ACIDS AND BASES

1. ACIDS AND BASES
CHAPTER 4
BANELE MBATHA

2. ARRHENIUS THEORY
Definitions
• Acid is a substance that dissosciates in water to produce hydrogen
ions (𝐻+)
• A base is a substance that dissosciates in water to produce
hydroxide ions (OH‾)

3. ARRHENIUS ACID (EXAMPLES)
• HCl(aq)→𝐻+(aq)+Cl‾(aq)
• In the equation above, HCL dissociate in water to produce hydrogen
ions and chloride ions.

4. ARRHENIUS BASES (EXAMPLES)
NaOH(aq)→𝑁𝑎+(aq)+OH‾(aq)
Above sodium hydroxide (NaOH) dissolves in water to form sodium ion and
hyrdroxide ions.

5. LIMITATIONS (ARRHENIUS THEORY)
• The Arrhenius theory is limited in that it can only describe acid-
base chemistry in aqueous solutions. Similar reactions can also
occur in non-aqueous solvents, however, as well as between
molecules in the gas phase. As a result, modern chemists usually
prefer the Brønsted-Lowry theory, which is useful in a broader
range of chemical reactions.

6. BRONSTED-LOWRY THEORY
• Acid is a proton donor
• A base is a proton acceptor

7. EXAMPLES
1. HCL(aq) + H20(l) → H3O(aq) + CL‾
• Above: HCL donated a proton (𝐻+) and became CL‾, so HCL is an
acid.
• H2O accepted a proton and became 𝐻3 𝑂+
, so water acted as a
base.
2. NH3(aq) + HCL(aq) → 𝑁𝐻4
+
(aq) + CL‾(aq)
In example HCL (acid) reacts with NH3 (base). NH3 gains a proton
and HCL loses a proton.

8. ACID-BASE PAIRS
• Conjugate acid- is a substance that has gained a proton. That is, it
is a base with a hydrogen ion added to it.
• Conjugate base- is a substance that has lost a proton.
• A base with its conjugate acid and an acid with its conjugate base
are called conjugate acid-base pairs.

9. EXAMPLES
DETERMINE THE CONJUGATE ACID BASE PAIRS:
• HCL(aq) + H2O(l) → CL‾(aq) + 𝐻30+(aq)
SOLUTION:
• HCL (acid) donated a proton and became CL‾ (conjugate base),
there fore HCL and CL‾ are conjugate acid-base pairs.
• H2O (base) gained a proton and became 𝐻30+ (conjugate acid),
therefore water and hydroxide ions are conjugate acid-base pairs.

10. CONTINUES
• H2O(aq) + NH3(aq) → OH‾(aq) + 𝑁𝐻4
+
(aq)
SOLUTION:
• H2O donates a proton and becomes OH‾, this means that H2O is the
acid and the hydroxide ion is the conjugate base, therefore H2O and
OH‾ are conjugate acid base pairs
• NH3 (base) gains a proton and becomes 𝑁𝐻4
+
conjugate acid . NH3 and
𝑁𝐻4
+
are therefore conjugate acid-base pairs.
• Note that the difference between an acid and its conjugate base (or
base and its conjugate acid) must always be one hydrogen ion.

11. AMPHOLYTES
• Some substances can act both as an acid or a base, meaning they
can accept a proton and they can also donate it. These substances
are called ampholytes.
• As we have seen above, water can either accept a proton and
form 𝐻30+
or donate a proton to form OH‾. Therefore,
water(H2O) is an example of ampholytes.

12. REFERENCES
• http://www2.latech.edu/~upali/chem481/chem481c4.pdf
• http://www.slideshare.net/notesmaster/c10-acids-bases-and-salts
• https://www.slideshare.net/smithdk/acids-and-
bases?qid=4cf49bd7-9ca6-4ce3-af5e-
742ee0e790ff&v=&b=&from_search=32