Acids & Bases

Substances that affect the pH of solutions.

What’s wrong in this picture?

Acids & Bases typically are, or behave as, IONIC compounds.

• Acids:• Are corrosive• Taste sour• React with

indicators• Neutralize bases

• Ex.

HCl (hydrochloric acid), H2SO4 (sulfuric acid)

Bases: Are slippery Taste bitter React with indicators Neutralize acidsEx.

NaOH (sodium hydroxide),

NH4OH (ammonium hydroxide)

Baking soda (NaHCO3)

Litmus is a vegetable dye obtained from certain lichens found principally in the Netherlands.

The difference between the aqueous solution processes of ionization and dissociation.

Typical with Acids Typical with Bases

NomenclatureAcids Most are

“hydrogen” bonded with an anion

Examples: HNO3 (nitric acid)

HC2H3O2 (acetic acid)

HCl (hydrochloric acid)

Bases Most are metal

hydroxidesExamples: NaOH (sodium hydroxide)

KOH (potassium hydroxide)

Ba(OH)2 (barium hydroxide)

NH4OH* (ammonium

hydroxide)

Definitions

AcidsArrhenius - acids donate H+ (in soln)

Bronsted-Lowery -acids donate H+ (in soln)

BasesArrhenius - bases donate OH- (in soln)

Bronsted-Lowery - bases accept H+ (in soln)

Coordinate covalent bond

Conjugate Acid-Base Pairs

The transfer of protons illustrates the characteristics of conjugate pairsHNO2 + H2O <==> H3O+ + NO2

-

NO2- is the conjugate base of HNO2

H3O+ is the conjugate acid of H2O

Protocity

Monoprotic HCl, HNO3

Diprotic H2CO3

Triprotic H3PO4

Acids can be classified according to the number of hydrogen ions (protons) they can transfer per molecule during an acid-base reaction.

Acid-Base Strength(You can dilute an acid or a base but you can’t change its strength)

Strong

“ions” completely dissociate in water

ACIDS:

HCl, HBr, HI,

HClO4, H2SO4, HNO3

BASES:

LiOH, NaOH, KOH,

Ca(OH)2, Sr(OH)2, Ba(OH)2

Weak “ions” partially

dissociate in waterAll non-strong acids & basesThese exist as equilibrium systems in solution, thus, their “weakness” exists within a range defined by their Keq values.

A comparison of the number of acidic species present in strong acid and weak acid solutions of the same concentration.

Weak A/B equilibrium

Two reactions (forward & reverse) occur at the same rate

HA <==>H+ + A-

BOH <=> B+ + OH-

Equilibrium expressions are ways to show the mathematical relationships

Keq = [Products]n

[Reactants]m

n & m are the coefficients of each substance

Ionization Constants for Acids & Bases

HA(aq) + H2O(l) <==> H3O+(aq) + A-(aq)

Ka = ---------------------

B(aq) + H2O(l) <==> BH+(aq) + OH-(aq)

Kb = ---------------------

Neutralization reactions- a special type of DR rxn

AX + BY --> AY + BX

HCl + KOH --> HOH + KCl

Acid + Base --> Water + Salt

To balance these rxns. Balance the H in the acid with the OH in the base :)!For a complete reaction, stoichiometric equivalents of the acid and base must be used.

Neutralization equations

HCl(aq) + NaOH(aq) --> H2O(l) + NaCl(aq)

H2SO4 + Ba(OH)2 -->

H3PO4 + KOH -->

HNO3 + Al(OH)3 -->

The acid-base reaction between sulfuric acid and barium hydroxide produces the insoluble salt barium sulfate.

Calculations

A sample of 0.0084 mol HCl is dissolved in water to make 1500 mL solution. Calculate the molarity of the HCl solution and the [H3O+].

Self-Ionization of Water (pH is a derivative of this concept)

Water molecules can break apart when they collide

H2O(l) <==> H+(aq) + OH-(aq)

Kw = ----------------

Kw = 1.0 x 10-14 M2

Adding an acid or a base changes the relative amounts of [H+] and [OH-] but not the value of Kw.

Ionic ConcentrationIf [H+] = [OH-] the solution is neutral

If [H+] > [OH-] the solution is acidic

If [H+] < [OH-] the solution is basic

[H+] x [OH-] = 1.0 x 10-14M2

The relationship between H3O+ and OH- in aqueous solution is an inverse proportion.

Calculations

If the [OH-] = 3.5 x 10-3 M, what is [H+]?

pH: a logarithmic scale of a solution’s hydrogen (hydronium) ion concentration (molarity)

This is a way to express the relative acidity/basicity of a solution.

pH = -log[H+] Therefore, each difference in pH of 1.0 is

equivalent to a concentration change by a factor of 10

High [H+] causes low pH Low [H+] causes high pH Therefore, strong acids have lower pH!

pOH = -log[OH-]

pH scale

0 - 14 is the usual range pH < 7 = acid pH > 7 = base pH = 7 = neutral

pH + pOH = 14

Calculations

If the [H+] = 3.35 x 10-5 M, what is the pH of the solution? On your calculator:

- log (3.35 x 10-5) =

Calculations

If the [OH-] = 2.8 x 10-4M, what is the pH of the solution?

pH --> [H+] calculations

What is the [H+] for a solution with a pH = 3.92?pH = -log[H+]

3.92 = -log[H+]

-3.92 =log[H+]

10-3.92= [H+]

[H+] = 1.20 x 10-4M

Practice: determine the [H+] for the solutions with the following values.

[H+] = 2.82 x 10-8 M

[H+] = 3.98 x 10-11 M

[H+] = 7.86 x 10-3 M

[H+] = 3.16 x 10-10 M

pH = 7.55

pH = 10.4

pH = 2.12

pOH = 4.5

Salt HydrolysisSome aqueous salt solutions have the ability to split (hydrolyze) water and form compounds which result in larger [H+] or [OH-] in the solution.Example: Aluminum chloride

AlCl3(aq) --> Al+3(aq) + 3Cl-(aq) Cation of WB Anion of WA

Aluminum ion will react with OH- in solution: Remember: H2O <==> H+ + OH-

Al+3(aq) + H2O(l) <==> Al(OH)3(aq) + 3H+(aq) Chloride ion will NOT react with H+ in solution!

Rules for Determining pHStrength wins!

Strong Acid + Strong Base --> Neutral sol’n HCl + NaOH --> NaCl + H2O

Strong Acid + Weak Base --> Acidic sol’n HCl + Al(OH)3 --> AlCl3 + H2O

Weak Acid + Strong Base --> Basic sol’n H2S + NaOH --> Na2S + H2O

Weak Acid + Weak Base --> depends on the salt

HNO2 + NH4OH --> NH4NO2 + H2O

BuffersBuffers are solutions in which the pH remains relatively constant when small amounts of acid or base are added

Two active chemical species: A substance to react with & remove added base A substance to react with & remove added acid.

Buffers are solutions of a weak acid and one of its conjugate base OR a weak base and one of its conjugate base.

Carbonic acid and Sodium bicarbonate

H2CO3 <==> H+ + HCO3- Ka = 1.7 x 10-3

NaHCO3 --> Na+ + HCO3-

Buffering Action in Human Blood

H2CO3 <==> H+ + HCO3-

High concentration High concentration

Ratio: 1 : 10Add a base [OH-] and the equilibrium position shifts ; pH doesn’t change muchAdd an acid [H+] and the equilibrium position shifts ; pH doesn’t change muchReason: high [ ] of acid and anion can accommodate large shifts of EQ position.Lots of acid is

produced in the body

daily.

Buffer Systems

Titration

At the completion of a neutralization reaction (equivalence point) the

# moles acid = # moles base

So,

MaVa = MbVb

but, keep the reaction stoichiometry in mind.

Diagram showing setup for titration procedures.

Chemical Titration

This process can be done for any reaction in which a stoichiometric equivalence is reached and can be identified by an indicator

At the equivalence point an indicator will change color permanently.

Calculations

How many mL of 0.10M NaOH solution are needed to neutralize 15 mL of 0.20M H3PO4 solution?