300 Chemistry
Acids and Bases
Intro to Acids and Bases
-2 groups of substances with distinctive properties
-An application of equilibrium concepts
Operational Definitions
Acids…
Bases…
Have a low pH
Have a high pH
Taste sour
Taste bitter
React with metals to produce a salt and
--hydrogen gas
React with bases to produce a salt and
React with acids to produce a salt and
water (neutralization reaction)
water (neutralization reaction)
Are electrolytes, conduct electricity
Are electrolytes, conduct electricity
Turn blue litmus paper red
Turn red litmus paper blue
--Feel slippery
Arrhenius Definition
Acid = a substance that produces H+ ions in aqueous solution
Ex: HCl → H+ + ClNote: a monoprotic acid only has 1 acidic proton (H+), diprotic has 2… etc.
Base = a substance that produces OH- ions in aqueous solution
Ex: NaOH → Na+ + OHBronsted-Lowry Definition
Acid = proton (H+) donor
Base = proton (H+) acceptor
Conjugate acid = what the base becomes after it accepts a proton
Conjugate base = what the acid becomes after it donates a proton
Ex: NH3 + H2O → NH4+ + OHAmmonia is the Bronsted-Lowry base (it will accept a proton)
Water is the Bronsted-Lowry acid (it will donate a proton)
Ammonium ion is the conjugate acid of ammonia (it has accepted a proton)
Hydroxide ion is the conjugate base of water (it has donated a proton)
Lewis Definition (very little on this one)
Acid = electron pair acceptor
Base = electron pair donor
Amphoteric substances
Amphoteric substance = amphiprotic substance = substance that can act as an acid or a base
(ex: water as it reacts with HCl is a base, as it reacts with NH3 is an acid)
Strength of Acids and Bases
Strong acids and bases are strong electrolytes, weak acids and bases are weak electrolytes
Recall…. (and memorize if you didn’t earlier)…
The six strong acids are: HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO3
(nitric), H2SO4 (sulfuric), and HClO4 (perchloric)
If it’s not a strong acid, it’s a weak acid!
Common weak ones you’ll see are HF (hydrofluoric), HC2H3O2 (acetic), H2CO3 (carbonic),
and H3PO4 (phosphoric)
The strong bases are: any alkali metal hydroxides (LiOH, NaOH, KOH, RbOH, etc.),
Ca(OH)2 (calcium hydroxide), Ba(OH)2 (barium hydroxide), and Sr(OH)2 (strontium
hydroxide)
If it’s not a strong base, it’s a weak base!
The most common weak one you’ll see if NH3 (ammonia)
Determining pH
For strong acids and bases, it is easy to determine their strength, using pH
For weak acids and bases, it is a bit more complex to determine their strength and pH (we’re
not going to worry about those)
Acid and Base Strength for Strong Acids and Bases
Strong acids and bases dissociate 100% (equilibrium position is far to the right, reaction goes
nearly completely to the products (ions))
So, the concentration of the “reactants” (undissociated acid or base) approach ZERO and K eq
or Ka (acid dissociation constant) approaches INFINITY
Strong acids have very large (infinite) Ka’s (weak acids have very low Ka’s)
Ex: HCl → H+ + Cl6 M HCl produces 6 M [H+] and 6 M [Cl-], but [HCl] will approach 0 because pretty much all
the HCl is broken up into the ions
Ka = [H+] [Cl-] / [HCl] = 36/0 = infinity!
This simplifies things for us and lets us determine the pH easily
pH = scale used to measure how acidic or basic a solution is
Ranges from 0 to 14 (below 7 is acidic, above 7 is basic)
(chart which we will add to as we go)
For all aqueous solutions water also dissociates a little: H2O ↔ H+ + OHWe can write Keq = Kw = [H+] x [OH-] = 1.0 x 10-14
[It sounds strange… but this means even in a basic solution, there are hydrogen ions, and
even in an acidic solution, there are hydroxide ions… they come from the water]
pH below 7 indicates an acidic solution ([H+] > [OH-])
pH of 7 indicates a neutral solution (not an acid or a base) ([H+] = [OH-] = 1.0x10-7)
pH above 7 indicates a basic (alkaline) solution ([OH-] > [H+])
Use the “pH loop” to solve problems… the equations are:
pH = -log[H+] pOH = -log[OH-]
pH + pOH = 14
[H+] = 10-pH [OH-] = 10-pOH
[H+] x [OH-] = 1.0 x 10-14
Note: The number of decimal places in your pH is equal to the number of sig figs in your
concentration!
(examples)
Neutralization and Hydrolysis
In a neutralization reaction, an acid and base react to produce some salt and water… at what
is called the “equivalence point”, we can say that:
Moles of H+ = Moles of OHFor a monoprotic acid and a base with one hydroxide, use the following equation:
Ma x Va = Mb x Vb
Note: Be careful with polyprotic acids and some bases, use stoichiometry of the balanced
equation for the reaction
(examples)
In general, an acid and a base will react to form a salt and water….but that salt is not always
neutral (can also be acidic or basic)…
Specifically:
Strong Acid + Strong Base = Neutral Salt + Water
(A solution of a salt that comes from a strong acid anion and a strong base cation will be
neutral)
Strong Acid + Weak Base = Acidic Salt + Water
(A solution of a salt that comes from a strong acid anion and a weak base cation will be
acidic)
Weak Acid + Strong Base = Basic Salt + Water
(A solution of a salt that comes from a weak acid anion and a strong base cation will be
basic)
In a hydrolysis reaction, a substance reacts with water to remove hydrogen ions from or
donate hydrogen ions to water… the resulting solutions may either be acidic or basic
Titrations and Indicators
Titration = a process used to determine the amount (concentration) of acid or base in a
solution
Indicator = a substance that marks the equivalence point of a titration (or identifies a pH
range) by changing color
Common indicators: litmus paper (red in acid, blue in base), phenolphthalein (clear in acid,
pink in base), and universal indicator (a rainbow of colors depending on the pH)
pH meter = device used to make rapid, accurate pH measurements
Titration curve = a graph used to show how the pH of the unknown solution changes as
various amounts of titrant are added
How they are used:
-A solution of known concentration (the titrant) is delivered from a buret into an unknown
solution containing an indicator
-At the endpoint of the titration, when the unknown substance is just consumed, the
stoichiometric (equivalence) point is reached, and the indicator changes color
-Then, the amount of titrant added can be used to determine the concentration of the
unknown