Properties of Acids and Bases SECTION 1

Transcription

1 Acids and bases

2 Properties of Acids and Bases SECTION 1

3 Many foods have acid in them Sour milk lactic acid Vinegar acetic acid Citrus fruits citric acid Apples malic acid Grape juice tartaric acid

4 Many household items are bases Ammonia Sodium hydroxide (lye) in drain and oven cleaners Milk of magnesia (antacid) to relieve stomach pain (Mg(OH) 2 )

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6 Acids First recognized as separate class of compounds b/c of common properties of aqueous solutions 1. Aqueous solutions of acids have a sour taste. Should never be used as a test to evaluate a chemical substance Many are corrosive (destroy tissue/clothing) Many are poisonous

7 2. Acids change the color of acid-base indicators. When ph paper used, paper turns certain color

8 3. Some acids react with active metals to release hydrogen gas. Metals above hydrogen in activity series go through single-replacement reactions with certain acids 4. Acids react with bases to produce salts and water. When equal amounts of acid and base reacted, previous properties disappear Acid is neutralized Produces water and ionic compound (salt)

9 5. Some acids conduct electric current. Acids that form many ions in aqueous solution are elctrolytes

10 Naming Acids (Acid Nomenclature) Binary acid à acid that contains only two different elements: hydrogen, and one of the more electronegative elements Ex. Hydrogen halides HF, HCl, HBr, HI

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12 Oxyacid à acid that is a compound of hydrogen, oxygen, and a third element, usually a nonmetal Ex. Nitric acid HNO3 One class of ternary acids (contain 3 elements) Usually written as one or more H followed by polyatomic ion

13 Name of oxyacid Name of oxyanion Prefix Suffix Prefix Suffix Hypo- -ous Hypo- -ite None -ous None -ite None -ic None -ate Per- -ic Per- -ate

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16 Some Common Industrial Acids Properties of acids make them important chemicals in lab and industry Sulfuric acid Nitric acid Phosphoric acid Hydrochloric acid Acetic acid

17 Sulfuric Acid Most commonly produced industrial chemical in the world More than 47 million tons made each year in US alone Used in large quantities in petroleum refining and metallurgy Also in manufacture of fertilizer

18 Also needed for large number of industrial processes Making metals Paper Paint Dyes Detergents Many chemical raw materials Used in car batteries

19 b/c it attracts water, concentrated H2SO4 is an effective dehydration agent Can remove water from gases that it doesn t react with Sugar and other organic compounds dehydrated by H2SO4

20 Nitric Acid Pure nitric acid is volatile, unstable liquid rarely used in industries or labs Stains proteins yellow

21 Acid has suffocating odor Stains skin Can cause serious burns Used in making explosives Also used to make rubber, plastics, dyes, pharmaceuticals

22 Initially it is clear If you leave it, it will turn yellow Slight decomposition to brown nitrogen dioxide gas

23 Phosphoric Acid Phosphorous is essential element for plants and animals Most phosphoric acid used directly for making fertilizers and animal feed Dilute phosphoric acid has pleasant but sour taste and is not toxic Used as flavoring agent in beverages Cleaning agent for dairy equipment Important in making detergents and ceramics

24 Hydrochloric Acid Stomach makes it to help digestion In industry, useful to pickle iron and steel (remove surface impurities) Used in industry as general cleaning agent Used in food processing Used in activation of oil wells, recovery of magnesium from sea water, and in making other chemicals

25 Dilute solution of HCl (referred to as muriatic acid) can be found in hardware stores Used to maintain correct acidity in swimming pools and cleaning stone (masonry) Vixol solution of HCl

26 Acetic Acid Concentrated acetic acid is clear, colorless, strongsmelling liquid known as glacial acetic acid Name comes from freezing point (17 ) Can form crystals in cold room Fermentation of certain plants makes vinegars containing acetic acid White vinegar 4-8% acetic acid

27 Important in industry in making chemicals used in making plastics It s a raw material in production of food supplements ex. Lysine (essential amino acid) Acetic acid is also used as fungicide

28 Bases 1. Aqueous solutions of bases taste bitter. Ever had soap in your mouth? NEVER use this as a test Many bases are caustic they attack skin and tissues 2. Bases change the color of acid-base indicators.

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30 3. Dilute aqueous solutions of bases feel slippery. Like soap 4. Bases react with acids to make salts and water. 5. Bases conduct electric current. Like acids, bases form ions in aqueous solutions so are electrolytes

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32 Arrhenius Acids and Bases Svante Arrhenius Swedish chemist, Understood aqueous solutions of acids and bases conduct electric current He concluded acids and bases must produce ions in solution

33 Arrhenius acid à chemical compound that increases the concentration of hydrogen ions in aqueous solution Acid ionized in solution and increases the number of H + present Arrhenius base à substance that increases the concentration of hydroxide ions (OH - ) in aqueous solution

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35 Some bases are ionic hydroxides These dissociate in solution to release OH - into solution Others are substances that react with water to remove H +, leaving OH - in solution

36 Aqueous Solutions of Acids The acids described by Arrhenius are molecular compounds with ionizable hydrogen atoms Their water solutions are called aqueous acids All pure aqueous acids are electrolytes

37 Acid molecules are polar enough so that one or more hydrogen ions are attracted by H2O Negatively charged anions are left behind Hydrogen ion in aqueous solution is best represented by hydronium, H3O +

38 Ex. HNO3 Hydrogen atoms combine with water to form hydronium HCl ionizes in a similar way

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40 Strength of Acids Strong acid à one that ionizes completely in aqueous solution Ex. HClO4, HCl, HNO3 Strength depends on polarity of bond between hydrogen and other elements how easily it breaks Strength increases with increasing polarity and decreasing bond energy

41 Weak acid à acids that are weak electrolytes Aqueous solutions contain hydronium ions, anions and dissolved acid molecules Ex. HCN (hydrocyanic acid)

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43 In aqueous solution, ionization and reverse reaction of HCN happen at same time Most of solution is made of hydrogen cyanide and water

44 Strong acids assumed to ionize completely to give up one H+ Number of H+ in formula does not affect strength

45 Molecules with multiple hydrogen atoms may not give them up easily Ex. Phosphoric acid has 3 hydrogens Doesn t give any of them up easily So is weak acid

46 Organic acids à contain acidic carboxyl group (-COOH) Generally weak acids Ex. Acetic acid (CH3COOH) Ionizes slightly in water

47 1 molecule of acetic acid contains 4 hydrogen atoms Only 1 ionizes Hydrogen in carboxyl is the acidic hydrogen Forms hydronium ion

48 Aqueous Solutions of Bases Most bases are ionic compounds made of metal cations and the hydroxide anion b/c they are ionic, they dissociate to some extent when in solution Alkaline à when a base completely dissociates in water to yield aqueous OH - ions

49 NaOH is water-soluble and dissociates as follows Remember sodium is an alkali metal All alkali metal hydroxides form alkaline solutions

50 Not all bases are ionic compounds Ammonia (common household cleaner) is molecular It s a base b/c it produces hydroxide ions when it reacts with water molecules

51 Strength of Bases Strength of base also depends on extent of dissociation into solution Ex. KOH is strong base it dissociates completely Strong bases are also strong electrolytes

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53 Bases that are not very soluble don t make a large number of OH - when added to water Some metal hydroxide are not very soluble in water They cannot make strong alkaline solutions Alkalinity depends on concentration of OH - in solution Unrelated to number of OH - ions in undissolved compound

54 Consider ammonia highly soluble but weak electrolyte Concentration of OH- ions in solution relatively low So, ammonia is weak base Many organic compounds that contain N atoms also weak bases

55 Acid-Base Theories SECTION 2

56 As scientists investigated acid-base behavior, they found some substances acted as acids or bases when not in water solution b/c Arrhenius definition requires aqueous solutions, definitions of acids and bases had to be revised

57 Brønsted-Lowry Acids and Bases Brønsted-Lowry acid à a molecule or ion that is a proton donor b/c H+ is proton, all Arrhenius acids donate a proton and so are Brønsted-Lowry acids as well Substances other than molecules can also donate protons These are not Arrhenius acids but are included in Brønsted-Lowry acids

58 Hydrogen chloride acts as Brønsted-Lowry acid when dissolved in ammonia HCl transfers proton to ammonia molecule Forms ammonium

59 Electron-dot formulas show similarity of reaction to reaction with HCl in water In both reactions, HCl is Brønsted-Lowry acid

60 Water can also be Brønsted-Lowry acid Ex. Water donating proton to ammonia molecule

61 Brønsted-Lowry base à molecule or ion that is a proton acceptor In reaction with HCl and ammonia, ammonia accepts proton from HCl It is a Brønsted-Lowry base

62 Arrhenius hydroxide bases (NaOH) not necessarily Brønsted-Lowry bases As compounds, they are not proton acceptors OH- made in solution is Brønsted-Lowry base It is the species that accepts proton

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64 Brønsted-Lowry acid-base reaction à protons transferred from one reactant (the acid) to another (the base)

65 Monoprotic Acids Monoprotic acid à an acid that can donate only 1 proton (hydrogen ion) per molecule Ex. HClO4, HCl, HNO3

66 Polyprotic Acids Polyprotic acid à an acid that can donate more than one proton per molecule Ex. H2SO4, H3PO4 Ionization occurs in stages Acid loses hydrogens one at a time

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68 Diprotic acid à acid that can donate 2 protons per molecule Triprotic acid à acid that can donate 3 protons

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70 Lewis Acids and Bases Both Arrhenius and Brønsted-Lowry definitions assume that acid contains or make hydrogen ions 3 rd classification (based on bonding and structure) includes acids as substances that do not have hydrogen at all Lewis definition emphasizes role of electron pairs in acidbase reactions Lewis acid à atom, ion, or molecule that accepts an electron pair to form a covalent bond

71 Broadest definition of 3 definitions Applies to any species that can accept electron pair to form covalent bond with another species Lone proton is Lewis acid in reactions where it forms covalent bond

72 The formula for Lewis acid doesn t need to have hydrogen Even silver ion can be Lewis acid

73 Any compound where central atom has 3 valence electrons and forms 3 covalent bonds can react as Lewis acid It accepts pair of electrons to form 4 th covalent bond (completes electron octect) Ex. Boron trifluoride excellent Lewis acid Forms 4 th covalent bond with many molecules and ions

74 Lewis definition applies to species in any phase (s, l, g, aq) Ex. BF3 and NH3 in gas phase

75 Lewis base à atom, ion, or molecule that donates an electron pair to form a covalent bond Anion is Lewis base in reaction where it forms covalent bond by donating electron pair Ex. BF3 reacting with F- F- donates electron pair to BF3 (acts as Lewis base)

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77 Lewis acid-base reaction à formation of one or more covalent bonds between an electron-pair donor and an electron-pair acceptor Even though 3 definitions are different, many compounds categorized as acids or bases according to all 3

78 Ex. NH3 Arrhenius base b/c OH- ions created when ammonia is in solution Brønsted-Lowry base b/c it accept a proton in acid-base reaction Lewis base in all reactions where it donates lone pair of electrons to form covalent bond

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80 Acid-Base Reactions SECTION 3

81 Conjugate Acids and Bases Brønsted-Lowry definitions provide basis for studying proton-transfer reactions If Brønsted-Lowry acid gives up proton, remaining ion/ molecule can accept it This makes it a conjugate base Conjugate base à the species that remains after a Brønsted-Lowry acid has given up a proton

82 Ex. Fluoride ion is conjugate base of HF In this reaction, water molecule is Brønsted-Lowry base It accepts proton to form H 3 O+ (which is acid)

83 Conjugate acid à species that is formed when a Brønsted-Lowry base gains a proton

84 In general, Brønsted-Lowry acid-base reactions are equilibrium systems Both forward and reverse reactions occur They involve two acid-base pairs (conjugate acid-base pairs)

85 Strength of Conjugate Acids and Bases Degree of reaction between Brønsted-Lowry acid and base depends on strengths of acids and bases Ex. HCL strong acid (gives up proton readily) Cl- has little tendency to attract and keep proton Cl- is very weak base

86 Important conclusion: the stronger an acid is, the weaker its conjugate base; the stronger a base is, the weaker its conjugate acid This allows strengths of different acids and bases to be compared to predict outcome of reaction

87 Ex. Perchloric acid (HClO4) and water Hydronium ion too weak an acid to compete with perchloric acid in donating proton (HClO4 stronger acid)

88 Perchlorate ion and H2O are both bases Because HClO4 is very strong acid, ClO4- is very weak base So H2O gets proton

89 H3O+ ion concentration lower than previous example b/c acetic acid is weak acid CH3COOH doesn t compete successfully with H3O+ ion in donating protons to base Acetate ion is stronger base than H2O So H2O molecule doesn t compete successfully with CH3COO- in accepting proton H3O+ is stronger acid CH3COO- is stronger base Reaction to left more favorable

90 Favored reactions always toward the weaker acid and base Second important conclusion: proton-transfer reactions favor the production of the weaker acid and the weaker base For reaction to reach completion, reactants must be stronger acids/bases than products

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92 Amphoteric Compounds Water can be acid or base Amphoteric compound à any species that can react as either an acid or a base

93 Whether an acid or base depends on strength of acid/ base it is reacting with If water reacts with compound that is stronger acid than itself, then it will be a base If water reacts with a compound that is a weaker acid than itself, it will be an acid

94 -OH in a Molecule Molecular compounds with OH groups can be acidic or amphoteric This is the hydroxyl group For compound to be acidic a water molecule must be able to attract a hydrogen atom from a hydroxl group This happens more easily when O-H bond very polar Any feature of molecule that increases polarity of O-H bond increases acidity of molecular compound

95 More-electronegative atoms in upper right corner of periodic table form compounds with acidic hydroxyl groups All oxyacids are molecular electrolytes that contain 1 or more O-H bonds Ex. Chloric and perchloric acids

96 Notice all oxygen atoms bonded to chlorine atom Each hydrogen bonded to oxygen atom Aqueous solutions of these are acids b/c O-H bonds broken as hydrogen attracted by water molecules

97 Behavior of compound affected by number of O atoms bonded to atom connected to OH group Larger number of O atom, more acidic compound will be Electronegative O atoms draw electron density away from O-H bond and make it more polar

98 Notice as more oxygen added, it becomes more acidic

99 In acetic acid not ethanol second O bonded to C connected to OH group This is why acetic acid is acidic but ethanol isn t

100 Neutralization Reactions Ex. Sodium bicarbonate and tartaric acid in baking powder When in solution, they react to make CO2 Escaping CO2 used to make breads rise Ex. Antacid (Eno) soothes overly acidic stomach

101 Strong Acid-Strong Base Neutralization Reaction between aqueous HCl and NaOH

102 Solution of 1 mol NaOH dissociates as follows: Solution of 1 mol NaCl dissociates as follows:

103 If two solutions are mixed, a reaction happens between aqueous ions NaCl and water are produced Overall ionic equation:

104 b/c they are on both sides of equation, Na + and Cl - are spectator ions Only participants in reaction are H3O + and OH - Net ionic equation:

105 There are equal numbers of H3O + and OH - ions in this reaction so they are completely converted to water Neutralization à the reaction of hydronium ions and hydroxide ions to form water molecules Salt is also produced Salt à ionic compound composed of a cation from a base and an anion from an acid

106 Acid Rain Many industrial reactions make gases like NO, NO 2, CO 2, SO 2 and SO 3 These dissolve in atmospheric water to make acidic solutions that fall as rain or snow

107 Marble in many buildings and statues are made of calcium carbonate CaCO3 Acid rain: Products: salt, water, and CO2

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