3 definitions of Acids/BasesnThe theory of acids/bases has changed over time due to research and experimentationn1) Arrehnius (oldest)n2) Bronstead-Lowrey (newer)n3) Lewis (newest)
1.Arrhenius Definition - 1887n Acids produce hydrogen cations (H+) or hydronium ions (H3O +) when dissolved in water (HCl → H+ + Cl-)n Bases produce hydroxide anions (OH-) when dissolved in water. (NaOH → Na + + OH-)n Only in aqueous solutions.n Aqueous solutions are liquids that have moleculess dissolved in WATER
Svante Arrheniusn He was a Swedish chemist (1859-1927), and a Nobel prize winner in chemistry (1903)n One of the first chemists to explain the chemical theory of the behavior of acids and bases
2. Brønsted-Lowry - 1923n A broader definition than Arrheniusn Acid is hydrogen-ion donor (H+ or proton); base is hydrogen-ion acceptor.n Acids and bases always come in pairs.n HCl is an acid. –When it dissolves in water, it gives it’s proton to water. HCl(g) + H2O(l) ↔ H3O+(aq) + Cl-(aq)n Water is a base; makes hydronium ion.
Johannes Brønsted Thomas Lowry (1879-1947) (1874-1936) Denmark England
Why Ammonia is a Basen Ammonia can be explained as a base by using Brønsted-Lowry:NH3(aq) + H2O(l) ↔ NH4 (aq) + OH (aq) 1+ 1-Ammonia is the hydrogen ion acceptor (base), and water is the hydrogen ion donor (acid).This causes the OH1- concentration to be greater than in pure water, and the ammonia solution is basic
Acids and bases come in pairsn A “conjugate base” is the charged particle of the original acid, after it donates it’s hydrogen ionn A “conjugate acid” is the charged particle formed when the original base gains a hydrogen ionn Thus, a conjugate acid-base pair is related by the loss or gain of a single hydrogen ion.
Acids and bases come in pairsn General equation is: HA(aq) + H2O(l) ↔ H3O+(aq) + A-(aq)n Acid + Base ↔ Conjugate acid + Conjugate basen NH3 + H2O ↔ NH4 1+ + OH 1- base acid c.a. c.b.n HCl + H2O ↔ H3O1+ + Cl1- acid base c.a. c.b.n Amphoteric – a substance that can act as both an acid and base- as water shows
3. Lewis Acids and Basesn GilbertLewis focused on the donation or acceptance of a pair of electrons during a reactionn Lewis Acid - electron pair acceptorn Lewis Base - electron pair donorn Most general of all 3 definitions; acids don’t even need hydrogen!
What Happens When an Acid Dissolves in Water? • Water acts as a base and abstracts a proton (H+) from the acid. • As a result, the conjugate base (Cl-) of the acid and a hydronium ion (H3O+) are formed. Acids and Bases
Arrhenius Acid Any substance that releases H+ ions as the only positive ion in the aqueous solution. 1+ 1- + + HCl H 2O H3O+ Cl-hydrogen chloride water hydronium ion chloride ion(an Arrhenius acid)
DefinitionsArrhenius - In aqueous solution… •Acids form hydronium ions (H3O+) HCl + H2O → H3O + Cl + – H H + – O O Cl Cl H H H H acid Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
DefinitionsArrhenius - In aqueous solution… •Bases form hydroxide ions (OH-) NH3 + H2O → NH4+ + OH - H H + – N O N O H H H H H H H H base Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Neutralization Neutralization is a chemical reaction between an acid and a base to produce a salt (an ionic compound) and water. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) base acid salt water Some neutralization reactions: H2SO4(aq) + 2 NaOH(aq) Na2SO4 + 2 H2O(l) sulfuric acid sodium hydroxide sodium sulfate water2 HC2H3O2(aq) + Ca(OH)2(aq) Ca(C2H3O2)2 + 2 H2O(l) acetic acid calcium hydroxide calcium acetate water
Salts NaClSalts - Ionic compounds containing a positive ion other thanthe hydrogen ion and a negative ion other than the hydroxideion. i.e., a metal and a non-metal NaCl(s) + H2O(l) Na1+(aq) + Cl1-(aq) Formulas and names of common salts SALT FORMULA Common Name sodium chloride NaCl (table) salt sodium nitrate NaNO3 Chile saltpeter sodium bicarbonate NaHCO3 baking soda potassium carbonate K2CO3 potash ammonium chloride NH4Cl sal ammoniac
Neutralization ACID + BASE → SALT + WATER HCl + NaOH → NaCl + H2O strong strong neutralHC2H3O2 + NaOH → NaC2H3O2 + H2O weak strong basic• Salts can be neutral, acidic, or basic.• Neutralization does not mean pH = 7. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
ACID + BASE → SALT + WATER HCl + NaOH → NaCl + H2O strong strong neutralHC2H3O2 + NaOH → NaC2H3O2 + H2O weak strong basic• Salts can be neutral, acidic, or basic.• Neutralization does not mean pH = 7. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
pH scale: measures acidity/basicity ACID BASE0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 NEUTRALAcids have a pH between 0 and 7Bases have a pH between 7 and 14“Neutral” pH is 7
Properties of Acidsn They taste sour (don’t try this at home).n They can conduct electricity. –Can be strong or weak electrolytes in aqueous solutionn React with metals to form H2 gas.n Change the color of indicators (for example: blue litmus turns to red).n React with bases (metallic hydroxides) to form water and a salt.
Properties of Acidsn They have a pH of less than 7 (more on this concept of pH in a later lesson)n They react with carbonates and bicarbonates to produce a salt, water, and carbon dioxide gasn How do you know if a chemical is an acid? –It usually starts with Hydrogen. –HCl, H2SO4, HNO3, etc. (but not water!)
Acids Affect Indicators, by changing their colorBlue litmus paper turns red incontact with an acid (and red paperstays red).
Acids React with Active MetalsAcids react with active metals toform salts and hydrogen gas:HCl(aq) + Mg(s) → MgCl2(aq) + H2(g)This is a single-replacement reaction
Acids React with Carbonates and Bicarbonates HCl + NaHCO3Hydrochloric acid + sodium bicarbonate NaCl + H2O + CO2 salt + water + carbon dioxide An old-time home remedy for relieving an upset stomach
Effects of Acid Rain on Marble (marble is calcium carbonate)George Washington: George Washington: BEFORE acid rain AFTER acid rain
Sulfuric Acid = H2SO44 Highestvolume production of any chemical in the U.S. (approximately 60 billion pounds/year)4 Used in the production of paper4 Used in production of fertilizers4 Used in petroleum refining; auto batteries
Nitric Acid = HNO34 Used in the production of fertilizers4 Used in the production of explosives4 Nitric acid is a volatile acid – its reactive components evaporate easily4 Stains proteins yellow (including skin!)
Hydrochloric Acid = HCl4 Used in the “pickling” of steel4 Used to purify magnesium from sea water4 Part of gastric juice, it aids in the digestion of proteins4 Sold commercially as Muriatic acid
Phosphoric Acid = H3PO4 4 A flavoring agent in sodas (adds “tart”) 4 Used in the manufacture of detergents 4 Used in the manufacture of fertilizers 4 Not a common laboratory reagent
Acetic Acid = HC2H3O2(also called Ethanoic Acid, CH3COOH)4 Used in the manufacture of plastics4 Used in making pharmaceuticals4 Acetic acid is the acid that is present in household vinegar
Properties of Bases (metallic hydroxides)n React with acids to form water and a salt.n Taste bitter.n Feel slippery (don’t try this either).n Can be strong or weak electrolytes in aqueous solutionn Change the color of indicators (red litmus turns blue).
Examples of Bases (metallic hydroxides) Sodium hydroxide, NaOH (lye for drain cleaner; soap) Potassium hydroxide, KOH (alkaline batteries) Magnesium hydroxide, Mg (OH)2 (Milk of Magnesia) Calcium hydroxide, Ca (OH)2 (lime; masonry)
Bases Affect IndicatorsRed litmus paperturns blue in contactwith a base (and blue Phenolphthaleinpaper stays blue). turns purple in base (pH above 9.0)
Bases Neutralize AcidsMilk of Magnesia containsmagnesium hydroxide,Mg(OH)2, which neutralizesstomach acid, HCl. 2 HCl + Mg(OH)2 Magnesium salts can cause diarrhea (thus they are used MgCl2 + 2 H2O as a laxative) and may also cause kidney stones.
Acids and Bases litmus paper pH less than 7 pH greater than 7 sour taste ______ bitter taste ______ bases react with ______ acids react with ______ proton (H1+) donor proton (H1+) acceptor turn litmus red turn litmus blue lots of H1+/H3O1+ lots of OH1– react w/metals don’t react w/metalsBoth are electrolytes. (they conduct electricity in soln)
Strengthn Acids and Bases are classified according to the degree to which they ionize or dissociate (break apart into ions) in water: –Strong are completely ionized in aqueous solution; this means they ionize 100 % –Weak ionize only slightly in aqueous solutionn Strength is very different from Concentration
Strengthn Strong- completely ionizes when dissolved (100 % ionization)n KOH (Potassium Hydroxide) is a strong base- it completely ionizes (100%) in water.n HCl(Hydrochloric Acid) is a strong acid - it completely ionizes (100%) in water
Concentrationn Concentration:How many acid/base molecules are present in a solutionn Concentrated = A LOT of acid/base molecules. High concentration is above 1.0 M (M = Molar = moles/L)n Dilute = A little of acid/base molecules. Low concentration is below 1.0 M
Strength vs. Concentrationn The words concentrated and dilute tell how much of an acid or base is dissolved in solution - refers to the number of moles of acid or base in a given volumen Thewords strong and weak refer to the extent of ionization of an acid or base. Strong = 80% - 100% ionizationn Weak = 10% - 30 % ionization
Common Acids (cont.)Weak Acids (dissociate very little)acetic acid: CH3COOH H1+ + CH3COO1– -- vinegar; naturally made by appleshydrofluoric acid: HF H1+ + F1– -- used to etch glasscitric acid, H3C6H5O7-- lemons or limes; sour candyascorbic acid, H2C6H6O6 -- vitamin Clactic acid, CH3CHOHCOOH-- waste product of muscular exertion
carbonic acid, H2CO3-- carbonated beverages-- CO2 + H2O H2CO3 rainwater H2CO3: beverage carbonation in air dissolves limestone (CaCO3) H2CO3: cave formation H2CO3: natural acidity of lakes
How to measure pH with wide-range paper1. Moisten the pHindicator paper stripwith a few drops of 2.Compare the color tosolution, by using a the chart on the vial –stirring rod. then read the pH value.
Some of the many pH Indicators and their pH range
Acid-Base Indicatorsn Although useful, there are limitations to indicators: –usually given for a certain temperature (25 oC), thus may change at different temperatures –what if the solution already has a color, like paint? – the ability of the human eye to distinguish colors is limited
Acid-Base Indicatorsn A pHmeter may give more accurate pH values –some are large, others portable –works by measuring the voltage between two electrodes; typically accurate to within 0.01 pH unit of the true pH –Instruments need to be calibrated
Indicators chemicals that change color, depending on the pHTwo examples, out of many: litmus………………… red in acid, blue in base acid base phenolphthalein…….. clear in acid, pink in base acid base l e a b pink r
Measuring pH litmus paper Basically, pH < 7 or pH > 7. phenolphthaleinpH paper -- contains a mixture of various indicators -- each type of paper measures a range of pH -- pH anywhere from 0 to 14universal indicator -- is a mixture of several indicators -- pH 4 to 10 4 5 6 7 8 9 10 R O Y G B I V
Measuring pH (cont.)pH meter -- measures small voltages in solutions -- calibrated to convert voltages into pH -- precise measurement of pH
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