Fall, GECH119 Solutions Dr. Ralph C. Gatrone Virginia State University Department of Chemistry and Physics

Fall, Chapter Objectives  Ionic Behavior  Polyatomic Ions  Intermolecular Interactions  Solubility  Electrolytes  Osmosis  What is an acid?  What is a base?  Acids react with bases  Acid strength  The pH scale  Buffer Solutions

Fall, Assignment  Read Chapters 6 and 7 in Investigating Chemistry: A Forensic Science Perspective  For future tests and quizzes you should be able to do problems: 1 – 33 in Chapters 6 and 1 – 16 and 28 in Chapter 7.

Fall, Pure Substances  Pure Substances –Elements  Consist of atoms –Covalent Compounds  Consist of molecules –Ionic Compounds  Consist of Ions

Fall, Chemical Bonding  Ionic Bonding  electromagnetic attraction between ions of opposite charge  Covalent Bonding  Sharing of electrons between nuclei  Polar Covalent Bonding  Unequal sharing of electrons between nuclei

Fall, Ionic compounds  Metals on left side form cations  Elements from right side form anions  Cations and anions attract forming ionic compounds  The charges in ionic compounds must equal zero (0)

Fall, Mummification  Egyptian process used Natron to dry body  Natron: sodium carbonate, sodium chloride, sodium bicarbonate and sodium sulfate  Salt is used to dry (preserve) protein.

Fall, Salt Dried Fish

Fall, Questions  How does salt dry a body?  How does salt absorb water?  Salts behave as desiccants.  Desiccants absorb water from surroundings.

Fall, Polarity of Water  Water has bent structure  Electrons are on one side.  Oxygen atom is electronegative.  Forms a dipole with partial charges on each end.

Fall, NaCl dissolves in water  Why?  Na + attracted to negative end of water (O)  Cl - attracted to positive end of water (H)  Ion – dipole attraction  Ions are hydrated

Fall, Ion Dipole Interactions  Weaker than ionic bonds  Large number of these interactions  Disrupts the ionic bond  Causes sodium chloride to dissolve in water  Produces an aqueous solution

Fall, NaCl Dissolving

Fall, Molecular Interactions  Before full discussion of solutions  Need to discuss molecular interactions  What are they?  Where do they come from?

Fall, Attractions Between Particles  All attractions are weaker than chemical bonds  What are these attractions?  How do they arise?

Fall, Attractions Between Particles (strength decreases down list)  Ion-dipole  Dipole – Dipole  Dipole – Induced dipole  Induced dipole – induced dipole Decreasing

Fall, Dipole – Dipole Attraction  Attraction between two polar molecules  Hydrogen bond  Occurs between molecules  Contain N, O, F  Electronegative atoms  Greater negativity causes electrons to be pulled closer to the atom

Fall, Hydrogen Bonding

Fall, Strength of H-Bond  Variable strength  Depends on strength of dipoles  How strongly nonbonding electrons can attract a hydrogen  Weaker than any covalent bond  Very pronounced effect  Structure of water  Structure of the DNA molecule

Fall, Induced Dipoles  Nonpolar molecules induced into a dipole  when it approaches a polar molecule  Electrons are pushed by negative end of water molecule  Induced dipole dipole attraction  Temporary and are very weak  Responsible for oxygen dissolving into water  Responsible for CO 2 dissolving into water (soda industry)

Fall, Temporary Dipoles  Randomness of electron motion  Can have electrons on one side of a nonpolar molecule  Results in a temporary dipole

Fall, Induced dipole – induced dipole  Attraction is the weakest of particle to particle attraction

Fall, Induced dipole – induced dipole  Important in large molecules  More space available  Methane (small molecule) is a gas  Octane (larger molecule) is a liquid

Fall, Solutions  Dissolve sucrose molecules into water  Sucrose is pulled into water by forming hydrogen bonds  Homogeneous mixture is produced  single phase (liquid)  solution

Fall, Solutions  Solid solutions –Gems, alloys  Liquid solutions –Salt water, sugar water  Gaseous solutions –Air (mixture of nitrogen, oxygen, other gases)

Fall, Solutions – Important Terms  Solvent – component in the largest amount (water)  Solute – the other lesser amount component (sugar)  Dissolve (Dissolving) – process of the solute dispersing into the solvent  Saturated – a solution which contains the maximum amount of solute  Unsaturated – a solution which contains less than the maximum amount of solute

Fall, Solutions – More Terms  Concentration – the amount of solute dissolved in an amount of solvent  Units of concentration can be –grams/liter (mass per volume) –moles/liter (number of particles per volume) –moles/liter given by the term molarity (M)

Fall, Solubility  Ability of a solute to dissolve  Solute dissolves easily – soluble  Solute dissolves with difficulty or not at all it is insoluble  Solubility – determined by interactions between solute particles and solute particles with the solvent

Fall, Example

Fall, Infinitely Soluble  Ethanol in Water  Interactions are very similar  Hydrogen bonds  Ethanol will dissolve into water in all proportions  Can reach a point where ethanol is the solvent

Fall, Effect of Temperature  As T increases solubility generally increases  Increase the motion of molecules  Disrupts solute interactions  Not always the case  T has little effect on NaCl dissolving

Fall, Saturated Solutions  Maximum amount of solute  On cooling the solute frequently comes out of solution  Solid coming out of solution is called a precipitate  We state that the solid has precipitated from solvent

Fall, Oxygen in water  Attraction of molecules is weak  Dipole – induced dipole  Water hydrogen bonds to water  Strong attraction  Oxygen is excluded  Oxygen is nearly insoluble  Sand and water are insoluble  Glass and water are insoluble

Fall, Solutions of Gases  Solubility of gases decreases as T increases  Warm soda loses carbonation faster than cold soda  Observed when you open a warm bottle of soda – gas escapes more quickly  Solubility of gases increases with pressure

Fall, Sudden release of pressure from a carbonated beverage. Sudden release of pressure from a carbonated beverage.

Fall, Salts in Water  Salts + water yields ions in water  Solution can conduct electricity  Salts and ions called electrolytes  Dissolved salts in body fluids maintain  Blood pressure, neural functions, healthy cells

Fall, Consider the Tomato  Growth requires  Sunshine  Fertilizer  Water  Soil  Salt concentration is critical  Too much salt – water is removed from the tomato leading to death

Fall, Tomato Pictures

Fall, How is water removed?

Fall, Osmosis  Movement of water across a semipermeable membrane from a region of low ion concentration to a region of high ion concentration  Semipermeable membrane – permits certain molecules to pass through  Ions are surrounded by water and are too large to pass through

Fall, Cell Membranes  Semipermeable

Fall, The Tomato  High salt concentration in soil  Water passes out of tomato into soil  Tomato dries and dies  Genetically engineering a salt tolerant tomato plant

Fall, Soaps  Molecule that has a polar and nonpolar end  Polar end dissolves into water (polar)  Nonpolar end dissolves grease (nonpolar)

Fall, Soap  Polar end interacts with water  Non-polar end interacts with dirt  Combination pulls dirt into water  Polar end must be water soluble  Favorable cation is Na +1  Water often has lots of Ca +2  This soap salt is not soluble in water  Leads to soap scum

Fall, Soap Scum  Hard water  Ca +2 and Mg +2 present  Forms an insoluble salt with soap  Soap Scum  Bind other anions to form insoluble salts  Block up pipes

Fall, Picture of soap scum

Fall, Hard water to Soft Water  Remove Ca +2 and Mg +2 ions  Softens the water  Add CaCO 3 :  done with detergents

Fall, Water Softeners

Fall, Water  Autoionization reaction  H 2 O H + + HO -  Concentrations are small in pure water  pH used to determine [H + ]  pH = 7, water is neutral  Add acid pH decreases (value less than 7)  Add base pH increases (value greater than 7)

Fall, What are Acids?  Bronsted – Lowry definition  Acids donate the hydrogen ion  Hydrogen ion (H + ) = a proton  Acids are sour tasting

Common Acids  Vinegar (acetic acid)  Lemons (citric acid)  Vitamin C (ascorbic acid)  Bile (hydrochloric acid) Fall,

Fall, What are Bases?  Bronsted – Lowry definition  Bases accept the hydrogen ion (H + )  Bases taste bitter  Bases have a slippery feeling  Common bases:  Baking soda (NaHCO 3 )  Drain cleaners (NaOH)

Fall, Common Bases  NaOH – sodium hydroxide  KOH – potassium hydroxide  NH 3 – ammonia  CaO – calcium oxide

Fall, Acid Base Reactions  Acids react with bases  Generally acids do not react with acids  Bases do not react with bases  H + is transferred to the base  Example,  HCl + H 2 O H 3 O + + Cl -  Water accepts the proton becoming  Hydronium ion

Fall, Acid – Base Behavior  Water can behave as an acid – donates H +  Water can behave as a base – accepts H +  Products of acid – base reactions  Behave as acids or bases as well  H 2 O + NH 3 HO - + NH 4 +  Reactions are often reversible

Fall, Water acts like an acid  H 2 O + NH 3 HO - + NH 4 +  Non-bonding electrons on N accept proton  Hydroxide ion forms  Ammonium ion forms

Fall, Water acts like a base

Fall, The hydroxide ion

Fall, Product from Acid Base Reactions  HCl + NaOH  HCl donates proton to hydroxide  Hydroxide accepts proton  Proton + hydroxide = HOH (water)  Chloride does nothing  Sodium does nothing  NaCl is a salt

Fall, Salts  Salts are less corrosive than the acids and bases they come from  Corrosive chemicals  Property to disintegrate or wear away a surface  There are many salts  NaCN – toxic salt  KNO 3 – fertilizer (salt peter) and in gun powder formulations  CaCl 2 – de-ice highways  NaF – prevents tooth decay

Fall, Neutralization Reactions  Acid + base  Equal proportions  Yields salt and water  Water is not always the product  HCl + NH 3 NH 4 + Cl -

Fall, Strength of Acids and Bases  Stronger acids donate proton easily  Stronger bases accept proton easily  Add acid to water  If acid remains, considered weak  If no acid remains, considered strong  Figures show these properties

Fall, Strong Acid

Fall, Weak Acid

Fall, Corrosion  How corrosive something depends  Amount of hydronium ion present  Dilute solutions of strong acids  Less corrosive than more concentrated  Fewer hydronium ions present  True for hydroxide ions from bases

Fall, Water  Water behaves as an acid  Water behaves as a base  Property is called amphoteric  Water reacts with itself  Gives hydronium and hydroxide ions  In pure water  [hydronium ion] = [hydroxide ion]

Fall, Acids  What happens when we add an acid to water?  The hydronium ion (H 3 O + ) forms.  Hydroxide ion concentration must decrease

Fall, Bases  What happens when we add base?  Form hydroxide ion  Hydronium ion concentration must decrease

Fall, Consequence  Adding acid, [H 3 O + ] increases  Solution is described as acidic  Adding base, [HO - ] increases  Solution is described as basic  Equal amounts, solution is neutral  Adding base to acid is neutralization

Fall, Measuring Acidity  pH is used to measure acidity  Acidic solutions  pH is < 7  Basic solutions  pH is > 7  Neutral solutions  pH = 7

Fall, Buffers  Buffers  Solutions that are resistant to changes in pH  Can absorb acid – no pH change  Can absorb base – no pH change  Vital to life processes  pH of blood kept between 7.35 – 7.45  Proteins denature slightly below and above this pH value