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  • 1

    ©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

    ©2007 Pearson Prentice Hall 2

    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.

  • 2

    ©2007 Pearson Prentice Hall 3

    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.

    ©2007 Pearson Prentice Hall 4

    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.

  • 3

    ©2007 Pearson Prentice Hall 5

    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.

    ©2007 Pearson Prentice Hall 6

    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.

  • 4

    ©2007 Pearson Prentice Hall 7

    Mummies and Salts

    • Questions remain

    about how mummies

    were made.

    • Embalming solutions

    contain salts like

    natron.

    ©2007 Pearson Prentice Hall 8

    Natron

    • Natron is a collection of

    salts used in the

    mummification process.

    • Some contain simple ions

    like chloride.

    • Other anions are

    poly(many)atomic:

    – CO3 2-, SO4

    2-

    • Polyatomic ions have

    several resonance dot

    structures.

  • 5

    ©2007 Pearson Prentice Hall 9

    Dot Structures for Ions

    ©2007 Pearson Prentice Hall 10

    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.

  • 6

    ©2007 Pearson Prentice Hall 11

    Expanding the Octet Rule

    • Maximum number of bonds formed by

    carbon is 4 – equivalent to 8 electrons.

    • Sulfate ion SO4 2- 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.

    ©2007 Pearson Prentice Hall 12

    Formulas of Ionic Compounds

    • Salts are neutral but ions are charged.

    • Charges of the ions must cancel out: • In MgCl2 Mg

    2+ charge is cancelled by 2 x Cl-

    • In general, the formula of the salt can be predicted using:

    – y+ is charge on cation

    – x- is charge on anion

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

    2-)

    A B x y

    x-y+

  • 7

    ©2007 Pearson Prentice Hall 13

    Common Polyatomic Ions

    ©2007 Pearson Prentice Hall 14

    Salts and Desiccation: Polarity

    of Water

    • Water is a bent

    molecule.

    • O is more

    electronegative than

    H and attracts

    electrons.

  • 8

    ©2007 Pearson Prentice Hall 15

    Water is a Polar Molecule

    • The charge

    distribution is uneven.

    • Water is a polar

    molecule.

    ©2007 Pearson Prentice Hall 16

    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.

  • 9

    ©2007 Pearson Prentice Hall 17

    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.

    ©2007 Pearson Prentice Hall 18

    The Dynamic Equilibrium

    • In a saturated solution the solid salt is in equilibrium with the dissolved salt:

    • There is constant exchange between the ions in the solid and in the solution: – Rate of ions entering

    solution = rate of ions entering solid.

  • 10

    ©2007 Pearson Prentice Hall 19

    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.

    ©2007 Pearson Prentice Hall 20

    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.

  • 11

    ©2007 Pearson Prentice Hall 21

    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.

    ©2007 Pearson Prentice Hall 22

    Salts are Electrolytes

    • Pure water does not

    conduct electricity.

    • Salts in solution

    contain ions.

    • Ions conduct

    electricity.

    • Salts are eelectrolytes.

  • 12

    ©2007 Pearson Prentice Hall 23

    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 of ions in bodily function.

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

    ©2007 Pearson Prentice Hall 24

    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?

    _

    _

    Molessolute Molarity

    Litersolution =

    0.520 0.0867

    6.00

    molNaCl MolarityM

    Lsolution = =

  • 13

    ©2007 Pearson Prentice Hall 25

    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.

    ©2007 Pearson Prentice Hall 26

    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

  • 14

    ©2007 Pearson Prentice Hall 27

    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.

    ©2007 Pearson Prentice Hall 28

    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.

  • 15

    ©2007 Pearson Prentice Hall 29

    Growing Tomatoes in Salty

    Soils • Excess salt in the soil

    causes passage of water from

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