Brønsted-Lowry Acids and Bases
Brønsted-Lowry acid is defined as a compound that donates a proton to another compound and known as a proton donor
Brønsted-Lowry base is the compound that accepts a proton from another compound and known as a proton acceptor
In the chemical equation below identify the Brønsted-Lowry acid.
C6H5OH + NH2− → C6H5O− + NH3
C6H5OH is losing an H+ ion thus donating a proton. Based on the definition above it is a Brønsted-Lowry acid. NH2− is accepting the H+ therefore it is a proton acceptor and can be concluded as a base using the definition for Brønsted-Lowry base.
Amphiprotic, or amphoteric species: Species that can either accept or donate protons.An example is shown in the figure below where bicarbonate and water can either be proton donor (acid) or acceptor (base), thus they both are amphiprotic.
The pH Scale
pH refers to the power of the hydronium ion which measured through the concentration of (hydronium) H3O+ ions. In other words pH scale helps to measure how acidic or basic a solution is. Lower the value of pH the more acidic a solution becomes. For example a solution with pH two is more acidic than a solution with pH of four. Neutral solution such as water (at 25 °C) has a pH value of seven, and basic solution has a pH that's greater than seven.
pH is calculated using the following formula:
pH = -log [ H3O+ ]
If the pH of a solution is given then the hydronium ion concentration can be calculated using the following formula:
[ H3O+ ] = 10-pH
Find the pH of pure water if it has the hydronium ion concentration of 1 X 10-7 M ?
[ H3O+ ] = 1 X 10-7 M
pH = -log [ 1 X 10-7 ] = -(-7.00) = 7.00
Pure water is a neutral solution therefore its pH is 7 at 25 °C.
Calculate the hydronium ion concentration of a solution if it has a pH of 7.3
[ H3O+ ] = 10-pH
[ H3O+ ] = 10-7.3
[ H3O+ ] = 5.0 x 10-8 M
The pOH Scale
Similarly the hydroxide ion concentration of a solution can be expressed using the pOH scale.
The following formula is used to calculate the pOH:
pOH = -log [ OH- ]
If the pOH of a solution is given then the hydroxide ion concentration can be calculated using the following formula:
[ OH- ] = 10-pOH
Calculate the pOH of a solution that has the hydroxide ion concentration of 1.25 x 10-6 M.
pOH = -log [ OH- ] = pOH = -log [ 1.25 x 10-6] = -(5.90) = 5.90
Calculate the hydroxide ion concentration of a solution that has a pOH of 4.37
[ OH- ] = 10-pOH =[ OH- ] = 10-4.37 = 4.27 x 10-5 M
The Relationship Between pH and pOH
At 25 °C a water solution's pH value and pOH value are related as shown below:
pH + pOH = 14
Calculate the pH of a solution that has a pOH of 11.75
pH =14 - pOH = 14 - 11.75 = 2.25
Figure 1: The pH scale. From Vecteezy.com (n.d.). Retrieved April 13, 2021 from https://www.vecteezy.com/free-vector/ph-scale
Relative Strengths of Acids and Bases
Relative strengths of acids and bases characterize how well an acid or base ionizes when dissolved in water. It can be quantized using the acid (Ka), base (Kb) ionization constants, or percent ionization.
Strong acids and bases: Strong acids or bases have 100% ionization in water
Examples of strong acids:
Examples of strong bases:
Weak acids and bases: They do not dissociate completely in the solution therefore the reaction is usually written as a reversible reaction
Conjugate base: Conjugate base is what remains after an acid donates a proton (H+) because it can now act as a proton acceptor (base) in the reverse reaction.
Conjugate acid: When a base accepts an H+ it becomes a conjugate acid because it can now donate a proton (H+)
Figure 2: Conjugate acid-base pairs. From LibreTexts (2019, August 22), Buffers and conjugate acid-base pairs. Chemistry. https://chem.libretexts.org/Courses/Sacramento_City_College/SCC%3A_CHEM_330_-_Adventures_in_Chemistry_(Alviar-Agnew)/07%3A_Acids_and_Bases/7.07%3A_Buffers_and_Conjugate_Acid-Base_Pairs
Acid ionization constant
The acid ionization constant can be calculated using the following equation for a reaction of acid HA.
HA(aq) + H2O(l) ⇌ H3O+(aq) + A−(aq)
Base ionization constant
The base ionization constant can be calculated using the following equation for a reaction of base HB.
B(aq) + H2O(l) ⇌ HB+(aq) + OH−(aq)
The ratio of the concentration of ionized weak acid to its initial acid concentration is known as percent ionization. It's another way of measuring the strength of weak acids and calculated using the following equation.
Mathematical product of conjugate acid-base pair is equal to Kw
Adding the above two equation gives the following:
Photo credit: Openstax
With that the Kw becomes:
Photo credit: Openstax
There are two different types of acids based on the number of protons they can give up:
Monoprotic acid: An acid that can only donate one proton.
Example: HCl, HNO3, HCN
Diprotic acid: An acid that consists of two hydrogen atoms and can donate two protons (hydrogen atoms).
First ionization: H2SO4(aq) + H2O(l) ⇌ H3O+(aq) + HSO4-(aq) Ka1 = more than 102
Second ionization: HSO4-(aq) H2O(l) ⇌ H3O+(aq) + SO42-(aq) Ka2 = 1.2 X 10-2
Triprotic acid: This one contains three hydrogen atoms and can donate three protons.
First ionization: H3PO4(aq) + H2O(l) ⇌ H3O+(aq) + H2PO4-(aq) Ka1 = 7.5 × 10−3
Second ionization: H2PO4-(aq) + H2O(l) ⇌ H3O+(aq) + HPO42−(aq) Ka2 = 6.2 × 10−8
Third ionization: HPO42−(aq) + H2O(l) ⇌ H3O+(aq) + PO43−(aq) Ka3 = 4.2 × 10−13