Acids and Bases (Grade 11 NSC Matric Physical Sciences): Revision Notes

Acids and Bases

What are acids and bases?

Chemical reactions are happening all around us constantly. Plants perform photosynthesis, car engines burn petrol with air, and our bodies carry out countless complex reactions. Two particularly important types of reactions that occur both in nature and in chemistry laboratories are acid-base reactions and redox reactions.

Acids and bases are substances that have distinct characteristics and can be identified by their behaviour in chemical reactions. Understanding these substances is essential because they play crucial roles in many chemical processes we encounter daily.

Common acids and bases

Here are some frequently encountered acids and bases with their chemical formulas:

Most acids share similar characteristics, and most bases also have common properties. Having clear definitions for acids and bases helps us identify them correctly in chemical reactions and predict how they will behave.

Defining acids and bases

The arrhenius definition

One of the earliest definitions came from Arrhenius in 1887. He observed that when certain substances dissolve in water, they split apart (dissociate) into H₃O⁺ and OH⁻ ions.

The water dissociation equation is: $2\text{H}_2\text{O}(l) \rightarrow \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq)$

According to Arrhenius:

• An acid is a substance that increases the concentration of H₃O⁺ ions when dissolved in water
• A base is a substance that increases the concentration of OH⁻ ions when dissolved in water

The brønsted-lowry definition

In 1923, Lowry and Brønsted developed a more comprehensive definition based on proton transfer rather than just water dissociation.

DEFINITION: Acids A Brønsted-Lowry acid is a substance that gives away protons (hydrogen cations H⁺) and is therefore called a proton donor.

DEFINITION: Bases
A Brønsted-Lowry base is a substance that takes up protons (hydrogen cations H⁺) and is therefore called a proton acceptor.

Worked examples of proton transfer

Special types of substances

Amphoteric substances

DEFINITION: Amphoteric An amphoteric substance is one that can react as either an acid or a base.

Water is a perfect example of an amphoteric substance. In Example 2, water acted as a base (accepting a proton), but in Example 3, water acted as an acid (donating a proton). Whether water behaves as an acid or base depends on what other substance is present in the reaction.

Amphiprotic substances

DEFINITION: Amphiprotic
An amphiprotic substance is one that can react as either a proton donor (Brønsted-Lowry acid) or a proton acceptor (Brønsted-Lowry base). Examples include water, hydrogen carbonate ion (HCO₃⁻) and hydrogen sulfate ion (HSO₄⁻).

Polyprotic acids

A polyprotic acid is an acid that has more than one proton available to donate. For example, sulfuric acid can donate one proton:

$\text{H}_2\text{SO}_4(aq) + \text{OH}^-(aq) \rightarrow \text{HSO}_4^-(aq) + \text{H}_2\text{O}(l)$

Or it can donate both protons:

$\text{H}_2\text{SO}_4(aq) + 2\text{OH}^-(aq) \rightarrow \text{SO}_4^{2-}(aq) + 2\text{H}_2\text{O}(l)$

Common polyprotic acids include H₂SO₄, H₂SO₃, H₂CO₃ and H₃PO₄.

Conjugate acid-base pairs

Let's examine what happens when we reverse an acid-base reaction.

Consider: $\text{HCl}(aq) + \text{NH}_3(aq) \rightarrow \text{NH}_4^+(aq) + \text{Cl}^-(aq)$

In this reaction:

• HCl acts as the acid (proton donor)
• NH₃ acts as the base (proton acceptor)

But what if we wrote the reverse reaction? $\text{NH}_4^+(aq) + \text{Cl}^-(aq) \rightarrow \text{HCl}(aq) + \text{NH}_3(aq)$

Now:

• NH₄⁺(aq) → NH₃(aq) (NH₄⁺ loses a proton, so it's the acid)
• Cl⁻(aq) → HCl(aq) (Cl⁻ gains a proton, so it's the base)

We call these related pairs conjugate acid-base pairs:

• HCl and Cl⁻ form one conjugate pair
• NH₃ and NH₄⁺ form another conjugate pair