普通化学 第七章 总结

Enthalpy as a State Function_ _ _

H is a state function.ΔH= Hfinal - Hinitial independent of path:

Standard-State Enthalpies of Reaction_ _ _ _

For a reaction carried out at a pressure of 1 bar,ΔH=ΔH°.ΔH° is called the standard-state enthalpy of reaction. 1 bar is part of the definition of standard conditions. Most tabulated enthalpies of reaction areΔH°.

Calculating Enthalpies of Reaction_ _

Easier to calculateΔH° than to measure it. These are calculations, not estimates.Break all the reactant bonds. Form all the product bonds.ΔH°= the difference in energy between these two processes.

Enthalpies of Atom Combination_ _

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N(g)+ 3 H(g)→ NH3(g) Notice these reactants are not in their diatomic elemental forms. If 1 mole of N atoms combine with 3 moles of H atoms to produce one mole of NH3 molecules, 1171.76 kJ of energy will be released.

Enthalpies of Atom Combination_ _ _ _ _ _

N(g)+ 3 H(g)→ NH3(g) Reaction is called atom combination. Not meant to reflect actual mechanism. The enthalpy associated with it is called the enthalpy of atom combination,ΔH°ac. In the above example,ΔH°ac= -1171.76 kJ. Why is it negative?

Enthalpies of Atom CombinationThe reverse reaction NH3(g)→ N(g)+ 3 H(g) is called atomization. The enthalpy change is called the enthalpy of atomization and in this example equals+1171.76 kJ.

Enthalpies of Atom Combination_ _

Can be used to study chemical processes 2H2(g)+ O2(g)→2H2O(g)

ΔH°=2ΔH°ac(H2O(g)) -2ΔH°ac (H2(g))-1ΔH°ac (O2(g) ) From Appendix B.13ΔH°=2(-926.29 kJ) -2(-435.30 kJ)-1(-498.340 kJ)ΔH°= -483.64 kJ

Enthalpies of Atom Combination_ _ _ _

Can be used to study chemical processes. 2H2(g)+ O2(g)→ 2H2O(g) Established this as exothermic. Determined that 483.64 kJ of energy will be released for each mole of O2(g) consumed.

Using Enthalpies of Atom Combination to Probe Chemical ReactionsIsomersHO2C C H C H H CO2H HO2C C C CO2H H

Using Enthalpies of Atom Combination to Probe Chemical Reactions_

KnowingΔH°ac for each isomer allows for _ the calculation of the enthalpy change associated with the transformationHO2C C H C H H CO2H HO2C C C CO2H H

Using Enthalpies of Atom Combination to Probe Chemical Reactions_ _

KnowingΔH°ac gives insight into average bond strengths. These provide a microscopic interpretation of overallΔH° for a reaction.endothermic or exothermic character magnitude ofΔH°

Using Enthalpies of Atom Combination to Probe Chemical Reactions 4HF(g)+ SiO2(g)→ SiF4(g)+ 2H2O(g)ΔH°= -103.4 kJ4HCl(g)+ SiO2(g)→ SiCl4(g)+ 2H2O(g)ΔH°=+139.6 kJ

Why the difference in sign?

Using Enthalpies of Atom Combination to Probe Chemical Reactions4HF(g)+ SiO2(g)→ SiF4(g)+ 2H2O(g)ΔH°= -103.4 kJ 4HCl(g)+ SiO2(g)→ SiCl4(g)+ 2H2O(g)ΔH°=+139.6 kJ

Why the difference in sign?The Si-F bond is stronger than the Si-Cl bond by an amount greater than the difference in bond strength between H-

F and H-Cl shown on the next slide.

Bond Length and the Enthalpy of Atom CombinationTable 7.5

Bond Length and the Enthalpy of Atom Combination_ _

Longer bonds tend to be weaker bonds. Multiple bonds tend to be stronger than single bonds.

Hess's Law_ _ _

An alternative method for calculatingΔ H° Does not useΔH°ac. Takes advantage of H being a state function.