Chapter 16 Acids and Bases

Characteristics Acids: Sour taste –Lemons, Oranges (citric acid) Bases: Slippery, bitter taste –Soaps Change the color of dyes

Characteristics Add acid to water makes water acidic Add base to water makes it more basic Add acid to base neutralizes NaOH + HCl  NaCl + H 2 O

Arrhenius Acids –Donate H + –Increase [H + ] Bases –Donate OH - –Increase [OH - ]

Bronsted/Lowry Acids –Transfer proton to another substance Bases –Accept protons from another substance

Water as a Bronsted/Lowry H 2 O  H + + OH - H + + H 2 O  H 3 O + Hydronium can bond to many other water molecules

Water and Hydronium H 2 O + HCl  H 3 O + + Cl - Where water is a base HCl is 100% ionized Hydronium is the conjugate acid Chloride is the conjugate base

Ammonia as a base HCl +NH 3  NH Cl - H 2 O +NH 3  NH OH - An acid and a base always work together to transfer protons.

Amphoteric Species A compound that can act as both an acid and a base under different conditions. Water –Base with HCl –Acid with NH 3

Conjugate acids and bases Every acid/base reaction has –Acid –Base –Conjugate acid –Conjugate base –H 2 O +NH 3  NH 4 + OH -

Practice What are the conjugate bases of the following acids? HClO 4 H 2 S PH 4 +

Practice What are the conjugate acids of the following bases?  CN -  SO 2 -2  HCO 3 -

Strong Acids 100% ionized HCl, H 2 SO 4, HNO 3 HCl  H + + Cl - –Reaction is not reversible –Cl - is a weak base and does not accept protons

Seven Strong Acids HCl HBr HI HClO 4, HClO 3 HNO 3 H 2 SO 4

Strong Bases 100% ionized Group 1 hydroxides, heavy Group 2 NaOH + H 2 O  NaH 2 O + + OH - –Conjugate acid is weak CH 3 - is a VERY strong base –No acidity and takes proton from water

Weak Acids Slightly dissociated CH 3 COOH + H 2 O  CH 3 COO - + H 3 O + Scale on page 598, fig 16.4

Generally HX + H 2 O  H 3 O + + X - If HX is stronger than water, rxn right If HX is weaker than water, rxn left Equilibrium favors transfer of proton from stronger acid to stronger base

Practice NH OH -  NH 3 + H 2 O –Does the equilibrium lie to the right or left? –Why

Autoionization of water H 2 O + H 2 O  H 3 O + + OH - This is a very fast reaction Only about 1 out of 10 9 water molecules are dissociated at any time

Generally Shown as: H 2 O  H + + OH -

Ion Product H 2 O + H 2 O  H 3 O + + OH - K c = [H + ] [OH - ] [H 2 O] But water is a constant so…

MEMORIZE THIS! K c [H 2 O] = [H + ][OH - ] K w = [H + ][OH - ] = 1 x At equilibrium [H + ]=[OH - ]= 1 x 10 -7

Practice Remember: at equilibrium [H + ]=[OH - ]= 1 x Identify the following as acid or base [H + ] = 1 E –5 [OH - ] = 3.2 E –9 [H + ] = 7.1 E –8 [OH - ] = 4.6 E –3

pH scale Cumbersome to always use exponents Use logarithmic scale to deal with exponent MEMORIZE: pH = -log [H + ]

So… If [H + ] = 1E -7 then –log[1E -7 ] = 7 Therefore… the pH of a neutral solution is 7 –(equal amounts of H + and OH - )

Calculations involving pH If given [OH - ] there are two methods to get pH 1. Use K w = [H + ][OH - ] = 1 x to solve for [H + ] Then use pH = -log[H + ]

Calculations involving pH What is pH if [OH - ] = 3.2 E -5 ? 2. pOH = - log[OH - ] –If K w = 1E -14, then pK w = pH + pOH = pK w

The pH scale If the pH < 7, the solution is acidic If the pH > 7, the solution is basic If the pH = 7, the solution is neutral See fig 16.5 p. 602

Practice Calculate the pH of lemon juice that has a [H + ] = 3.8 E -4 M

How do you undo a logarithm? If pH = -log [H + ] Then [H + ] = 10 -pH

Practice What is the [H + ] of a solution that has a pH of What is the [OH - ] of each solution?

Practice What is the pH of M HCl? What is the pH of M Ca(OH) 2 What is [HNO 3 ] if pH = 2.45

Quiz 1.What is the pH of a solution that has [OH - ] = 4.33 x 10 –6 M? 2.What is the [H + ] of a solution that has a pH of What is the pOH of a solution that has a [H + ] = x Identify all of the above as either acid/base

Weak Acids… Write the acid ionization expression for HCHO 2, formic acid, dissolving in water. This is an equilibrium expression. This gets the designation K a

HCHO 2  H + + CHO 2 - A student prepared a 0.10 M solution of formic acid and measured its pH. At 25 o C, it was found to be Calculate K a ICE Box

Niacin… A 0.20 M solution of niacin has a pH of what is the K a for niacin? ICE Box

Practice Calculate the percentage of HF molecules ionized in: 1.A 0.10 M HF solution 2.A 0.01 M HF solution

Weak Bases Substances that react with water to form a cation and OH - NH 3(aq) + HOH (l)  NH 4 + (aq) + OH - (aq) Write the K b expression.

Types of Weak Bases Amines: compounds that have a NH 2 component –i.e. methylamine CH 3 NH 2 Anions of weak acids –i.e. ClO 2 - from chlorous acid –Dissolve NaClO 2 in water…

K a and K b Write the appropriate K expression for NH 4 + (aq)  NH 3(aq) + H + (aq) Write the appropriate K expression for NH 3(aq) + HOH (l)  NH 4 + (aq) + OH - (aq)

Hess’ Law Add the previous equations together… what do you get? When two reactions are added to give a third reaction, the equilibrium constant for the third reaction is equal to the product of the equilibrium constants for the two added reactions.

Then… The product of the acid-dissociation constant, K a, for an acid and the base- dissociation constant, K b for its conjugate base is the ion-product constant for water. So… K a x K b = K w

Application… K a x K b = K w = 1.00 E -14 If you know one, you can solve the other. Can be modified… take the ‘p’ of each and you get… pK a + pK b = pK w = 14.00

Practice What is the value for Ka for NH 4 + ? For NH 3, Kb= 1.8 E -5 The equation is: NH 3(aq) + HOH (l)  NH 4 + (aq) + OH - (aq)

Salts Ions can exhibit acid/base behavior when they react with water Only anions/cations of weak acids/bases

Basic Solutions Generally, anions, X-, from weak acids behave as follows in water, X - + HOH  HX + OH -

Application/Practice Predict whether NaHPO 4 will form a basic or acidic solution when it dissolves in water. Write two equations… Find K a and K b

Rules 1.Salts derived from a strong base and a strong acid give pH = 7. Neither the anion or cation react with water. 2.Salts derived from a strong base and a weak acid. Anion from acid hydrolyzes in water. pH above 7 1.i.e. NaClO 2.Ba(C 2 H 3 O 2 ) 2

Rules 3.Salts derived from a weak base and a strong acid. The cation is a strong conjugate acid. pH below 7 i.e. NH 4 Cl i.e. Al(NO 3 ) 3

Rules 4. Salts derived from a weak base and a weak acid. Both cation and anion hydrolyze. pH depends on the value of K a or K b. i.e. NH 4 CN – find appropriate K a and K b and compare.

Practice Will the following salts give acidic, basic, or neutral solutions? 1.NaNO 3 2.KBrO 3.CH 3 NH 3 Cl 4.NH 4 NO 3