enthalpies of solution and entropy

what is the enthalpy of solution (ΔH_sol)? give an example eqn and state whether it is exo or endo:

• ΔH when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute soln

• NaCl _(s) → Na⁺_(aq) + Cl^-_(aq)

• varies :(

give the general cycle for the calculation of an enthalpy of solution:

give the eqn for the calculation of the enthalpy of solution:

enthalpy of solution = enthalpy of lattice dissociation + enthalpy of hydration

describe what happens when ionic compounds dissolve in water:

• polar water molecules attracted to charged ions

• δ- O atoms in H₂O molecules are attracted to the +ve ions and the δ+ H atoms in H₂O molecules are attracted to the -ve ions

why are enthalpy of hydration values exothermic?

E released when ions are attracted to polar water molecules

why is the enthalpy of hydration of F^- more exo than the enthalpy of hydration for Cl^-?

• smaller ion w/ same charge has a higher charge density

• so -ve charge on ion attracts δ+ H in H₂O more strongly

what is entropy change? give its symbol and units:

• measure of disorder - the greater the disorder, the greater the entropy

• ΔS

• J K^-1 mol^-1

generally, how does the entropy of a solid compare to that of a liquid or gas?

S of solid > S of liquid > S of gas

is the entropy value for ionic, metallic and macromolecular structures typically high or low? why?

• low

• have regular structures

is the entropy value for simple molecular substances high or low? why?

high - gases and liquids less ordered

describe and explain the shape of this graph:

• at 0K, the S of a perfect crystalline substance is 0

• graph shows a general increase in S as temperature increases

• w/in a solid, S increases w/ temp as particles gain E and vibrate more

• as solid melts and forms a liquid, there is a large increase in S at the mpt

• there is a very large increase in S at the bpt as gases are very disordered

give the eqn used to calculate the entropy change of a reaction:

ΔS^θ = ΣS^θ_products - ΣS^θ_reactants - take molar ratios into account

how can we predict the entropy change of a system?

increase (+ve):

• decomposition reactions

• increase in moles of gas

• overall no. of particles increases

(very situationist)

describe the chelate effect:

• bidentate/multidentate ligands replace monodentate ligands to form a more stable complex

• because there is an increase in entropy or more particles formed