20 B Week IV Chapters 11

• Chapter 10, 11( except 11.4 and and 11.6 -7)

• Intermolecular potentials.

ion-dipole, ion-ion

• Solutions. Interactions in solution, Boiling Point Elevation, Freezing Point Depression and Osmotic Pressure, Electrolyte solutions

• Dissolution reactions(rxns) and Arrhenius type Acid/Base rxns

Midterm Friday:

Chaps 9(no 9.7), 10, 11.-11.3

One side of 1 page notes(must be hand written), closed bookReview Session Today @ 2-3 pm, in FRANZ 1178

Fig. 10-6, p. 450

In Solutions, for example when NaCl(s) is dissolved in H2O(l).

+ H2O

NaCl(s) + H2O(l) Na+(aq) +Cl-(aq)

(aq) means an aqueous solution, where water is the solvent,major component.The solute is NaCl, which is dissolved, minor componentWater molecules solvates the ions the Cation (Na+) and the Anion (Cl-). The forces at play here areIon dipole forces

Dissolution of a polar solid by a polar solid by a polar liquidA non-polar liquid e.g., benzene, would not dissolve NaCl?

Fig. 10-6a, p. 450

+-2∂

-2∂

-2∂

-2∂

+∂+∂

+∂

Solvated Na+

First Solvation Shell of theSolvent

Fig. 11-3, p. 480

2nd solvation shell

Dissolution of K2SO4 in water

In a solution of solvent A and solute B, the important thermodynamic variables are V, T, molar Concentration of [A] and [B] and the relative composition X

The Molarity, moles of A or B/ Liters of Solution [A]=nA/Vsol and [B]=nB/Vsol in units of mols L-1

The composition in Mole Fraction: XA= nA/(nA + nB) and XB= nB/(nA + nB)

XA + XB =1 and XA=1 -XB

Because Volume depends on the Temperature, the Molarity is not always theBest variable, the Molality

{B} = Moles of Solute/ kg of Solvent mol kg-1

{B}= nB//kg of solvent, since the density of water is defined as kg/L,

Therefore, for ideal aqueous solutions the Molality is, in effect, the moles of solute/Liters of solvent.

The dissolution reaction with water as a solvent: A(s)A(aq) A is a molecular solid and the molecular units, or monomers, do not dissociate in solution. Like fructose with lots of OH ( hydroxyl groups) for forming H-bonds and which makes the monomer more stable in aq soln

Electro Staticpotential

Fructose C6H12O6

Hydrated Fructose C6H12O6

H-bonds

The Hydrogen bonds between the water molecules of the solvents makes the molecule more stable in solution than in the solid

The dissolution of salts such as NaCl(s), NaOH(s)(basic in soln, sodium hydroxide) and NH4Cl(acidic in soln, Ammonium Chloride )

AB(s) A+(aq) + B+(aq)

In some case like NaCl no reaction occurs with the solvent just hydration

+ H2O

NH4Cl(s) NH+4(aq) + Cl-(aq) NH+

4(aq) + H2O(l)H3O+((aq) + NH3((aq)

Notice that the Number of Elements and the Amount of Charge are balanced On both sides reaction the rxn

Such a reaction has the correct Stoichiometry and is said to be balanced

+

Lone pair

Proton Transfer

NH+4(aq) + H2O(l) H3O+(aq) + NH3(aq)

By definition, the Ammonium ion NH+4 is an Arrhenius acid

(Svante Arrhenius) since it increased the concentration(molarity) of the H3O+(aq), the hydronium ion in solution by reacting with the solvent, H2O in the case

+

In the general an Acid AH, the solute, in the Arrhenius sense will react with the solvent H2O, in this case, it could have been

AH(aq) + H2O(l) H3O+(aq) + A-(aq)

A strong acid such as HCl will completely dissociate in solution

HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

The reaction goes to completion: all the HCl molecules produce H3O+(aq)

For a relatively weak Acids such as HF, the reaction does not go to completion

HF(g) + H2O(l) H3O+(aq) + F-(aq)

And there is still lots of HF in solution solvated by H2O molecules, the rxn therefore goes to equilibrium

HF forms a strong H-bonded network

Table 11-1, p. 483

Types of Acids

The Neutralization reaction H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

where H2O(l) amphoteric acts both as an acid as well as a base ths reaction goes to equilibrium

H+ +

NaOH(s) Na+(aq) + OH-(aq)

Where OH-, the hydroxide ion, is a strong base

In acid base reactions for example:

HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

HCl is the strong Acid and H2O is the Base

HCl + NaOH H2O + NaCl Na+(aq) + Cl-

(aq)

Which is just salt water

Fig. 11-3, p. 480

Electrolyte Solutions can Carry currentDissolution of K2SO4(s) 2K+(aq) + SO-

4(aq)

The Solubility limit of Potassium Sulfate in aqueous solnis 120 gL-1 at 25 °CMolecules that dissolve in solution to produce ions are calledElectrolytes.

Some compounds have rather limited solubility in water e.g., example of BaSO4(s) can dissolve to 2.5 mgL-1

BaSO4(s) Ba++(aq) + SO2-4(aq)

The equilibrium of this reaction is therefore on the side of

Ba++(aq) + SO2-4(aq) BaSO4(s)

And the solid would precipitate out of soln