Chapter 15
Electrolytes, Acids and Bases
Acid (Latin acidus - sour): sour
taste; turns plant dye litmus red;
dissolves metals producing H2 gas.
Substances that are neither acids nor
bases were called neutral substances.
Electrolytes
Nonelectrolytes
Acetone (C3H6O)
Vinegar (HC2H3OH)
Carbon monoxide
Hydrogen chloride (HCl)
Ethanol (C2H5OH)
Baking soda (NaHCO3)
Table salt (NaCl)
Methane (CH4)
Milk of magnesia (Mg(OH)2) Sucrose (C12H22O11)
Sulfuric acid (H2SO4)
Turpentine (C10H22)
Base: bitter taste; turns plant dye
litmus blue; aqueous solutions feel
slippery to touch.
Electrolyte conducts electric current,
non-electrolyte does not.
Michael Faraday
(early 1800)
proposed that
some mobile
charged particles
must exist in
solution: ions.
Svante Arhenius hypothesized that ions ‘Radical, ridiculous’ Ph.D.
come from the dissociation of the solute. thesis, Nobel prize later.
An electrolyte is a solute that dissolves in A nonelectrolyte is a solute that dissolves
water and dissociates into ions, yielding in water without producing ions. The
a solution that conducts electricity.
solution consists of intact solute molecules.
Ionic compounds are usually metal plus
Water soluble molecular substances
nonmetal or group of nonmetals.
usually consist entirely of nonmetals.
Exceptions: HX (X-halogen) - polar covalent,
Quick quiz: find electrolytes from the list:
produce acids in H2O; ammonium salts.
Al(NO3)3, (CH3)2O, (NH4)2SO4, CH3OH, HBr
Weak and Strong Electrolytes
HCl and HF are both hydrogen halides.
A strong electrolyte is one that
completely dissociates into ions
upon dissolving in water.
A weak electrolyte is one that only
partially dissociates into ions upon
dissolving in water.
HCl(g)
Dissolves in H2O
H+(aq) + Cl-(aq)
100% dissociation
All HCl molecules dissociate, no HCl
molecules – light bulb shines strongly.
HF(g)
Dissolves in H2O
H+(aq) + F-(aq)
Far more undissociated HF molecules
than ions – light bulb barely lights.
Arhenius defined acid as an
electrolyte that produces H+
ions when dissolved in water.
In fact, H+ are always hydrated with
a water molecule, it is more precise
to say that the ions are H3O+ ions.
A strong acid is a water-soluble
compound that dissociates
extensively to produce a large
Strong acids
#H3O+
Weak acids
+
Hydrochloric (HCl) 1 1
Hydrofluoric (HF)
number of H3O (hydronium) ions.
Hydrobromic (HBr) 1 1
Hypochlorous (HClO)
Depending on the maximum number
Hydroiodic (HI)
1 1
Acetic (HC2H3O2)
of H3O+ ions that can be produced
1 2
Carbonic (H2CO3)
Nitric (HNO3)
from 1 mol of acid, it is referred to as Sulfuric (H SO
2 3
Phosphoric (H3PO4)
2
4)
monoprotic, diprotic or triprotic
(1, 2 or 3 moles of H3O+). Polyprotic acids are not always stronger than monoprotic.
Dissociation constant, Keq,
shows how many protons can
be obtained from 1 mol of acid.
Weak acid
H3PO4 + H2O
H3O+ + H2PO4-
Keq = 7.5 x 10-3
H2PO4- + H2O
H3O+ + HPO42-
Keq = 6.2 x 10-8
H3O+ + PO43-
Keq = 4.2 x 10-13
Strong acid
H3O+ + HSO4-
Keq > 1.0 x 103
H3O+ + SO42-
Keq = 1.2 x 10-2
Phosphoric acid is considered a
HPO42- + H2O
weak acid because even the first
dissociation step is weak.
The 3rd dissociation step is the weakest. H2SO4 + H2O
Sulfuric acid is considered strong acid
HSO4- + H2O
because of the first dissociation step.
The second dissociation step is weak.
Bases are opposite of acids. When Arhenius’ definition: Any electrolyte that contains
a metal ion and hydroxide group and produces
added in a proper amount to an
acidic solution, the acidic properties hydroxide (OH-) ions when dissolved in water.
are destroyed; neutralization
reaction produces salt and water.
All bases given here are strong
and completely dissociated.
Bases can be monobasic,
Lithium hydroxide, LiOH
di- and tri- basic.
Potassium hydroxide, KOH
Calcium hydroxide, Ca(OH)
Sodium hydroxide, NaOH
Magnesium hydroxide, Mg(OH)2
Barium hydroxide, Ba(OH)2
2
Quick quiz:
What do you get when you add
2 HNO3(aq) + Ba(OH)2(aq) 2 H2O(l) + Ba(NO3)2(aq)
HNO3(aq) to Ba(OH)2(aq)?
Arhenius theory cannot explain
basicity of NH3 as it has no OH- ions.
Brønsted and Lowry independently
explained it, defining a base as a
substance that removes H3O+.
To accept a proton (H+) from the
hydronium ion, the base must have a
free electron pair. When a proton
leaves H3O+ ion, H2O stays behind.
Base - anything that accepts a proton.
Acid - anything that donates a proton.
This new theory can explain acetyleneamide reaction, and formation of
ammonium chloride in gas phase.
Note that acetate and carbonate ions
come from dissociation of weak acids!
When a solution is acidic neutral basic
Parenthesis
mean:
concentration
[H3O+]
>
=
<
Weak bases
Ammonia, NH3
Acetate ion, C2H3O2Carbonate ion, CO32-
[OH-]
H2O + H2O
Keq = 1.8 x 10-5
Keq = 5.6 x 10-10
Keq = 2.1 x 10-4
H3O+ + OH-
[H3O+] x [OH-] K = K x [H O]2 = [H O+] x [OH-]
w
eq
2
3
Water also dissociates by the Keq = [H O] x [H O]
2
2
constant
process called auto-ionization.
pH Scale
Pure water is neutral because autoionization produces equal
concentrations of H3O+ and OH- ions:
[H3O+] = [OH-] = 1 x 10-7 M
Acid solution: [H3O+] > 1 x 10-7 M.
Basic solution: [OH-] > 1 x 10-7 M.
When [H3O+] rises, [OH-] lowers to satisfy
the ionic product of water, Kw = 1 x 10-14.
Performing calculations with so small
concentrations is tedious. Instead,
scientists use logarithms.
pH = - log [H3O+]
Taking a logarithm is asking “to what power
must I raise 10 to get the displayed number?”
Example: log 558 = 2.75, because 102.75 = 558.
Note that each onefold decrease in pH
represents tenfold increase in acidity.
To convert from pH to molar H3O+
concentration, use the formula:
[H3O+] = 10-(pH)
Kw = Keq x [H2O]2 = [H3O+] x [OH-]
Kw = [H3O] x [OH-] = [1 x 10-7] x [1 x 10-7]
Kw = 1 x 10-14
Reactions of Acids and Bases
acid + metal hydrogen + ionic salt
2 HCl(aq) + Ca(s) H2(g) + CaCl2(aq)
acid + base salt + water
HBr(aq) + KOH(aq) KBr(aq) + H2O(l)
acid + metal oxide salt + water
2 HCl(aq) + Na2O(aq) 2 NaCl(aq) + H2O(l)
acid + carbonate salt + H2O + CO2
2 HCl(aq) + Na2CO3(aq) 2 NaCl(aq)
+ H2O(l) + CO2(g)
2 KOH(aq) + 2 Al(s) + 6 H2O(l) 2 KAl(OH)4(aq)
amphoteric hydroxides are capable of + 3 H2(g)
reacting as a base or an acid
Reactions of amphoteric hydroxides
Zn(OH)2(s) + 2 HCl(aq) ZnCl2(aq) + 2 H2O(l)
Zn(OH)2(s) + 2 NaOH(aq) Na2Zn(OH)4(aq)
Reactions with certain metals
2 NaOH(aq) + Zn(s) + H2O(l)
Na2Zn(OH)4(aq) + H2(g)
Titrations
Stoichiometric
procedure to
determine solution
concentrations.
Done by acid / base
neutralization.
Needed:
1. buret with known
conc. of titrant
2. Known volume of
the titrate in the
Erlenmeyer flask
3. Indicator (usually
phenolphtalein).
Fill the buret with the titrant
to 0.00 mL mark, drain it by
opening the stopcock. Stop
when indicator changes
color and read the final
volume of the titrant.
Balanced eq.: NaOH(aq) + HCl(aq) H2O + NaCl(aq)
moles of (known) base = moles of (unknown) acid Mbase x Vbase = Macid x Vacid
0.100 mol NaOH 1 mol HCl
1
x1 mol NaOH x
MHCl = 0.02250 L NaOH x 1L NaOH
0.05000 L HCl
Writing Net Ionic Equations
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) Na+(aq) + Cl-(aq) + H2O(l)
spectator ions cancel
H+(aq) + OH-(aq) H2O(l)
Formula eq.
Total ionic eq.
Net ionic eq.
All three equations must have balanced number of atoms and electrical charges.
Strong electrolytes are written in their ionic form.
Weak electrolytes, nonelectrolytes, precipitates and gases are written in their molecular form.
Net ionic equation includes only substances that have undergone a chemical change.
2 AgNO3(aq) + BaCl2(aq) Ba(NO3)2(aq) + 2 AgCl(s)
2 Ag+(aq) + 2 NO3-(aq) + Ba2+(aq) + 2 Cl-(aq) Ba2+(aq) + 2 NO3-(aq) + 2 AgCl(s)
AgCl precipitates, Ba2+ and NO3- are spectator ions.
Ag+(aq) + Cl-(aq) AgCl(s)
Try H2SO4 + Ba(OH)2; Mg + HCl, Na2CO3 + H2SO4.
Colloidal Dispersions
Colloids are particles that are intermediate between true
solution and suspension (e.g. fine sand in water, which settles
down once the shaking stops). Colloids (e.g. milk) neither settle
down, nor form a true solution; the colloidal particle sizes are
between true solute ions or molecules, and particles of
mechanical suspension (i.e. between 10-4 and 10-7 cm) with a
diameter of ~ 500 nm. Thus, colloidal particle is 1000 times
larger in diameter, and about 109 times larger in volume.
Ba2+(aq) + SO42-(aq) BaSO4(s)
H+(aq) + OH-(aq) H2O(l);
2 H+(aq) H2(g);
2H+(aq) + CO32-(aq) H2CO3(aq)
H2CO3(aq) H2O(l) + CO2(g)
Colloidal particles can be
removed by dialysis (as in
artificial kidneys). Colloidal
dispersion shows Tyndal effect
(light scatter).
Resisting pH Changes – Buffer Solutions
A buffer is a solution of a weak acid
and its conjugate base. Addition of
a strong acid or base to a buffer
changes pH only slightly.
Human blood (pH = 7.4) may not change
by more than 0.4 pH units. Gastric juice
has pH as low as 1.3. Only a fraction of a
drop of gastric juice would change the pH
of blood enough to kill you, if it were not
buffered by HCO3- / CO32- system.
Each acid has a conjugate base,
and each base has conjugate acid.
The difference is in one proton.
A buffer replaces added strong
acid with its weak acid, or added
strong base with its weak base.
OH- + HC2H3O2 C2H3O2- + H2O
Added
base
Weak acid
in buffer
Conjugate
weak base
Neutral
water
H3O+ + C2H3O2- HC2H3O2 + H2O
Added
acid
Weak base
in buffer
Conjugate
weak acid
Neutral
water
Weak acid conjugate has all properties of a
base. Strong acid conjugate is neutral.
In acetate buffer, the weak
acid, HC2H3O2 goes after
any added OH- ions.
Simultaneously, the acid
conjugate, C2H3O2- goes
after any added H3O+.
No buffer entirely cancels out
the effect of adding strong acid
or base. pH still changes, but a
lot less than in water.
Buffer is exhausted when either
weak acid or its conjugate gets used up.
HW (p.377) 1, 3, 5(a-c), 13, 15, 33
1. Identify the conjugate acid=base pairts in each of the following equations:
NH4+ + OHHC2H3O2 + H2O
C2H3O2- + H3O+
NH3 + H2O
H2PO4- + OHHPO42- + H2O
HCl + H2O
Cl- + H3O+
3. Complete and balance these equations: Zn(s) + HCl(aq) Na2CO3(aq) + HC2H3O2(aq) Al(OH)3(s) + H2SO4(aq) KOH(aq) + H3PO4(aq) Ca(HCO3)2(s) + HBr(aq) 5. For the first three of the formula equations in Q.3, write total and net ionic eq.
13. Calculate the [H+] for:
black coffee, pH = 5.0
a solution with a pH of 8.5;
a solution with a pH of 1.2
15. Determine the molar concentrations of each ion present in the solutions that result
from each of the following mixtures (disregard the concentration of H+ and OHfrom water and assume volumes are additive):
55.5 mL of 0.50 M HCl and 75.0 mL of 1.25 M HCl
125 mL of 0.75 M CaCl2 and 125 mL of 0.25 M CaCl2
35.0 mL of 0.333 M NaOH and 22.5 mL of 0.250 M HCl
12.5 mL of 0.500 M H2SO4 and 23.5 mL of 0.175 M NaOH
33. Determine whether each of the following is a strong aicid, weak acid, strong base
or weak base. Then write an equation describing the process that occurs when the
substance is dissolved in water: NH3; HCl; KOH; HC2H3O2.