give the limitations of Arrhenius theory
Answers
Answer:
Explanation:
Arrhenius theory is known to be the most limited of the three theories since it requires the solutions to be aqueous. It only applies to substances that produce Hydrogen ions (H+) or hydroxide ions (OH−).
1- An acid is expected to be an acid in any solvent. But that’s not the case nowadays. For example HCL acts as an Arrhenius acid when dissolved in water. However when HCL is dissolved in benzene there is no dissociation. This is against Arrhenius theory; Arrhenius states that dissociation occurs in any aqueous solution. The properties of acid and bases play a critical role.
2- Arrhenius did not explain in his theory the behavior of acids and bases in a non aqueous solution For example, the dissociation of acetic acid in methanol it could be written as CH3CO2H + CH3OH ⇄ CH3CO2− + CH3OH
3- In Arrhenius theory all salts should produce solutions that are neither acidic nor basic. But there are some exceptions against this theory. For example if equal amounts of HCl and ammonia react, the solution is slightly acidic. If equal amounts of acetic acid and sodium hydroxide react, the resulting solution is basic. Arrhenius theory does not include any explanation for this.
4- The need for hydroxide as the base led Arrhenius to propose the formula NH4OH as the formula for ammonia in water. This led to the misunderstanding that NH4OH is the actual base. But the actual base is NH3.
5- As you read before the bare proton H+, cannot exist for a long time in a water solution. Therefore this reaction: H2O + H+ à H3O+ Occurs most of the time.
Later on, it was realized that Arrhenius theory was rather limiting as his ideas only are valid in aqueous solutions. Although many acid-base reactions do occur when water is the solvent, this is not always the case.
Scientists realized that water is important when it comes to acidic solutions. They then concluded that
not only hydrogen must be present in an aqueous solution but something else as well. They made this conclusion by looking at hydrogen chloride in gas-form, HCl (g,). Hydrogen chloride itself is not acidic but when it is dissolved in water an acidic solution is produced. There was not only this flaw to Arrhenius’ discovery. They realized that there existed acid-base reactions which were taking place in solvents other than water. Not only that, but they reacted without any solvent at all.
One problem is that NH3, Ammonia, cannot be dissociated according to Arrhenius’ theory. Ammonia is a base. However, for it to be a base it has to contain oxygen and hydrogen, which it doesn’t. This means that according to Arrhenius theory ammonium can’t dissociate since it can’t produce hydroxide ions (OH-). In other words, Arrhenius theory does in this case not work. His theory did simply not explain why ammonia (NH3) was a base.
NH3à Doesn’t work
Hydrochloric acid can be neutralized by either sodium hydroxide or ammonia solution.
Chemical equation for the reactions:
NaOH (aq) + HCl (aq) à NaCl (aq) + H2O (l)
NH3 (aq) + HCl (aq) à NH4Cl (aq)
In the first reaction above (the sodium hydroxide example), the hydrogen ions from the acid will along
However, as seen in the second reaction above (the ammonia case), no hydroxide ions are appeared. There are no hydrogen- and hydroxide ions involved in this case, since there isn’t any solution. According to the Arrhenius theory, this is not an acid-base reaction even though it’s producing the same product as when the two substances were in solution.with the hydroxide ions from the base, react together and form H2O, the molecule water and the salt NaCl. This example is in line with the Arrhenius theory as a hydrogen ion is produced by hydrochloric acid (the acid) and a hydroxide ion by sodium hydroxide (the base).
Yet ammonia is able to create hydroxide ions if it reacts with water (H2O). What occurs in this reaction
is that the water will give away one of its hydrogen atoms to the ammonia. Ammonia will no longer be ammonia; instead it will become NH4+, an ammonia ion, in other words, ammonium. A hydrogen ion,
H+, has no electrons in its outer shell. Thus in this case it will simply just bond with the two remaining electrons from nitrogen. Since the ratio of hydrogen to oxygen in water is 2:1 and one hydrogen atom goes to the ammonia, one hydrogen atom and one oxygen atom will be left (hydroxide, OH-). In this way hydroxide ions are produced nevertheless this does not work in Arrhenius’ dissociation method.
NH3 (g) +H2O (l) ↔ NH4+ (aq) + OH- (aq)
This reaction is a reversible one. In a typical dilute solution about 99% of the ammonia will remain in being ammonia molecules. The remaining 1% will change into ammonium, leading to hydroxide ions being produced. Therefore this chemical reaction can be connected to the Arrhenius theory.
Ammonia will in aqueous solutions act like a base, although it does not produce hydroxide ions. Ammonia is clearly not the only substance that acts like this, as there are other substances which behave like acids or bases without conforming to the Arrhenius definition.
The disadvantage with Arrhenius’ theory was that the ideas within it later on were found out to be quite limiting. However, as this theory was the first comprehensive theory in history, it afterward became followed by other somewhat more developed theories. Furthermore, this theory can indeed be seen more as an advantage than a disadvantage, as despise its weaknesses, it made other scientists propose more general definitions of acids and bases.
The limitations of the first acid-base theory indicated a need for a more comprehensive and developed one. This definition would in 1923, be made by two chemists, the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry, and is for that reason called the Brønsted-Lowry theory. The main difference between Arrhenius’ theory and Brønsted-Lowry theory is that while both of them define an acid as a proton donor, the Brønsted-Lowry theory defines a base as a proton acceptor whereas the Arrhenius theory defines it as a hydroxide donor.