Download - The Chemistry of Everything Kimberley Waldron...The Chemistry of Everything Kimberley Waldron Chapter 4 Salt Behavior of ions, acids and bases and the notion of equilibrium ©2007

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©2007 Pearson Prentice Hall 1

The Chemistry of Everything

Kimberley Waldron

Chapter 4 Salt

Behavior of ions, acids and bases

and the notion of equilibrium


Chapter Topics

• Ionic liquids, ionic interactions, delocalized

electronic charge.

• Polyatomic ions.

• Polar molecules, dipoles, ion–dipole interactions,

solubility.

• Electrolytes, molarity.

• Osmosis.

• Autoionization of water, acids and bases, pH.

• Acid rain.

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Salt: The Staff of Life

• Solid salt is a hard white powder.

• In solution in water, salt becomes a

collection of ions.

• Ions in solution can have powerful effects

on matter.

• Ionic solutions are essential for life.

• Acids and bases are special cases of ionic

solutions.


More than Morton’s

• A salt is any ionic solid

formed by neutralization

of an acid by a base.

• It contains a positive ion

(cation) and negative ion

(anion).

• Formation of salt from

elements involves.

• Transfer of electrons from

metal to non-metal.

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Ionic Liquids?

• Densely packed ionic

lattices make traditional

salts high melting point

solids.

• New ionic compounds are

liquids at room

temperature.

– Ions are very large and

don’t pack together.

– Charge density is low.

– Attractions between ions

are weak.


Applications of Ionic Liquids

• Environmentally friendly

alternative to organic

solvents.

• Wide liquid range.

• Millions of variations by

changing the cation.

• Separating heavy metal

ion contaminants (lead

etc.) from water. The

lead dissolves better in

the ionic liquid.

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Mummies and Salts

• Questions remain

about how mummies

were made.

• Embalming solutions

contain salts like

natron.


Natron

• Natron is a collection of

salts used in the

mummification process.

• Some contain simple ions

like chloride.

• Other anions are

poly(many)atomic:

– CO32-, SO4

2-

• Polyatomic ions have

several resonance dot

structures.

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Dot Structures for Ions


Dot Structures for Ions don’t

Follow the Rules of Neutral

Molecules

• Addition of charge (+ or -) changes the

bonding.

• Negative ion bond number decreases:

– C usually makes 4 bonds but in CN- makes 3.

– O usually makes 2 bonds but in OH- makes 1.

• Positive ion bond number increases:

– N usually makes 3 bonds but in NH4+ makes

4.

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Expanding the Octet Rule

• Maximum number of bonds formed by

carbon is 4 – equivalent to 8 electrons.

• Sulfate ion SO42- contains 6 bonds to

sulfur.

• Sulfur is larger than carbon and can

accommodate more atoms.

• Rule 1 for valence electrons is followed.

• Rule 2 for number of bonds is not.


Formulas of Ionic Compounds

• Salts are neutral but ions are charged.

• Charges of the ions must cancel out:• In MgCl2 Mg2+ charge is cancelled by 2 x Cl-

• In general, the formula of the salt can bepredicted using:

– y+ is charge on cation

– x- is charge on anion

– If x = y then both are given value of 1 (exceptperoxide which is O2

2-)

A Bx y

x-y+

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Common Polyatomic Ions


Salts and Desiccation: Polarity

of Water

• Water is a bent

molecule.

• O is more

electronegative than

H and attracts

electrons.

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Water is a Polar Molecule

• The charge

distribution is uneven.

• Water is a polar

molecule.


Water Molecules Hydrate

Ions

• Why do tightly bounds ionic crystal lattices dissolve?

• Ion – dipole interactions aid the solvation process:– The negative O atoms on water attach to the positive ions.

– The positive H atoms on water attach to the negative ions.

• Like dissolves like:– Ionic compounds dissolve in polar solvents but not in non-polar

solvents.

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Saturation and Limits on

Solubility

• Water molecules are

needed to solvate the

ions.

• Solubility of salt is

limited by availability

of water molecules.

• SSaturation is when

solution has reached

the solubility limit.


The Dynamic Equilibrium

• In a saturated solution thesolid salt is in equilibriumwith the dissolved salt:

• There is constantexchange between theions in the solid and inthe solution:– Rate of ions entering

solution = rate of ionsentering solid.

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Equilibrium Everywhere

• The state of equilibrium is everywhere in chemistry:

– Solid in equilibrium with liquid at melting point.

– Gas in equilibrium with liquid at boiling point.

– Reactants in equilibrium with products in a reaction.


What’s This to do with

Mummies?

• Water is required by bacteria to

decompose the body.

• The preserving salt (natron) absorbs the

water.

• The excess salt means equilibrium is

never reached and no water is left for the

bacteria.

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Chemistry and the Crumbling

Temple

• The temple at Luxor

is crumbling into dust.

• Salt becomes lodged

in crevices.

• Salt absorbs water.

• Expands and breaks

the stone structure.


Salts are Electrolytes

• Pure water does not

conduct electricity.

• Salts in solution

contain ions.

• Ions conduct

electricity.

• Salts are eelectrolytes.

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Electrolytes and Body

Function• Ions in the body are essential for blood pressure control and neural

function.

• Exercise depletes the body of ions.

• Hence the need for electrolytes.

• The history of Gatorade derives from the recognition of the role ofions in bodily function.

• Na+, K+, Mg2+ and Cl- are most important ions


Measuring Concentration

• In chemistry, the most common way of

measuring concentration is molarity:

• Unit is M. mM means millimolar.

• Example: What is concentration of solution

that contains 0.520 mol NaCl in 6.00 L?

_

_

MolessoluteMolarity

Litersolution=

0.5200.0867

6.00

molNaClMolarityM

Lsolution= =

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Salt and Growing Tomatoes

• Salt content of soil is critical for growing a

good tomato.

• Salt level in soil controls moisture level.

• Too much salt in salt draws moisture out

of the plant.


Factors Affecting Fluid Flow

• Cell membranes regulate

flow cell contents

• Semi-permeable

membrane prevents flow

of ions but allows

passage of water

• Osmosis is the flow of

water through a semi-

permeable membrane

from a dilute to a more

concentrated solution

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Osmotic Pressure

• The equilibration of the solution concentrations by

osmosis leads to a height difference in the solutions.

• This is tantamount to a pressure.

• The osmotic pressure is the pressure required to prevent

osmosis occurring.


Osmosis Dictates Water

Transport in the Body

• If cell salt

concentration is

higher water flows

into the cell – cell

expands.

• If cell salt

concentration is

lower, water flows out

from the cell – cell

shrinks.

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Growing Tomatoes in Salty

Soils• Excess salt in the soil

causes passage ofwater from plant toequalizeconcentrations.

• Lower soil saltconcentrationmaintains desirablelevel of water in plant.


Genetic Engineering and Salty

Soils

• Genetic engineering

makes plants with

higher natural salt

contents to permit

growth in salty soils.

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Hard Water and Bad Hair

• Hard ions make hardwater:– Ca2+, Mg2+, Fe#+

• Small highly chargedions form precipitateswith soap.

• Water softeningworks by ionexchange – hard forsoft ions.


Mechanics of Ion-Exchange

• Ion-exchange resins

contain soft ions (Na+)

attracted to negative

tethers.

• Exchange involves

the Na+ ion trading

places with the Ca2+

or Mg2+ ions.

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Ions in Pure Water

• Even a sample of pure

water contains ions.

• This is auto-ionization:

– H+ ions are responsible for

acid behavior.

– OH- ions are responsible

for basic behavior.

• Forward reaction:

– H2O H+ + OH-

• Reverse reaction:

– H+ + OH- H2O


Hydrogen Ions and the pH

Scale

• At equilibrium concentration of H+ and OH-

ions = 1 x 10-7 M (1 in 550,000,000)

• pH is a simple way of reporting the H+ ion

concentration:

pH = -log10[H+]

• Example:

[H+] = 1.0 x 10-7M

pH = -log10(1.0 x 10-7) = 7.00

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Significance of p

• In general:

p ≡ -log10

• pOH = -log10[OH-]

• In pure water [H+] =[OH-] = 1.0 x 10-7 M

• pH = pOH = 7 = neutral

• pH < 7 means [H+] > [OH-]

• pH > 7 means [H+] < [OH-]


Acids and Ions

• Acids are substancesthat produce H+ ionsin solution.

• Water solvates theions.

• In a typical acidsolution the H+ ionsprovided by the waterare negligible– IN 0.001 M HCl, [H+] =

10-3 M, pH = 3

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Slippery when Basic

• Basic solutions contain OH-

• Basic solutions have pH > 7– Example:

– Solution 0.055 M OH- ion has pOH = 1.26

– pH + pOH = 14

– pH = 12.74

• Tartness = acidity; caustic = basic


Significance of pH units

• What is more acidic: tomato juice, pH =

4.5 or orange juice pH = 3.5?

– Tomato juice: [H+] = 10-4.5 = 3.2 x 10-5 M

– Orange juice: [H+] = 10-3.5 = 3.2 x 10-4 M

– Orange juice is 10 times more acidic

• One pH unit = factor of ten change in [H+]

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Measuring pH

• A pH meter contains

a small tube with acid.

• The pH meter is able

to detect the

difference in

concentration

between H+ ions

inside the tube and

outside.


Detecting Acidity

• Indicators aresubstances thatreveal pH by color.

• When pH changesfrom acid to basis, theindicator colorchanges.

• Example:– Phenolphthalein is

colorless in acid butpink in base.

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An Indicator for all Occasions

• Individual indicators can only indicate possible ranges ofpH.

• Different indicators can be selected for different pHranges.

• A universal indicator has several colors that give moreprecise measurements of pH over the whole range.


Strong Acids and Bases

• SStrong acids and

bases dissolve

completely in water.

• All of the molecules

are ionized.

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Interaction of Air and Water

• The pH of water is influenced by the air above it.

• Natural rainwater is slightly acidic because of CO2.

• Some gases, especially the products of industry are

strongly acidic.

• SO2 and NO2 are the molecules responsible for acid rain.


Sources of Acid Rain

• SO2 comes from

burning fossil fuels

(especially coal)

– In air becomes SO3

– In water becomes

H2SO4 (strong acid)

• NO2 comes from

automobiles:

– In water becomes

HNO3 (strong acid)

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Acid Rain Stresses the

Environment

• Aquatic life cannot tolerate low pH.

• Penetration depth for light changes which

affects growth of plants.

• Situation is reversible although full

recovery can take years.

• Environmental legislation has been

effective in reducing acid rain.

Download - The Chemistry of Everything Kimberley Waldron...The Chemistry of Everything Kimberley Waldron Chapter 4 Salt Behavior of ions, acids and bases and the notion of equilibrium ©2007

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