Arrhenius Theory

Arrhenius Theory Definition

Our journey into the Arrhenius theory begins with Svante Arrhenius, the chemist who gave the theory its name. Swedish scientist Svante Arrhenius was born on February 19, 1859, and died on October 2, 1927, at the age of 68 years. Arrhenius was an accomplished chemist during his lifetime. It's no wonder he won the Nobel Prize for Chemistry in 1903!

_{Figure 1. Svante Arrhenius,}

I'm sure you're wondering what exactly Arrhenius did to earn his place among the greatest scientists in history. Well, Svante Arrhenius was the first chemist to classify Acids and Bases! Now, let's look at the definition of the Arrhenius theory of acids and bases.

Arrhenius Theory of Acid and Base

According to the Arrhenius theory, Arrhenius acids dissociate in water to form hydrogen (H⁺) ions. In other words, an Arrhenius acid increases the Concentration of H⁺ ions when dissolved in water (H₂O). For example, when hydrobromic acid (HBr) dissolves in water, it dissociates into hydrogen and bromine ions.

$$ \text{HBr} \xrightarrow{\text{H}_{2}\text{O}}\text{ H}^{+}\text{ + Br}^{-} $$

Now, an Arrhenius base dissociates in water to form hydroxide (OH^-) ions, increasing the Concentration of hydroxide ions in solution. A common example of an Arrhenius base is potassium hydroxide (KOH). When KOH dissolves in water, it separates into potassium cations (K⁺) and hydroxide anions (OH^-).

$$ \text{KOH } \xrightarrow{\text{H}_{2}\text{O}}\text{ K}^{+}\text{ + OH}^{-} $$

Let's look at a problem!

Arrhenius Theory Limitations

Now, like everything in this world, the Arrhenius theory is not perfect, and it has some limitations. According to the Arrhenius definition of acids and bases, the H⁺ cation and the OH^- anion are fundamental to the acid/base concept. So, it can only explain the behavior of acids and bases that actually contain H⁺ and OH^-.

However, there are acids and bases that do not fit this theoretical description. For example, aqueous NaHSO₄ is acidic, while aqueous Na₂CO₃ is basic.

Another limitation of the Arrhenius theory of acids and bases is that it fails to describe how acids and bases behave in non-aqueous solutions.

It also fails to explain the behavior of H⁺ in water. As it turns out, H⁺ cannot exist as ions in water because these ions are attracted to the polar water molecules, forming hydronium ions (H₃O⁺). To account for this behavior of H+, scientists can come up with the modified Arrhenius theory.

For example, the ionization of carbonic acid in water yields H₃O⁺ ions and HCO₃^- ions.

$$ \text{H}_{2}\text{CO}_{3}(aq)\text{ + H}_{2}\text{O}(l) \longrightarrow \text{H}_{3}\text{O}^{+}(aq) \text{ + HCO}_{3}^{-}(aq)$$

Arrhenius Theory of Electrolytic Dissociation

In 1903, Svante Arrhenius became the recipient of the Nobel Prize of Chemistry, for this development of the electrolytic theory of dissociation, which is a theory that describes aqueous solutions in terms of acids (Arrhenius acids) and bases (Arrhenius bases).

This theory also aimed to explain why some solutions are able to conduct electricity, and it proposed that these ions present in solution allow for electricity to flow!

The Arrhenius theory of electrolytic dissociation is greatly used in the laboratory when dealing with electrolysis, a chemical process that involves passing an electric current through an aqueous solution that contains ions to split up compounds into its elements.

Arrhenius Theory Equation

Last but not least, let's discuss the Arrhenius equation, also attributed to Svante Arrhenius. This equation, however, is not related to either the Arrhenius theory of acids and bases or the Arrhenius theory of electrolytic dissociation, as it is associated with reaction rates. Reaction rates help scientists predict how fast or slow a chemical reaction will take to complete.

During a chemical reaction, changing the temperature in which a reaction is taking place will affect the rate constant, \(k\). More specifically, increasing the temperature will cause the value for the rate constant (\(k\)) to increase, making the reaction go faster.

This information led Arrhenius to derive the Arrhenius equation, which relates rate constant and temperature.

The Arrhenius equation is shown below:

$$ k = \text{A}e^{\frac{\text{-E}_{a}}{\text{RT}}} $$

Where:

• \(k\) is the first order rate constant
• A is the frequency factor (also called the pre-exponential factor)
• \(e\) is the natural log base
• \(\text{-E}_{a}\) is the Activation Energy (kJ/mol)
• T is the absolute temperature (K)
• R is the universal molar gas constant, given as 8.314 J/mol·K.

The main use of the Arrhenius aqueous is to calculate the Activation Energy of a chemical reaction, and the way this is done is by using a ln \(k\) vs. 1/T graph. The slope of this graph is considered to be \(\frac{\text{-E}_{a}}{\text{R}}\). So, if the know that the value for R is 8.314 J/mol·K, we can calculate the activation energy of the reaction!

Figure 3. Arrhenius equation graph

I hope now you're more confident about your understanding of the Arrhenius theory and Svante Arrhenius' accomplishments!