6.1 – Introduction to Acids and BasesUnit 6 – Acids and Bases

Introduction In your years of studying chemistry, you

have probably come across a few common acids and bases:

Acids:Hydrocholoric acid (HCl)Sulfuric acid (H2SO4)Nitric acid (HNO3)Acetic acid (HC2H3O2)

Bases:Sodium Hydroxide (NaOH)Potassium Hydroxide (KOH)Calcium hydroxide Ca(OH)2

Ammonia (NH3)

Introduction

Acids and bases are special substances with very distinct properties. It is good think of acids and bases as opposites.

Key characteristics of acids Key characteristics of bases

- Sour taste (eg. Lemons, grapefruit, vinegar, sour milk)

- React with active metals such as zinc and magnesium to produce hydrogen gas

- Form electrolytic solutions (conduct electricity) because they produce ions

- Cause certain dyes to change color (litmus paper turns red)

- Neutralized by bases (neutralized means that the substance no longer has acidic or basic properties)

- Bitter taste- Generally no noticeable reaction

with active metals- Form electrolytic solutions (conduct

electricity) because they produce ions

- Cause certain dyes to change color (litmus paper turns blue)

- Slippery feel (eg. soapy feel)- -Neutralized by acids

Arrhenius’ Theory

Looking at our list of acids and bases, what do you see that is common between the acids? Most of the bases?

Acids:Hydrocholoric acid (HCl)Sulfuric acid (H2SO4)Nitric acid (HNO3)Acetic acid (HC2H3O2)

Bases:Sodium Hydroxide (NaOH)Potassium Hydroxide (KOH)Calcium hydroxide Ca(OH)2

Ammonia (NH3)

Arrhenius’ Theory

In the 1880’s, Svante Arrhenius determined that acids had their characteristic properties due to the presence of hydrogen ions, H+.

Likewise, he discovered the properties of bases are due to the presence of hydroxide ions, OH-.

These two observations together is known as the Arrhenius Theory of Acids and Bases.

Dissociation

Dissociation will be important in this unit.

Remember that this process is when an ionic compound is mixed with water. Dissociation of ionic compounds occurs when water

molecules “pull apart” the ionic crystal.

This occurs due to strong attractions between the polar ends of the water molecule and the positive and negative ions within the crystal.

Water molecules then surround the positive cations and negative anions

KOH(s) K+(aq) + OH-

(aq)

Note that bases undergo dissociation.

Dissociation

There are two important things to notice about writing dissociation equations: Generally DO NOT include H2O as a

reactant. We know something has been dissolved in water when we see the (aq) notation. We will make some exceptions later to this rule

Ion charges MUST BE included!

Ionization

Ionization is the process of dissolving molecular compounds (covalently bonded) in water to produce ions.

Most molecular compounds do not undergo ionization. However, acids ALWAYS do. In fact, all acids produce hydrogen ions in a

solution.

HCl(g) H+(aq) + Cl-(aq)

H2SO4(g) 2H+(aq) + SO4

2-(aq)

Ionization

So what is actually happening? Evidence suggests that the hydrogen ion

actually bonds to a water molecule forming a hydronium ion, H3O+.

Ex: HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

Ex: H2SO4(g)+ 2H2O(l) 2 H3O+(aq) +

SO42-

(aq)

You should be comfortable using either method of representation: one will mean the same as the other.

Why the Arrhenius Theory Isn’t Good Enough

Up until this point, we have said that a substance that produces H+ ions is an acid and one that produces OH- ions is a base

So… why is NH3 considered a base?

This may be a problem at first, but lets look at what happens when we add ammonia to water:

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

The Arrhenius Theory is unable to explain this occurrence. Luckily we have an alternative theory that works just fine…

Bronsted and Lowry Theory of Acids & Bases

2 chemists working independently, Johannes Bronsted and Thomas Lowry, came up with what is now known as the “Bronsted-Lowry Theory of Acids and Bases.” This theory states that acids are substances

that can DONATE a hydrogen ion, and bases are substances that can ACCEPT a

hydrogen ion.

Proton Donation

How are acids “donors?” HCl H+ + Cl-

This shows that HCl produces an H+, but to donate implies that something will receive the H+. So, we can see the donation with the ionization equation:

HCl + H2O H3O+ + Cl-

Proton Acceptance Getting back to ammonia , we will see how a base

can accept a hydrogen ion.

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

Notice that the ammonia has become an ammonium ion by accepting a H+ from the water.

The H+ that came from the water left its electrons behind with the remaining OH-, which gives us an H+ and an OH-.

Conjugate Acid-Base Pairs

Now, if NH3 can become NH4+ by gaining a

hydrogen ion, then lets consider the reverse – that is, NH4 should be able to change back to NH3 by losing a hydrogen ion. Since we have defined NH3 as a base because

it can accept an H+, then its partner ion, NH4+

can be considered an acid since it can give up an H+ to become NH3.

Conjugate Acid-Base Pairs

Let’s consider water now. In the same equation, H2O gives up an H+ to ammonia – therefore, we should be able to consider H2O an acid.

However, in the reverse reaction, H2O’s partner ion, OH-, accepts the H+ from NH4

+ to become water. This accepting of H+ makes it a base!

These two examples are called conjugate acid-base pairs.

Conjugate Acid-Base Pairs Conjugate acid-base pairs differ from each other

by the presence or absence of a single hydrogen ion (or proton).

Every acid has a conjugate base, and every base has a conjugate acid.

We can now express these equations with a double arrow, since it represents acid-base equilibrium

Examples

Example 1: Write the conjugate bases for the following acids:

A) HF

B) H2SO4

Answers

A) F-

B) HSO4-

Examples

Example 2: Write the conjugate acids for the following bases:

A) PO43-

B) SO42-

Answers

A) HPO4-2

B) HSO4-

Amphoteric Substances

Notice that in the ammonia example, water acted as a acid. However, how does water react in the following reaction?

HCl(g) + H2O(l) H3O+(aq) + Cl-

(aq)

Since water is accepting an H+ it is considered a base.

“Amphoteric substances” are those that act as an acid in one reaction but a base in another.

Example Example 3: In the following two reactions

which substance is amphoteric? When is it an acid? A base?

A) HSO4- + H3O+ H2SO4 + H2O

B) HSO4- + OH- SO4

2- + H2O

Answer: Forward: HSO4

-, A = base, B = acid

Reverse: H20, A = Base, B = acid