Acids and Bases_2025.pdf engineering chemistry

this is the engineering chemistry 2a notes on acids and bases

1. Lewis Acid-Base Reactions and
Acidity/Basicity Trends
Compiled by: H Lubanyana, 2025

2. What is a Lewis acid
Lewis says:
A Lewis acid is an electron pair acceptor
A Lewis base is an electron pair doner
Arrhenius says:
Acid is a compound that increases the concentration of
hydrogen ion (H +) in aqueous solution
A base is a compound that increases the concentration of
hydroxide ion (OH −) in aqueous solution
Lowry- Bronsted definition:
An acid is a proton donor and a base is a proton acceptor
Chapter 3 2

3. Arrhenius acids and bases
Chapter 3 3
Examples of Arrhenius Acids:
1.Hydrochloric acid (HCl) : HCl → H⁺ + Cl⁻
2.Sulfuric acid (H₂SO₄) : H₂SO₄ → 2H⁺ + SO₄²⁻
3.Nitric acid (HNO₃) : HNO₃ → H⁺ + NO₃⁻
4.Perchloric acid (HClO₄) : HClO₄ → H⁺ + ClO₄⁻
5.Acetic acid (CH₃COOH) : CH₃COOH ⇌ H⁺ + CH₃COO⁻ (Weak
acid, partially ionizes)
Examples of Arrhenius Bases:
1.Sodium hydroxide (NaOH) : NaOH → Na⁺ + OH⁻
2.Potassium hydroxide (KOH) :KOH → K⁺ + OH⁻
3.Calcium hydroxide (Ca(OH)₂) : Ca(OH)₂ → Ca²⁺ + 2OH⁻
4.Ammonium hydroxide (NH₄OH) : NH₄OH ⇌ NH₄⁺ + OH⁻ (Weak
base, partially ionizes)
5.Lithium hydroxide (LiOH) : LiOH → Li⁺ + OH⁻

4. Acidity strength
Acids and Bases 4

5. How to Determine the Strength of an
Acid
The strength of an acid depends on how completely it ionizes in
water. Strong acids fully dissociate into ions, while weak acids only
partially dissociate. Here’s how you can determine the strength of
an Arrhenius acid:
1. Degree of ionisation
Strong acids ionize completely in water (100% dissociation).
Example:HCl → H
+
+ Cl
−
Weak acids ionize partially in water
Example:
CH
3COOH ⇌ H
+
+ CH
3COO
−
The double arrow (⇌) shows that equilibrium is established,
meaning not all the acid molecules ionize.
Chapter 3 5

6. 2. Acid Dissociation Constant (Ka)
Ka measures how much the acid ionizes in water.
Higher Ka = Stronger Acid (more dissociation)
Lower Ka = Weaker Acid (less dissociation)
pKa is the negative logarithm of Ka.
pKa = −log(Ka​)
Lower pKa = Stronger Acid
Chapter 3 6

7. Example 1
The reaction below went to equilibrium in solution X. It was found that, at
equilibrium, the concentration of [HF] was 0.250M, [H
3O
+
] was 0.0134M,
and [F
-
] was 0.0134M. What is the pKa of this reaction?

HF
(aq) + H
2O
(l)  H
3O
+
(aq)
+ F
-
(aq)
Chapter 3 7
Ka =
[H3O+]x[F−]
[HF]
√
=
(0.0134 ???????????? 0.0134)
0.250

= 7.18 x 10
-4
√

pKa = -LogKa
= -Log (7.18 x 10
-4
)
= 3.144 √

8. Example 2
Lactic acid (HC3H5O3) has a Ka value of 1.4 x10
-4
and hydrazidic acid (HN3) has a Ka value
of 1.9 x10
-5
. Calculate the pKa value for each acid and determine which acid produces a
weak conjugate base.
Chapter 3 8
pKa = - Log Ka
= -Log (1.4 x10
-4
)
= 3.85 (Lactic acid) √

Lactic acid will produce a weak conjugate base √
pKa = - Log Ka
= -Log (1.9 x10
-5
)
= 4.72 √

9. 3. Electronegativity of the Conjugate
Base
The more stable the conjugate base (A⁻), the stronger the
acid.
Example: HCl is stronger than HF because Cl⁻ is larger and
better stabilizes the negative charge than F⁻ .
Example:
Chapter 3 9

10. 4. Bond Strength (H–X Bond Strength in
Binary Acids HX)
Weaker bonds → Stronger acids (easier to donate H⁺).
Acid strength increases down a group in the periodic table:
HI > HBr > HCl > HF
Iodine has a weaker bond to H than Fluorine, so HI is the
strongest).
Chapter 3 10

11. 5. Oxoacids (H-O-Y Type Acids, e.g.,
HNO₃, H₂SO₄)
If Y is highly electronegative, it pulls electron density
away, making it easier to lose H⁺. More oxygen atoms
increase acidity because they stabilize the conjugate
base by resonance.
Example:
HClO
4​> H
2​SO
4​> HNO
3​> HClO
(Perchloric acid is strongest because it has the most
oxygen atoms).
Chapter 3 11

12. Lewis acids and bases examples
Which of the following are potential Lewis acids, and which are
potential Lewis bases?
Chapter 3 12
Lewis base √ b) Lewis acid √ c) Lewis base √ d) Lewis base √
(a)
CH
3CH
2
N CH
3
CH
3
(b)
H
3C C
CH
3
CH
3
(c)
P
(d)
Br

13. Acidity strength
Chapter 3 13
Write the following organic compounds, in order of acidity, from the strongest to the weakest:



COOH COOH OH
A B
C

B √ C √ A √
…………… ………………… ………………..
Strongest Weakest

14. Chapter 3 14

15. Chapter 3 15
The Relationship Between Structure and Acidity
Acidity increases going down a row of the periodic table
Bond strength to hydrogen decreases going down the row and
therefore acidity increases

16. Chapter 3 16
Curved arrows show movement of
electrons to form and break bonds

17. Chapter 3 17
The Use of Curved Arrows in Illustrating Reactions
Curved arrows show the flow of electrons in a reaction
An arrow starts at a site of higher electron density (a covalent
bond or unshared electron pair) and points to a site of electron
deficiency
Example: Mechanism of reaction of HCl and water

18. Example 1
Write the reaction equation that occurs between sodium hydroxide and ethanoic acid using
curved arrow notation. Also assign appropriate formal charges and label the acid/ base
conjugation.
Chapter 3 18
Na OH+H C
H
H
C
O
O H
C
H
H
H
C
O
O
-
Na
+
+H
2O
Conjugate base
conjugate acid

19. Example 2
When methyl alcohol is treated with NaH, the product is
CH
3O
-
Na
+
(and H
2) and not Na+- CH
2OH (and H
2). Explain
why this is so.
The proton in the methyl alcohol is very acidic because
it is attached to an electron negative atom (oxygen)
compared to the proton attached to a much less
negative atom which is carbon. Thus, It will donate the
proton and accept the sodium ion to form a sodium salt
and a hydrogen molecule.
Chapter 3 19

20. Examples
Chapter 3 20
1.Write the reaction equation that occurs between ethanol and aluminium chloride using curved arrow
notation. Also assign appropriate formal charges.
2.Write the following organic compounds, in order of acidity, from the weakest to the strongest.
3.Formic acid (HCO2H) has a Ka value of 1.77 x 10-4 and trifluoro-acetic acid (CF3CO2H) has a Ka value
of 1. Calculate the pKa value for each acid and determine which acid produces a strong conjugate base.
4.Write the conjugate base for the following acids.,
4.1 HI ………………….
4.2 CH2OH …………………..

COOH
4.3

21. 4.4 Write the balanced equation for the neutralization of
magnesium hydroxide and nitric acid.
4.5 Define an Arrhenius base and Arrhenius acid.
4.6 Study the molecules provided below and identify the
weakest base. Support your answer.
Chapter 3 21