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Chemistry - The Arrhenius Theory

The Arrhenius Theory was the first modern acid-base theory developed. In this theory an acid is a substance that , when dissolved in water, yields H + ions, and a base is a substance that, when dissolved in water, yields OH ­ ions. Hydrogen chloride gas (HCl (g)) can be considered as a typical Arrhenius acid, because when this gas dissolved in water, it ionizes (forms ions) to give the H+ ion.

According to the Arrhenius theory, sodium hydroxide is classified as a base, because when it dissolves, it yields the hydroxide ion:

NaOH (aq) -> Na + + OH ­

Arrhenius also classified the reaction between an acid and base as a neutralization reaction, because if you mix an acidic solution with basic solutions, you end up with a neutral solution composed of water and a salt.

HCl (aq) + NaOH (aq) -> H 2O (l) + NaCl (aq)

The following equation shows the reaction and production of ions (notice where the water comes from):

H + + Cl- + Na + -> H 2O (l) + Na + + Cl-

As you can see, the water is formed from combining the hydrogen and hydroxide ions. The net-ionic equation is the same for all Arrhenius acid-base reactions:

H + + OH- -> H 2O (l)

The Arrhenius theory is still used quite a bit, but like all theories it has some limitations, for example, look at the following reaction (note the gas phase reaction between ammonia and hydrogen chloride gases):

NH 3(g) + HCl (g) -> NH 4+ + Cl- -> NH 4Cl (s)

The two clear, colourless gases mix, and a white solid of ammonium chloride forms. The equation shows the intermediate formation of the ions in the equation so that you can better see what is actually happening.

The HCl transfers an H + to the ammonia. That is basically the same thing that happens in the HCl/NaOH reaction, but the reaction involving the ammonia cannot be classified as an acid-base reaction, because it does not occur in water, and it does not involve the hydroxide ion.

In both reactions the same basic process is taking place. In order to account for these similarities, a new acid-base theory was developed, it is known as the Bronsted-Lowery theory.

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