BE is the measure of the STRENGTH of a covalent bond  NOTE: Bond dissociation enthalpy calculations are for gasses only!!!!  Bond length gets shorter as bond enthalpy get larger!! Lattice Enthalpy - the change in the energy occurs when an ionic crystalline solid is separated into isolated gaseous ions MX (s) M (g) + X (g) Magnitude of Lattice Enthalpies:  Depends on the charge of the ions and their separation 1 Entropy: Spontaneous Reactions: - is there a thermodynamic function that will indicate whether or not a reaction will occur spontaneously? “spontaneously” means the reaction will occur with NO external intervention System spontaneously change in such a manner to decrease their capacity for change The reverse process is NOT SPONTANEOUS Heat Flow  Heat is the product of 2 factors  Intensity factor (e.g. T (temperature))  Quantity (or capacity) factor  Second term is ENTROPY (S)  Entropy is a STATE function  A measure of randomness or disorder  Measure of the probability of number of equivalent arrangements of the same energy  Disorder: 2 types  POSITIONAL: distribution of species in space  THERMAL: distribution of energy among species, or distribution over energy levels  S is a measure of ENERGY DISPERSION 2nd Law of Thermodynamics: In a spontaneous process, the ENTROPY of the UNIVERSE INCREASES Delta S universe = delta S of system + delta S surroundings must be greater than 0.  Spontaneous process: Delta S universe is greater than 0  Equilibrium: Delta S universe = 0  Non- spontaneous process: Delta S universe is less than 0  Reverse direction For the universe, we need to consider both the system and the surroundings 3rd Law of Thermodynamics: A perfect crystalline substance at 0 Kelvin has S=0  Therefore S is measurable (actual values)  S values are ALWAYS POSITIVE  S values are stable forms of pure elemesnts at 298 K, 1 atm are NOT 0 o We have positive values because we are not at absolute 0 2  Boiling point – a lot of disorder 0 Factors affecting m : (qualitative approach) 1. S increases with melting/ vaporization m  Increases with disorder 0 2. S mncreases when soild / liquid is dissolved in water or other liquid solvents 3. S mecreases when a gas is dissolved in water or other solvents  Less disorder 4. S increases with increasing mass ( energy levels more closely spaces as mass increases) m  Increasing molar mass
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