solubility and qualitative analysis of solutions 3

5/31/2018 Solubility and Qualitative Analysis of Solutions 3

Solubility and Qualitative Analysis of Solutions

Solution Properties

Homogeneous mixtures of substances composed of at least one solute(substance that getsdissolved) and one solvent(substance that does the dissolving).

An aqueous solution is a homogeneous mixture using water as the solvent. clear (transparent) coloured or colourless conductive/nonconductive acidic, basic, or neutralElectrolytes are compounds that conduct electricity in aqueous solutions, while nonelectrolytes do not

conduct electricity.

Intermolecular Forces

the force of attraction and repulsion between molecules generally much weaker than covalent and ionic bonds

(intramolecular forces)

1. dipole-dipole forces a force of attraction between polar molecules where the

oppositely charged dipoles attract one another

the greater the polarity of the molecule, the greater thestrength of the dipole-dipole force

hydrogen bonds

a specific type of dipole-dipole forces that are stronger due to larger differences inelectronegativity in the covalent bonds (N-H, O-H, and F-H)

these highly polar covalent bonds result in strong dipoles forming on the hydrogen atom andhighly electronegative atom (either N, O, or F), producing very strong dipole-dipole bonds

2. London dispersion forces weak intermolecular forces

in nonpolar molecules

due to the simultaneousattraction of the elecrons of

one molecule by the

positive nuclei in thesurrounding molecules

by increasing thenumber of electrons in

the molecule, thestrength of London

forces increases


Explaining Solutions

For solutions to result, intermolecular forces must form between the particles of the solvent and the

particles of the solute.

The intermolecular forces that are produced by the solution should be equivalent to (or greaterthan) the intermolecular forces between solute particles.

Polar molecular compounds Dipole-dipole forces (or hydrogen bonds) attract polar particles to each other. A polar solvent can form dipole-dipole forces (or hydrogen bonds) with polar particles.

Generally, polar compounds will dissolve into polar solvents, but not into nonpolar solventsNonpolar molecular compounds

London dispersion forces attract nonpolar particles to each other. Polar particles are unable to participate in London dispersion forces due to the transient nature

of these intermolecular forces.

Nonpolar solutes prefer to dissolve in nonpolar solvents rather than polar solvents.Ionic compounds Electrostatic forces of attraction hold ionic compounds together. A polar solvent can form ion-dipole forces with the charged particles of the ionic compound.

If the electrostatic forces are weaker than the ion-dipole forces, the ionic compound wontdissolve.

If the electrostatic forces are stronger than the ion-dipole forces, the ionic compound wontdissolve.

Further complicating solution formation, many solvents and solutes contain both polar and nonpolar

portions (and in some instances, ionic portions).

This generally allows these compounds to form solutions in varying amounts with both polar andnonpolar compounds.

The influence of the polar portion diminishes as the size of the nonpolar portion increases. Similarly, the addition of more polar portions lessens the nonpolar character of the compound.Often, determining whether two compounds will form a solution with one another requires simpleexperimentation.

London Dispersion Forces

The intermolecular forces acting within nonpolar compounds are London dispersion forces.

temporary, transient displacements of electron clouds around atoms result in short-lived dipoles and the attractive forces will eventually be lost weak intermolecular forces

Nonpolar Mixtures

London dispersion forcesare

believed to be responsible fornonpolar solutes dissolving in

nonpolar solvents.

transient dipoles (constantly moving) only compounds that allow for transient dipoles can perform

this interaction

nonpolar dissolves nonpolar


The Importance of Being Water

Known as the universal solvent, water dissolves more substances than any other liquid.

1. highly polar compound (large difference in electronegativity)

O

HH

...

. two stronglypolar bonds

two electron

lone pairs

++

-

2. capacity for hydrogen bonds (two donor, +, and two acceptor, -)

OHH

...

. can form 4

H bonds

3. small size of molecules water molecules can work inside larger molecules to find sites for intermolecular interactions capable of forming dipole-dipole, H-bonds, and ion-dipole forces


Solubility

Solubilityis a quantitative property of a solute that measures how much of that substance can bedissolved in a solvent at a specific temperature and pressure.

measured in g/100 mL A saturated solutioncontains the maximum amount of a solute at a specific temperature andpressure.

unsaturated solutions can dissolve more solute supersatured solutions contains more than the maximum

Solubility Curves

A solubility curve shows the relationship between thesolubility of a solute and the temperature of the solution.

As temperature is increased, the solubility of solids tends toincrease.

Solubility of Gases in Water

GasSolubility (g/100 mL) at specified temperature

0oC 20

oC 50

oC

N2(g) 0.0029 0.0019 0.0012

O2(g) 0.0069 0.0043 0.0027

CO2(g) 0.335 0.169 0.076

NH3(g) 89.9 51.8 28.4

Gases have greater solubility at lower temperatures. As the solvent temperature increases, dissolved gas

molecules gain energy and will more easily escape the

intermolecular forces form with the solvent.

Solubility of Liquids

liquids that mix with one another in all proportions aremiscible and will form a single, homogeneous layer

liquids that separate into separate layers instead of mixing(heterogeneous) are immiscible


Solubility Categories

Qualitative definitions at room temperature and pressure can be used to describe the solubility of solidcompounds.

high solubilityindicatessaturation point is greaterthan 0.1 mol/L

low solubility indicatessaturation point is less than

0.1 mol/L

insoluble meansnegligiblesolubility (saturation point is

less than 1 mg/L)

The reason 0.1 mol/L is used:

most ionic compounds have solubilities significant larger or smaller than this convenient for experimental purposes

Precipitation

To precipitate(verb) means to form a solid from aqueous solution when the maximumconcentration has been exceeded.

A precipitate(noun) is a solid formed in a chemical reaction or through a decrease in solubility.


Double Displacement

AX + BY AY + BX

metal displaces metal; nonmetal displaces nonmetallead(II) nitrate + sodium iodide

lead(II) iodide + sodium nitrate

the formation of a precipitate (solid) will cause double displacement reactions to occur need to use the solubility rules to determine if a precipitate will form the neutralization of an acid by a base will cause double displacement reactions to occur

sodium hydroxide + sulfuric acidsodium sulfate + water

the formation of a gas will cause double displacement reactions to occursodium sulfide + hydrochloric acidsodium chloride + hydrogen sulfide

Chemical Equations in Solution

potassium iodide + silver nitratepotassium nitrate + silver iodideKI(aq)+ AgNO3(aq)KNO3(aq)+ AgI(s)

Total ionic equation shows all high solubility ionic compounds in their dissociated form.

K+

(aq)+ I-(aq)+ Ag

+(aq)+ NO3

-(aq)K

+(aq)+ NO3

-(aq)+ AgI(s)

Spectator ions are present in a reaction system, but does not change during the course of the chemical

reaction.

need not be written in the chemical equation (remove K+(aq)and NO3-(aq))I-(aq)+ Ag

+(aq)AgI(s)

Net ionic equation is a way of representing a reaction by writing only those ions or neutral substances

specifically involved in an overall chemical reaction.

Writing Net Ionic Equations

Step 1 Write the balanced chemical equation with full chemical formulas for all reactants andproducts.

Step 2 For all ionic compounds with high solubility, rewrite the formulas as dissociated ions,

to show the total ionic equation.

Step 3 Cancel identical amounts of identical entities appearing on both the reactant and product

sides of the total ionic equation.

Step 4 Write the net ionic equation, by removing all cancelled entities, and reduce thecoefficients if necessary.


Qualitative Chemical Analysis

Tests used for the identification of the specific substances present.

(e.g. flame tests, solubility, litmus paper, conductivity)

Colours of Solutions Colours of Flames

Ion Solution Colour Ion Flame Colour

Groups 1, 2, 17 colourless H+

(aq) colourlessCr

(aq) blue Li

(aq) bright red

Cr+

(aq) green Na+

(aq) yellow

Co+

(aq) pink K+

(aq) violet

Cu+

(aq) green Ca+

(aq) yellow-red

Cu+

(aq) blue Sr+

(aq) bright red

Fe

(aq) pale green Ba

(aq) yellow-green

Fe+

(aq) yellow brown Cu+

(aq) blue (halides)/green (others)

Mn

(aq) pale pink Pb

(aq) light blue-grey

Ni+

(aq) green Zn+

(aq) whitish green

CrO4-

(aq) yellow

Cr2O7-

(aq) orange

MnO4-

(aq) purple

Sequential Qualitative Chemical Analysis

based on precipitation of low-solubility productssolution known to contain

Pb and/or Sr

add NaCl

white precipitate no precipitate

solution containedlead(II) ions,

precipitated as PbCl

no lead(II) ionswere present

filter

add Na SO

white precipitate no precipitate

solution containedstrontium ions,

precipitated as PbCl

no strontium

were presentions

sequential qualitative analysis for anions in solution

solubility and qualitative analysis of solutions 3

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