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Arrhenius Theory of Acids and Bases Chemistry Tutorial

Key Concepts

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Definition of Arrhenius Acids and Bases

In 1887, Svante Arrhenius (a Swedish chemist) showed that strong electrolytes were completely dissociated, that is, when a strong electrolyte dissolves in water it breaks up into ions which can carry an electric current in solution.

Strong acids like aqueous solutions of hydrochloric acid (HCl(aq)), nitric acid (HNO3(aq)) and sulfuric acid (H2SO4(aq)) are also strong electrolytes, that is, HCl(aq), HNO3(aq) and H2SO4(aq) are all good conductors of electricity.
Therefore, these strong acids must dissociate completely in water to produce ions.
When we write a chemical equation to describe these dissociation reactions we use a one-headed arrow (→) to show that all the acid has dissociated and that no molecules of the acid are present in solution as shown in the examples below:

HCl(aq) → H+(aq) + Cl-(aq)

HNO3(aq) → H+(aq) + NO3-(aq)

H2SO4(aq) → 2H+(aq) + SO42-(aq)

The resultant solution, the solution resulting from the dissociation of a strong acid in water, contains cations (positively charged ions) and anions (negatively charged ions).

Notice that each aqueous solution of strong acid above dissociates to produce the same cation, hydrogen(1+) ions (H+(aq)).
Arrhenius used this to define what an acid is: an acid is a compound which contains hydrogen and dissociates in water to produce hydrogen(1+) ions, H+(aq).

Very few acids are strong electrolytes, most acids are weak electrolytes, that is, aqueous solutions of most acids do not conduct electricity well.
Arrhenius explained this by defining a weak acid as a compound which contains hydrogen but which does NOT fully dissociate, that is, there are fewer hydrogen(1+) ion (H+(aq)) in solution.
For example, an aqueous solution of acetic acid (ethanoic acid, CH3COOH(aq)) does not conduct electricity as well as an aqueous solution of hydrochloric acid (HCl(aq)) of the same concentration (at the same temperature and pressure).
This is because HCl(aq) fully dissociates so there are no H-Cl molecules in the solution, the solution contains hydrogen(1+) ions, H+(aq), and chloride ions, Cl-(aq).
But CH3COOH(aq) does NOT fully dissociate so in the solution there are undissociated CH3COOH(aq) molecules as well as the acetate ions (ethanoate ions, CH3COO-(aq)) and hydrogen(1+) ions (H+(aq)).
We represent the dissociation of acetic acid as a reversible reaction, a reaction that achieves an equilibrium position in which the resultant solution contains reactant (acid) molecules as well as cations and anions using the double-arrow (⇋) as shown below:

CH3COOH(aq) ⇋ CH3COO-(aq) + H+(aq)

For aqueous solutions of the same concentration at the same temperature and pressure, the strong acid will contain more hydrogen(1+) ions than a weak acid, and, the strong acid will be a better conductor of electricity than the weak acid.

An Arrhenius acid is a compound that contains hydrogen and which dissociates in water to produce hydrogen(1+) ions, H+(aq)

  • A strong Arrhenius acid fully dissociates into ions in water.

    HA(aq) → H+(aq) + A-(aq)

  • A weak Arrhenius acid only partially dissociates into ions in water.

    HA(aq) ⇋ H+(aq) + A-(aq)

Similarly, Arrhenius found that strong bases were also strong electrolytes.
Aqueous solutions of sodium hydroxide, NaOH(aq), and potassium hydroxide, KOH(aq), are good conductors of electricity, therefore these hydroxide containing compounds must dissociate in water to produce ions:

NaOH(aq) → Na+(aq) + OH-(aq)

KOH(aq) → K+(aq) + OH-(aq)

An Arrhenius base is a compound that contains hydroxide and which dissociates in water to produce hydroxide ions, OH-(aq)

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Reactions of Arrhenius Acids and Bases: Neutralisation

An Arrhenius acid reacts with an Arrhenius base in a neutralisation reaction (neutralization reaction).

Recall from the discussion above that an Arrhenius acid is a hydrogen-containing compound that dissociates in water to produce hydrogen(1+) ions, H+(aq):

HA(aq) → H+(aq) + A-(aq)

This means that an aqueous solution of Arrhenius acid will always contain hydrogen(1+) ions, H+(aq).

Also recall that an Arrhenius base is a hydroxide-containing compound that dissociates in water to produce hydroxide ions, OH-(aq):

MOH(aq) → M+(aq) + OH-(aq)

This means that an aqueous solution of Arrhenius base will always contain hydroxide ions, OH-(aq).

So, when any Arrhenius acid reacts with any Arrhenius base, we can say that the reaction is between the H+(aq) of the acid and the OH-(aq) of the base.
The product of this reaction will be a molecule containing 2 hydrogen atoms and 1 oxygen atom, that is a water molecule, H2O(l):

H+(aq) + OH-(aq) → H2O(l)

The acid is said to be neutralised (neutralized) by the base.
The base is said to be neutralised (neutralized) by the acid.

This final aqueous solution will also contain the anions (A-(aq)) from the acid and the cations (M+(aq)) from the base.
Together these ions are represented as an aqueous compound dissolved in water with the chemical formula MA(aq).
The compound MA was referred to as a salt by Arrhenius.
Arrhenius defined a salt as the compound whose ions are left over when an acid has been neutralised by a base.
It is therefore possible to write the following equations to represent a neutralisation reaction:

Arrhenius Neutralisation Reaction
general word equation: acid + base → salt + water
general chemical equation: HA(aq) + MOH(aq) → MA(aq) + H2O(l)

For example, when an aqueous solution of sodium hydroxide (NaOH(aq)) is added to an aqueous solution of hydrochloric acid (HCl(aq)) a neutralisation reaction occurs.
The products of this neutralisation reaction are water (H2O(l)) and the salt, sodium chloride, dissolved in water (NaCl(aq)).
The chemical equations below describe this neutralisation reaction:

general word equation: Arrhenius acid + Arrhenius base → water + salt
word equation: hydrochloric acid + sodium hydroxide → water + sodium chloride
chemical equation: HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)

The Arrhenius acid, HCl(aq), has been neutralised by the Arrhenius base, NaOH(aq).

The Arrhenius base, NaOH(aq), has been neutralised by the Arrhenius acid, HCl(aq).

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Amphoteric Substances

An amphoteric substance is one that can act as either an Arrhenius acid or as an Arrhenius base, that is, an amphoteric substance will react with either an Arrhenius acid or with an Arrhenius base.

Consider solid zinc hydroxide, Zn(OH)2(s).
Zn(OH)2(s) looks like it should be an Arrhenius base because it contains hydroxide ions (OH-).
But does it behave like an Arrhenius base? Does it "neutralise" an Arrhenius acid?

If we add an Arrhenius acid such as an aqueous solution of hydrochloric acid, HCl(aq), to Zn(OH)2(s), then the Zn(OH)2(s) will dissolve in the acid. The products of the reaction can be represented as water (H2O(l)) and Zn(OH)+(aq).
This process is shown in the chemical equation given below:

Zn(OH)2(s) + H+(aq) → Zn(OH)+(s) + H2O(l)

Clearly, Zn(OH)2(s) is acting like an Arrhenius base because it is reacting with an Arrhenius acid (H+(aq)) to produce water.

However, if we add an Arrhenius base such as an aqueous solution of sodium hydroxide, NaOH(aq), to Zn(OH)2(s), then the zinc hydroxide will dissolve. The product of this process is Zn(OH)3-(aq).
This process is shown in the chemical equation given below:

Zn(OH)2(s) + OH-(aq) → Zn(OH)3-(aq)

Zn(OH)2(s) is reacting with the Arrhenius base (OH-(aq)) so Zn(OH)2(s) is acting like an Arrhenius acid.

Zn(OH)2(s) is amphoteric, it can act like an Arrhenius acid or it can act like an Arrhenius base.

Other examples of amphoteric metal hydroxides include the hydroxides of beryllium, aluminium, tin, and lead.

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Sample Questions with Worked Solutions

Question: Classify HSO4-(aq) as either an Arrhenius acid or an Arrhenius base.

Solution:

  1. Define "Arrhenius acid and Arrhenius base":

    An Arrhenius acid contains hydrogen and dissociates in water to produce hydrogen(1+) ions (H+(aq)).

    An Arrhenius base contains hydroxide and dissociates in water to produce hydroxide ions (OH-(aq)).

  2. Decide the composition of HSO4-(aq)

    (i) contains hydrogen (H)

    (ii) does NOT contain hydroxide (OH-)

  3. Decide whether HSO4-(aq) is an Arrhenius acid or an Arrhenius base:

    HSO4-(aq) cannot be an Arrhenius base because it does NOT contain hydroxide (OH-)
    HSO4-(aq) does contain hydrogen so it could be an Arrhenius acid.
    HSO4-(aq) could dissociate in water to produce hydrogen(1+) ions (H+(aq)) and sulfate ions (SO42-(aq)) so it is an Arrhenius acid.

  4. HSO4-(aq) is an Arrhenius acid because :

    (i) it contains hydrogen
    AND
    (ii) it produces hydrogen(1+) ions (H+(aq)) when dissolved in water

Question: In aqueous solution, [B(OH)3(aq)] dissociates according to the chemical equation shown below:

[B(OH)3(aq)] ⇋ [BO(OH)2]-(aq) + H+(aq)

Classify [B(OH)3(aq)] as either an Arrhenius acid or an Arrhenius base.

Solution:

  1. Define "Arrhenius acid and Arrhenius base":

    An Arrhenius acid contains hydrogen and dissociates in water to produce hydrogen(1+) ions (H+(aq)).

    An Arrhenius base contains hydroxide and dissociates in water to produce hydroxide ions (OH-(aq)).

  2. Decide whether [B(OH)3(aq)] is an Arrhenius acid or an Arrhenius base based on its dissociation products:

    [B(OH)3(aq)] dissociates to produce hydrogen(1+) ions (H+(aq))

  3. [B(OH)3(aq)] is an Arrhenius acid because it produces hydrogen(1+) ions (H+(aq)) when dissolved in water