To do or not to do?---------That is a question too…. Qn. 10 To do or not to do?---------That is a question too…. Liu Jiani Sherry (14)

a) L.E.: It is the energy evolved when 1 mole solid ionic compound formed from its gaseous constituents. WHY?

Standard conditions Why solid? Recall what is missing from the definition? Ionic compounds are operationally defined as solids at stp, with the exceptions of a few composed of cation and anionic complex…. Standard conditions

Why gaseous constituents? L.E. refers to the energy released from a bond formation. In gaseous state, ions have minimal interaction when they form the solid. So they bond energy evolved is then a good indication of the actual strength.

b) LE is proportional to (q+ * q-)/ (r+ + r-) Charges of Li =Na Ionic radius of Li > Na smaller inter-ionic distance between LiCl and bigger electrostatic forces of attraction between its ions Thus, larger magnitude and more exothermic.

When a bond forms, PE is released as KE.

-L.E: Solid ionic compound to gaseous ions Delta E of solution: dissolution of ionic compound in water to get aqueous ions Delta E of Hydration: hydration of individual gaseous ions to form Ion-Dipole interactions with water

Energy/ KJ per mol Li+ (g)+ Cl- (g) ∆H hyd (Li+) =-499 ∆H hyd (Li+) =-499 L.E of LiCl=-848 Li+ (aq)+ Cl- (g) ∆H hyd (Cl-) =-381 LiCl (s) ∆H sol LiCl(s) =-36 Li+ (aq)+ Cl- (aq) ∆H sol LiCl(s) = -L.E.+∆H hyd (Li+) + ∆H hyd (Cl-) =848 + (-499) + (-381) = -36KJ/mol

Energy/ KJ per mol Na+ (g)+ Cl- (g) L.E of NaCl=-776 L.E of NaCl=-776 ∆H hyd (Na+) =-390 Na+ (aq)+ Cl- (g) ∆H hyd (Cl-) =-381 Na+ (aq)+ Cl- (aq) ∆H sol NaCl(s) =+5 NaCl (s) ∆H sol NaCl(s) = -L.E.+∆H hyd (Na+) + ∆H hyd (Cl-) =776 + (-390) + (-381) = +5KJ/mol

Last part… Final temp. solution=? Q=mc ∆T -36000J/mol * [1.0/(6.9+35.5)]mol = (100*4.2* ∆T)J ∆T=2.02 degree celsius Final temp.=298K+2.02K =300.02K =300K (3sf)