The Chemistry of Everything Kimberley Waldron. Chapter Topics

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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 1 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 Pearson Prentice Hall 2 1

2 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 Pearson Prentice Hall 3 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 Pearson Prentice Hall 4 2

2007 Pearson Prentice Hall 5 Applications of Ionic Liquids Environmentally friendly alternative to organic solvents. Wide liquid range.

3 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. Ionic Liquids? 2007 Pearson Prentice Hall 5 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 Pearson Prentice Hall 6 3

4 Mummies and Salts Questions remain about how mummies were made. Embalming solutions contain salts like natron Pearson Prentice Hall 7 Natron Natron is a collection of salts used in the mummification process. Some contain simple ions like chloride. Other anions are poly(many)atomic: CO 3 2-, SO 4 2- Polyatomic ions have several resonance dot structures Pearson Prentice Hall 8 4

Dot Structures for Ions 2007 Pearson Prentice Hall 9 Dot Structures for Ions don t Follow the Rules of Neutral Molecules Addition of charge (+ or -) changes the bonding.

5 Dot Structures for Ions 2007 Pearson Prentice Hall 9 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 NH 4 + makes Pearson Prentice Hall 10 5

6 Expanding the Octet Rule Maximum number of bonds formed by carbon is 4 equivalent to 8 electrons. Sulfate ion SO 2-4 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 Pearson Prentice Hall 11 Formulas of Ionic Compounds Salts are neutral but ions are charged. Charges of the ions must cancel out: In MgCl 2 Mg 2+ charge is cancelled by 2 x Cl - In general, the formula of the salt can be predicted using: y y+ is charge on cation A + x B - x- is charge on anion x y If x = y then both are given value of 1 (except peroxide which is O 2 2- ) 2007 Pearson Prentice Hall 12 6

7 Common Polyatomic Ions 2007 Pearson Prentice Hall 13 Salts and Desiccation: Polarity of Water Water is a bent molecule. O is more electronegative than H and attracts electrons Pearson Prentice Hall 14 7

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

8 Water is a Polar Molecule The charge distribution is uneven. Water is a polar molecule Pearson Prentice Hall 15 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 Pearson Prentice Hall 16 8

Saturation and Limits on Solubility Water molecules are needed to solvate the ions. Solubility of salt is limited by availability of water molecules.

9 Saturation and Limits on Solubility Water molecules are needed to solvate the ions. Solubility of salt is limited by availability of water molecules. Saturation is when solution has reached the solubility limit Pearson Prentice Hall 17 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 Pearson Prentice Hall 18 9

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.

10 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 Pearson Prentice Hall 19 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 Pearson Prentice Hall 20 10

2007 Pearson Prentice Hall 21 Salts are Electrolytes Pure water does not conduct electricity.

11 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 Pearson Prentice Hall 21 Salts are Electrolytes Pure water does not conduct electricity. Salts in solution contain ions. Ions conduct electricity. Salts are electrolytes Pearson Prentice Hall 22 11

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.

12 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 +, Mg 2+ and Cl - are most important ions 2007 Pearson Prentice Hall 23 Measuring Concentration In chemistry, the most common way of measuring concentration is molarity: Molessolute _ Molarity= Litersolution _ Unit is M. mm means millimolar. Example: What is concentration of solution that contains mol NaCl in 6.00 L? molnacl MolarityM = = Lsolution 2007 Pearson Prentice Hall 24 12

13 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 Pearson Prentice Hall 25 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 semipermeable membrane from a dilute to a more concentrated solution 2007 Pearson Prentice Hall 26 13

2007 Pearson Prentice Hall 27 Osmosis Dictates Water Transport in the Body If cell salt concentration is higher water flows

14 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 Pearson Prentice Hall 27 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 Pearson Prentice Hall 28 14

Lower soil salt concentration maintains desirable level of water in plant.

15 Growing Tomatoes in Salty Soils Excess salt in the soil causes passage of water from plant to equalize concentrations. Lower soil salt concentration maintains desirable level of water in plant Pearson Prentice Hall 29 Genetic Engineering and Salty Soils Genetic engineering makes plants with higher natural salt contents to permit growth in salty soils Pearson Prentice Hall 30 15

Hard Water and Bad Hair Hard ions make hard water: Ca 2+, Mg 2+, Fe #+ Small highly charged ions form precipitates with soap. Water softening works by ion exchange hard for soft ions.

16 Hard Water and Bad Hair Hard ions make hard water: Ca 2+, Mg 2+, Fe #+ Small highly charged ions form precipitates with soap. Water softening works by ion exchange hard for soft ions Pearson Prentice Hall 31 Mechanics of Ion-Exchange Ion-exchange resins contain soft ions (Na + ) attracted to negative tethers. Exchange involves the Na + ion trading places with the Ca 2+ or Mg 2+ ions Pearson Prentice Hall 32 16

17 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: H 2 O H + + OH - Reverse reaction: H + + OH - H 2 O 2007 Pearson Prentice Hall 33 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 = -log 10 [H + ] Example: [H + ] = 1.0 x 10-7 M ph = -log 10 (1.0 x 10-7 ) = Pearson Prentice Hall 34 17

18 Significance of p In general: p -log 10 poh = -log 10 [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 - ] 2007 Pearson Prentice Hall 35 Acids are substances that produce H + ions in solution. Water solvates the ions. In a typical acid solution the H + ions provided by the water are negligible IN M HCl, [H + ] = 10-3 M, ph = 3 Acids and Ions 2007 Pearson Prentice Hall 36 18

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.

19 Slippery when Basic Basic solutions contain OH - Basic solutions have ph > 7 Example: Solution M OH - ion has poh = 1.26 ph + poh = 14 ph = Tartness = acidity; caustic = basic 2007 Pearson Prentice Hall 37 Significance of ph units What is more acidic: tomato juice, ph = 4.5 or orange juice ph = 3.5? Tomato juice: [H + ] = = 3.2 x 10-5 M Orange juice: [H + ] = = 3.2 x 10-4 M Orange juice is 10 times more acidic One ph unit = factor of ten change in [H + ] 2007 Pearson Prentice Hall 38 19

outside. 2007 Pearson Prentice Hall 39 Indicators are substances that reveal ph by color.

20 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 Pearson Prentice Hall 39 Indicators are substances that reveal ph by color. When ph changes from acid to basis, the indicator color changes. Example: Phenolphthalein is colorless in acid but pink in base. Detecting Acidity 2007 Pearson Prentice Hall 40 20

A universal indicator has several colors that give more precise measurements of ph over the whole range.

21 An Indicator for all Occasions Individual indicators can only indicate possible ranges of ph. Different indicators can be selected for different ph ranges. A universal indicator has several colors that give more precise measurements of ph over the whole range Pearson Prentice Hall 41 Strong Acids and Bases Strong acids and bases dissolve completely in water. All of the molecules are ionized Pearson Prentice Hall 42 21

SO 2 and NO 2 are the molecules responsible for acid rain.

22 Interaction of Air and Water The ph of water is influenced by the air above it. Natural rainwater is slightly acidic because of CO 2. Some gases, especially the products of industry are strongly acidic. SO 2 and NO 2 are the molecules responsible for acid rain Pearson Prentice Hall 43 Sources of Acid Rain SO 2 comes from burning fossil fuels (especially coal) In air becomes SO 3 In water becomes H 2 SO 4 (strong acid) NO 2 comes from automobiles: In water becomes HNO 3 (strong acid) 2007 Pearson Prentice Hall 44 22

23 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 Pearson Prentice Hall 45 23

A solution is a homogeneous mixture of two or more substances.

UNIT (5) SOLUTIONS A solution is a homogeneous mixture of two or more substances. 5.1 Terminology Solute and Solvent A simple solution has two components, a solute, and a solvent. The substance in smaller