THE ARRHENIUS ACID-BASE THEORY

A. The history of Arrhenius

Svante August ARRHENIUS was born in 1859 near Uppsala. He was an infant prodigy having taught himself to read at the age of three and graduating from high school as the youngest and brightest in his class. While attending the University of Uppsala he began to study how electricity passed through solutions. Faraday (1830's) had worked out the laws of electrolysis and from these laws it seemed that electricity like matter existed in forms of tiny particles. Faraday had mentioned "ions" (from the greek word for "wanderer") the particles that carried the electricity through the solution. However, the question remained as to just what the ions were. It was Arrhenius who explained what these ions were. Note: In chemistry an ion is a charged particle (positive or negative).

 

Arrhenius considered why salts such as NaCl when dissolved in water conducted electricity producing 'an electrolyte solution' while other chemicals substances such as sugar (sucrose) did not conduct electricity and produced a 'non-electrolyte' solutions?

 

From the work of Raoult (1886) it was found that when a non-electrolytic substance such as sucrose was dissolved in water, it lowered the freezing point of the water and the lowering was proportional to the number of particles present in solution. But what about the electrolytes? NaCl is composed of a fixed number of elements. As it turned out the amount of lowering induced by dissolved NaCl was twice the amount that it ought to have been. The explanation was that one particle of NaCl gave rise to two particles as did KBr and NaNO3. But with substances like BaCl2 and Na2SO4 they produced three times the lowering that one had expected. Each of the chemicals gave rise to three particles. Arrhenius decided that the only explanation was that NaCl broke up into two particles, namely a positve sodium or Na particle and a negative chloride or Cl particle. Of course the solutions did not contain metallic sodium or gaseous chlorine so the sodium and chloride particles must be carrying electric charges and that was why NaCl solutions could transmit an electric current.

 

NaCl(aq) → Na+(aq) + Cl-(aq)  A mixture of positive and negative ions are produced as sodium chloride dissolves in water.

(aq) = aqueous/ in water

 

The positively charged sodium or Na ion and the negatively charged chloride or Cl ion had properties very different from the unchanged atoms. The BaCl2 was split into three particles, a positive Ba ion with a double charge and two single charged negative chloride ions.

 

The equation for this can be written as BaCl2(aq) → Ba2+(aq) + 2Cl-(aq)

 

This was a revolutionary concept!

 

Most chemists of that time still believed Dalton's view that the atom was structureless and indivisible. They could not accept that atoms could be electrically charged. Where would the electric charge come from? How could a stable substance like NaCl break up in so mild a substance as water. One of Arrhenius' teacher dismissed him when he tried to explain his theory.

 

In 1884 Arrhenius prepared his PhD on the theory of ionic dissociation. After a four hour examination he was awarded the lowest passing grade (a class honours). Fortunately two German physical chemists, Van't Hoff and Ostwald became intrigued with this new theory and they colaborated with Arrhenius.

 

In 1890 Thomson's discovery of the electron and Becquerel's discovery of radioactivity showed that the atom was not structureless after all and that they were made up of electrically charged particles (negative electrons). It was now easy to see that a negative chloride ion was a chlorine atom that had obtained one more electron than its fair share while the positive sodium ion was the sodium atom with a missing electron. If the sodium and chlorine particles were held together by the attraction of electric charges, the water could separate them producing a solution of negatively and positively charged ions. Suddenly, Arrhenius' ionic theory made sense. In 1895 he was appointed Professor at the University of Stockholm. In 1903 the same PhD thesis that had scored a bare pass won him the Nobel Prize in Chemistry. There was some discussion as to whether the prize should be recorded as the prize in Chemistry or in Physics. It was awarded in Chemistry. 

 

The Arrhenius theory of electrolytes (1887) is concerned with the formation, number and speed of ions in solution. The key to the theory is the behaviour of the dissolved substance (solute) and the liquid (solvent) both of which are capable of dissociating into ions. It postulates that there is an equilibrium between undissociated solute particles and their ions, whose movement or migration can conduct an electric current through the solution.

 

Arrhenius later died in Stockholm in 1927. 

 

The theory of electrolyte behavior by Arrhenius helped define the modern concept of acids and bases

 

B. The concept of an acid and a base.

  • An acid is a compound which dissociates in water to give hydrogen ions: H+
  • A base is a compound which dissociates in water to give hydroxide ions: OH-

 

C. The dissociation or splitting apart of strong acids and bases.

It was found from measurements of electrical conductivity, that solutions of different acids and bases dissociate or split apart to different extents, even though the solutions may be of the same concentration. This introduces the idea of relative strengths of acids or bases. The greater the amount of dissociation of the acid molecules in water, the stronger the acid. Strong acids completely ionize in water and produce many hydrogen ions.

 

Hydrochloric acid: HCl(aq) → H+(aq) + Cl-(aq) 100% dissociation.

Sulfuric acid: H2SO4(aq) → 2H+(aq) + SO42-(aq)

Sodium hydroxide: NaOH(aq) → Na+(aq) + OH-(aq)

 

 

C. The dissociations or splitting apart of weak acids.

Studies show that weak acids partially ionize and produce relatively few hydrogen ions in water.

Acetic acid: CH3COOH(aq) → CH3COO-(aq) + H+(aq) 1% dissociation

 

The Arrhenius theory of acids and bases explained why the pH of equal concentrations of acids could be different.

The pH of a 0.1M solution of hydrochloric acid is 1. However, the pH of a 0.1 M solution of acetic acid is much higher, around 4 as there are many less hydrogen ions produced because of the weak dissociation of acetic acid.   

In summary, 

  • strong acids completely dissociate or ionize in water, producing more H+ ions and a lower pH.
  • weak acids partially dissoicate or ionize in water, producing less H+ ions and a higher pH.

 

D. Problems with Arrhenius theory

While the Arrhenius theory of acids and bases proved to be valuable there are a number of inadequacies.

  1. The theory is restricted to solutions in water only and did not extend to other solvents.
  2. No account is made of the role played by water molecules in the dissociation or splitting apart of acids and bases. 
  3. Some weak acids and bases do not contain H or OH groups so they cannot dissociate directly to form H+ and OH- ions. eg. Aluminum ions, Al3+ are acidic and ammonia, NH3 is a basic in water.

 

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