Properties of Acids Acids have a sour taste They react with “active” metals –i.e. Al, Zn, Fe, but not Ag or Au 2 Al + 6 HCl  AlCl H 2 –Corrosive They react with carbonates, producing CO 2 –marble, baking soda, chalk, limestone CaCO HCl  CaCl 2 + CO 2 + H 2 O They change the color of vegetable dyes –blue litmus turns red They react with bases to form ionic salts

Properties of Bases They are also known as “alkalis” They taste bitter –alkaloids = plant products that are alkaline often poisonous Base solutions feel slippery They change the color of vegetable dyeschange the color of vegetable dyes –different color than acids –red litmus turns blue They react with acids to form ionic salts –neutralization

Arrhenius Theory Bases dissociate in water to produce OH - ions and cations –ionic substances dissociate in water NaOH(aq) → Na + (aq) + OH – (aq) Acids ionize in water to produce H + ions and anions –because molecular acids are not made of ions, they cannot dissociate –they must be pulled apart, or ionized, by the water HCl(aq) → H + (aq) + Cl – (aq) –in formula, ionizable H written in front HC 2 H 3 O 2 (aq) → H + (aq) + C 2 H 3 O 2 – (aq)

Arrhenius Acid-Base Reactions The H + from the acid combines with the OH - from the base to make a molecule of H 2 O –it is often helpful to think of H 2 O as H-OH The cation from the base combines with the anion from the acid to make a salt acid + base → salt + water HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l)

Problems with Arrhenius Theory The Arrhenius Theory does not explain why molecular substances, like NH 3, dissolve in water to form basic solutions – even though they do not contain OH – ions. It does not explain acid-base reactions that do not take place in aqueous solution. The H + ions do not exist in water. Acid solutions contain H 3 O + ions –H + = a proton! –H 3 O + = hydronium ions

Brønsted-Lowery Theory In a Brønsted-Lowery Acid-Base reaction, an H + is transferred –It does not have to take place in aqueous solution –It is a broader definition than the Arrhenius Theory provides Acid is a H donor, base is a H acceptor –base structure must contain an atom with an unshared pair of electrons In the reaction, the acid molecule gives an H + to the base molecule H–A + :B  :A – + H–B +

Brønsted-Lowery Acid-Base Reactions One of the advantages of the Brønsted-Lowery theory is that it allows reactions to be reversible H–A + :B → :A – + H–B + The original base has an extra H + after the reaction – so it could act as an acid in the reverse process And, the original acid has a lone pair of electrons after the reaction – so it could act as a base in the reverse process :A – + H–B + → H–A + :B A double arrow “  ” is usually used to indicate a process that is reversible

Conjugate Pairs In a Brønsted-Lowery Acid-Base reaction, the original base becomes an acid in the reverse reaction, and the original acid becomes a base in the reverse process Each reactant and the product it becomes is called a conjugate pair The original base becomes the conjugate acid; and the original acid becomes the conjugate base H–A + :B  :A– + H–B+ acidbaseconjugate base conjugate acid HCHO2 + H2O  CHO2– + H3O+ acid baseconjugate base conjugate acid H2O + NH3  HO – + NH 4 + acidbase conjugate base conjugate acid

Conjugate Acid-Base Pairs AH + B A - + HB + ACID proton donor BASE proton acceptor CONJUGATE BASE proton acceptor CONJUGATE ACID proton donor ACIDCONJUGATE BASE BASECONJUAGE ACID ++

Types of Electrolytes salts = water soluble ionic compounds –Most are strong electrolytes acids = form H 3 O +1 ions in water solution bases = combine with H 3 O +1 ions in water solution –increases the OH -1 concentration may either directly release OH -1 or pull H off H 2 O to form OH -

Strong or Weak A strong acid is a strong electrolyte –practically all the acid molecules ionize, → a strong base is a strong electrolyte –practically all the base molecules form OH – ions, either through dissociation or reaction with water, → a weak acid is a weak electrolyte –only a small percentage of the molecules ionize,  a weak base is a weak electrolyte –only a small percentage of the base molecules form OH – ions, either through dissociation or reaction with water, 

Relationship between Strengths of Acids and their Conjugate Bases The stronger an acid is, the weaker is the attraction of the ionizable H for the rest of the molecule. The better the acid is at donating H, the worse its conjugate base will be at accepting a H: strong acid HCl + H 2 O → Cl – + H 3 O + weak conj. base weak acid HF + H 2 O  F – + H 3 O + strong conj. Base A strong acid is one for which a forward reaction predominates. The relatively weak conj. Base has a low attraction for proton. In contrast, a weak acid is one for which the reverse reaction predominates. The relatively strong conjugate base has a strong attraction for protons.

Common Acids

Structures of Acids Binary acids have acid hydrogens attached to a nonmetal atom –HCl, HF Oxyacids (most common) have acid hydrogens attached to an oxygen atom - H 2 SO 4, HNO 3 Note that sulfuric acid is diprotic acid: can furnish two protons. Carboxylic acids (organic acids) have a -COOH (carboxyl group) HC 2 H 3 O 2, H 3 C 6 H 5 O 3 only the first H in the formula is acidic the H is on the -COOH

Common Bases

Structure of Bases Most ionic bases contain OH - ions –NaOH, Ca(OH) 2 Some contain CO 3 2- ions –CaCO 3, NaHCO 3 Molecular bases contain structures that react with H + –mostly amine groups

Amphoteric Substances amphoteric substances can act as either an acid or a base –having both a transferable H and an atom with a lone pair of electrons HCl(aq) is acidic because HCl transfers an H + to H 2 O, forming H 3 O + ions –water acts as a base, accepting an H + HCl(aq) + H 2 O(l) → Cl – (aq) + H 3 O + (aq) NH 3 (aq) is basic because NH 3 accepts an H + from H 2 O, forming OH – (aq) –water acts as acid, donating H + NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH – (aq) Or:H 2 O(l) + H 2 O(l)  H 3 O + (aq) + OH – (aq) In this reaction one water molecule acts as an acid by furnishing a proton, and the other acts as a base by accepting the proton.

Autoionization of Water Water is actually an extremely weak electrolyte –therefore there must be a few ions present About 1 out of every 10 million water molecules form ions through a process called autoionization H 2 O  H + + OH – (for simplicity) H 2 O(l) + H 2 O(l)  H 3 O + (aq) + OH – (aq) All aqueous solutions contain both H + and OH – –the concentration of H + and OH – are equal in water –[H + ] = [OH – ] = °C

Ion Product of Water The product of the H + and OH – concentrations is always the same number. The number is called the ion product of water and has the symbol K w [H + ] x [OH – ] = 1 x = K w As the [H + ] increases the [OH – ] must decrease so the product stays constant –inversely proportional Neutral solutions have equal [H + ] and [OH – ] –[H + ] = [OH – ] = 1 x Acidic solutions have a larger [H + ] than [OH – ] –[H + ] > 1 x ; [OH – ] < 1 x Basic solutions have a larger [OH – ] than [H + ] –[H + ] 1 x 10 -7

pH The acidity/basicity of a solution is often expressed as pH pH = -log[H + ], [H + ] = 10 -pH –exponent on 10 with a positive sign –pH water = -log[10 -7 ] = 7 –need to know the [H + ] concentration to find pH pH 7 is basic and pH = 7 is neutral

Converting Between [H + ] and pH [H + ] = 1.23 x 10 -4, Calculate the pH Type 1.23 x then ‘log’, then ‘-’ pH = 3.91 pH = 9.4, Calculate [H + ] Type ‘-9.4’, then ‘2nd F’, then ‘10 x ’ [H + ] = 3.98 x Are these solutions acidic, basic, or neutral? The first is acidic, the second is basic NOTE: To find the pH of a strong acid, use the concentration of the acid itself to be equal to the concentration of H+, since strong acids undergo complete ionization into [H+] and the anion.

Converting Between [H + ] and pH Complete the following table: [H + ] pH Basic/neutral/acidic? 4.28 x

pH and pOH The p scale is also used to find the pOH. pOH= -log [OH - ] Example: find the pH and the pOH for a solution of 1.0x10 -4 M H + pH = - log = 4 pOH = -log = 10 (Note that pH + pOH =14) We know [H+] [OH - ] = 1.0x If we take the –log of both sides of the equation: pH + pOH = 14

Buffers Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They resist changing pH by neutralizing any added acid or base. Buffers are made by mixing together a weak acid and its conjugate base –or weak base and its conjugate acid How Buffers Work The weak acid present in the buffer mixture can neutralize added base. The conjugate base present in the buffer mixture can neutralize added acid. The net result is little to no change in solution pH.