3. High Temperature Gases FPK1 2009/MZ 1/23

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Milton Gilmore
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1 3. High Temerature Gases FP 9/MZ /3

2 Terms and Concets Dissociation Diatomic element gases Bond energy, dissociation energy (enthali) Flood s dissociation diagram Vaorization, eaoration, boiling, sublimation Heat of eaoration, enthaly of eaoration FP 9/MZ /3

3 Bond dissociation enthaly Only a limited number of enthalies of formation hae been measured, and there are many reactions for which H o f data is not aailable for one or more reagent. When this haens, H o rxn for the reaction can not be redicted. The enthaly of reaction can be estimated using bonddissociation enthalies. By definition, the bond-dissociation enthaly for an X-Y bond is the enthaly of the gas-hase reaction in which this bond is broken to gie isolated X and Y atoms. XY(g) X(g) + Y(g) FP 9/MZ 3/3

4 Bond dissociation enthaly The bond-dissociation enthaly for a C-H bond can be calculated by combining H o f data to gie a net equation in which the only thing that haens is the breaking of C-H bonds in the gas hase. CH 4 (g) C(s) + H (g) Ho = mol x 74.8 kj/mol CH 4 C(s) C(g) Ho = mol x kj/mol C H (g) 4 H(g) Ho = 4 mol x 7.65 kj/mol H CH 4 (g) C(g) + 4 H(g) Ho = 66.9 kj If it takes 66 kj/mol to break the four moles of C-H bonds in a mole of CH4, the aerage bond-dissociation enthaly for a single C-H bond is about 45 kj/mol. Bond-dissociation enthalies are always ositie numbers because it takes energy to break a bond. When a table of bond energies is used to estimate the enthaly associated with the formation of a bond, the sign becomes negatie because energy is released when bonds are formed. FP 9/MZ 4/3

Series," National Bureau of Standards, No.

5 Reference: Huheey, s. A- to A-34; T.L. Cottrell, "The Strengths of Chemical Bonds," nd ed., Butterworths, London, 958; B. deb. Darwent, "National Standard Reference Data Series," National Bureau of Standards, No. 3, Washington, DC, 97; S.W. Benson, J. Chem. Educ., 4, 5 (965). FP 9/MZ 5/3

6 Element gases Many elements form two-atomic molecules; these dissociate: O O Na Na (g) Equilibrium: (I) (II) (III) P i ni n tot Na Na n Na tot n Na n Na, degree of dissociation FP 9/MZ 6/3

7 Degree of dissociation s The degree of dissociation in gases is denoted by the symbol α where α refers to the ercentage of gas molecules which dissociate. Various relationshis between and α exist deending on the stoichiometry of the equation. FP 9/MZ 7/3

8 Degree of dissociation s : examle N O 4 (g) NO (g) When assuming mol N O 4 that dissociates Left N O 4 will be (-α), NO formed will be α Thus n NO4 =(- α); n NO = α n tot = n NO4 +n NO =- α + α =+ α =( NO ) / NO4 FP 9/MZ 8/3

9 FP 9/MZ 9/3 Degree of dissociation s And rearranging gies tot tot tot O N NO O N x x NO tot tot tot tot tot

10 Degree of dissociation s 4 tot We aroximate: and further: ln H RT S 98 5 cal mol S R We can generalize: f H 98, T tot Degree of dissociation can be estimated if we know H 98 and secify T and tot (see Flood s diagram). FP 9/MZ /3

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13 Boiling oint The boiling oint of a liquid is the temerature at which the aor ressure of the liquid equals the enironmental ressure surrounding the liquid A liquid in a acuum enironment has a lower boiling oint than when the liquid is at atmosheric ressure. A liquid in a high ressure enironment has a higher boiling oint than when the liquid is at atmosheric ressure. In other words, the boiling oint of liquids aries with and deends uon the surrounding enironmental ressure. FP 9/MZ 3/3

14 Heat of aorisation The enthaly of aorization, (symbol H), also known as the heat of aorization or heat of eaoration, is the energy required to transform a gien quantity of a substance into a gas. It is often measured at the normal boiling oint of a substance; although tabulated alues are usually corrected to 98, The heat of aorization is temerature-deendent, though a constant heat of aorization can only be assumed for small temerature ranges The heat of aorization diminishes with increasing temerature and it anishes comletely at the critical temerature (Tr=) because aboe the critical temerature the liquid and aor hases don't coexist anymore. FP 9/MZ 4/3

15 FP 9/MZ 5/3 Determining T b Vaorization equilibrium: NaCl (s,l) NaCl (g) At boiling oint (T B ), NaCl = atm, thus: R S RT H RT G NaCl e e e ln S H T S T H R S RT H R S RT H B B B B NaCl Clausius Claeyron

16 For ideal gases: S 5 cal mol We get: T B 5 mol cal H Boiling oint (ery) roughly linearly deendant on H. FP 9/MZ 6/3

17 H FP 9/MZ 7/3

18 H Hydrogen bonding gies larges H Polar substances hae higher H than similar non olar substances Similar bonding in similar comounds gies similar H NH 3 lowest H amonh hydrogen bounded molecules F, Cl, Br, I regular increase in H Methane, ethane, roane regular increase in H FP 9/MZ 8/3

19 FP 9/MZ 9/3 Determining T b and H: Neat aroach H can be determined by measuring x at different temeratures. Tb = boiling tem at atm (bar) ln ln ln H T T R T T R H R S RT H

20 Ex. A liquid has aour ressure of 54mmHg at 5 C and 648mmHg at 45 C. What is H and T b? FP 9/MZ /3

21 Heat of sublimation The heat of sublimation, or enthaly of sublimation, is defined as the heat required to sublime one mole of the substance at a gien combination of temerature and ressure, usually standard temerature and ressure (STP). The enthaly of sublimation is the heat of sublimation for aorizing one mole of the solid substance under three secific conditions: () the ressure remains constant, () the only ossible work that occurs is exansion against the atmoshere (so-called PdV work) and (3) the temerature remains constant during the rocess. A heat of sublimation for a substance is only alid for conersion of the ure solid to the ure gaseous state of the substance. FP 9/MZ /3

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23 Terms and Concets Dissociation Diatomic element gases Bond energy, dissociation energy (enthali) Flood s dissociation diagram Vaorization, eaoration, boiling, sublimation Heat of eaoration, enthaly of eaoration FP 9/MZ 3/3

dn i where we have used the Gibbs equation for the Gibbs energy and the definition of chemical potential

dn i where we have used the Gibbs equation for the Gibbs energy and the definition of chemical potential Chem 467 Sulement to Lectures 33 Phase Equilibrium Chemical Potential Revisited We introduced the chemical otential as the conjugate variable to amount. Briefly reviewing, the total Gibbs energy of a system