Chemistry C.8 Acids and Bases
an acid can be defined as a substance that dissociates in water to form hydrogen ions (H+) and anions, while a base dissociates into hydroxide ions (OH-) and cations.
acids donate H+ while bases accept H+
Conjugate acid-base pairs
The acid and base pair in a reaction that differ by one proton so acids react to form bases and vice versa
Substances that act as acids and bases as a Bronsted-Lowry acid, must be able to dissociate and release H+. As a Bronsted-Lowry base, must be able to accept H+
acids accepts H+ while bases donates H+
most widely used indicators that change color reversibly according to the concentration of H+ ions in the solution
dye derived from lichens which turns pink in the presence of acid and blue in the presence of alkalis
formed by mixing several indicators and so changes many times across a range of different acids and alkalis
compound formed when the hydrogen of an acid is replaced by a metal or another positive ion
Acids react with _______, _________, and __________ to form salts.
metals, bases, and carbonates
describes the relationship between an acid and its salt
species which do not change during a reaction and can be cancelled out
Strength of an acid
a measure of how readily it disassociates in aqueous solution
a variable depending on the number of moles per unit volume, according to how much solute has bee added to the water
Common strong acids (3)
HCl, HNO3, H2SO4
Weak acids (4)
CH3COOH and other organic acids, anything ending in COOH, H2CO3, H3PO4
Strong base (5)
LiOH, NaOH, KOH, Ba(OH)2, G1 metals
Weak base (3)
NH3, C2H5NH2 and other amines, Ca(OH)2
measure of H+ concentration
pH = -log10[H+]
directly reads the [H+] through a special electrode
can record accuracy to several decimal points
Kc = [H+] [OH-] / [H2O]
the ionic product constant of water
at 25oC, Kw = 1.00 x 10-14
in pure water, [H+] ___ [OH-].
[H+] = [OH-]
reaction for the disassociation of water is ________
an increase in temperature will shift the equilibrium to the ________ and ________ the value of Kw. There will be a _________ in pH
right and increase. There will be a decrease in pH.
the relationship between H+ and OH- is ____________
Acid solutions are defined as those in which [H+] > [OH-]. What will the pH be?
pH < 7
Neutral solutions are defined as those in which [H+] = [OH-]. what will the pH be?
pH = 7
Alkaline solutions are defined as those in which [H+] < [OH-]. What will the pH be?
pH > 7
Ka = [H3O+] [A-] / [HA]
Ka is the acid disassociation constant. It has the fixed value for a particular acid at a specified temperature.
The higher the value of Ka at a particular temperature, the greater disassociation, and so the stronger the acid.
Kb = [BH+] [OH-] / [B]
Kb is the base disassociation constant. It has a fixed value for a particular base at a specified temperature
The higher the value of Kb at a particular temperature, the greater the ionization and hence the stronger the base
pKa or pKb
pKa = -logKa
Ka = 10^-pka
pKb = -logKb
Kb = 10^-pKb
pKa + pKb =
pKa + pKb = pKw
something that acts to reduce the impact of one thing on another
How do buffers work?
mixture of two solutions, composed in such a way that they each contain two species of a conjugate acid-base pair.
made by mixing an aqueous solution of a weak acid with a solution of its salt and a strong alkali
made by mixing an aqueous solution of a weak base with its salt and a strong acid
buffer solutions are a mixture containing both an acid and a base of a weak conjugate pair. The buffer's acid neutralizes added alkali, and the buffer's base neutralizes added acid, and so pH change is resisted.
How to determine pH of a buffer solution
[H+] = Ka x [HA]initial / [MA]initial
MA = salt
pH = pKa + log [salt]/[acid]
Factors that influence buffers (2)
dilution and temperature
Effect of dilution on a buffer
diluting a buffer does not change the pH, but it does alter the amount of acid or base it can absorb without significant changes in pH (buffer capacity). This depends on the molar concentrations of its components, so decreases as they are lowered by dilution
The anion (A-) is a conjugate base of the parent acid. When the acid is weak, this acid is strong enough to hydrolyse water.
A- (aq) + H2O (l) → HA (aq) + OH-
the release of OH- causes the pH of the solution to increase.
The cation (M+) is a conjugate of the parent base. When the base is weak and his conjugate is non-metal, it is able to hydrolyse water
M+ (aq) + H2O (l) → MOH (aq) + H+ (aq)
the release of H+ causes the pH of the solution to decrease
metal ions which are able to hydrolyze water include: Be2+, Al3+, Fe3+
the overall pH of a salt solution depends on the relative hydrolysis of its anions and cations, and these can be deduced from the relative strengths of the parent acids and bases.
the calculation of the exact concentration of one solution when the other is known
The equivalence point
1. The equivalence point is where neutralization has occurred exactly, so the solution contains salt and water only
how do indicators signal change in pH
by applying Le Chatelier's principle, we can predict how this equilibrium will respond to a change in the pH of the medium
1. increasing [H+]: the equilibrium will shift to the left
2. decreasing [H+]: the equilibrium will shift to the right
change point/end point
when the equilibrium is balanced so the acid and its conjugate are equal
How indicators can be used to signal the equivalence point in titrations
1. determine what combination of weak and strong acid and base reacting together
2. deduce the pH of salt solution and equivalence
3. choose an indicator with an end point in the range of equivalence point by consulting data tables
if the equilibrium lies to the right, the acid has disassociated fully and it is a ________ acid
if the equilibrium lies to the right, the acid has disassociated fully and it is a strong acid
if the equilibrium lies to the left, the acid has only partially disassociated and it is a _____ acid
if the equilibrium lies to the left, the acid has only partially disassociated and it is a weak acid