Acid-alkali reaction


Acid-alkali reaction

An acid-alkali reaction is a special case of an acid-base reaction, where the base used is also an alkali. When an acid reacts with an alkali it forms a metal, salt and water. Acid-alkali reactions are also a type of neutralisation reaction.

In general acid-alkali reactions can be simplified to

:OH(aq) + H+(aq)H2O

by omitting spectator ions.

Acids are generally pure substances which contain hydrogen ions (H+) or cause them to be produced in solutions. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) are common examples. In water, these break apart into ions:

HCl → H+(aq) + Cl(aq) OR H2SO4 → H+(aq) + HSO4(aq)

An alkali is a base, more precisely a base which contains a metal from column 1 or 2 of the periodic table (the alkali metals or the alkaline earth metals). The Science Module 3 within The Digital Brain [http://www.digitalbrain.com/document.server/subjects/ks3sci/su3/mod3/acidmet.htm] (Science KS3 SU3 Module 3, Acids and Bases) and Doc Brown (http://www.docbrown.info/, 2000-2008, p. 3) define alkalis as "soluble bases", which means they must be able to dissolve in water. Bases generally are defined as substances which contain hydroxide ion (OH-) or produce it in solution. Therefore, we may also speak of hydroxide bases which dissolve in water, and thus these would also be alkalis. Some examples, then, of alkalis would be sodium hydroxide (NaOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)2), and calcium hydroxide (Ca(OH)2). Note that only hydroxides with an alkali metal—column 1—are very soluble in water; hydroxides with an alkaline earth metal—column 2—are not as soluble. Some sources will even say the alkaline earth metal hydroxides are insoluble.

To produce hydroxide ions in water, the alkali breaks apart into ions as below:

NaOH → Na+(aq) + OH-(aq)

However, at http://dictionary.sensagent.com/alkali/en-en/, alkalies have a broader definition which includes carbonates (CO32-) bonded to a column 1 metal, an ammonium ion (NH4+), or an amine (NHx radical) as the positive ion. Examples of alkalis would then also include Li2CO3, Na2CO3, and (NH4)2CO3.

There seems to be conflicting information on acid-base reactions being neutralization reactions. Some sources define a neutralization reaction as the reaction between an acid and a base which produces a salt and water. Yet in the book Chemical Misconceptions: Prevention, Diagnosis and Cure by K. Tabor (2002), it is noted that “the term neutralization is usually reserved for acid-alkali reactions.” Thus this does not make acid-alkali a "type" of neutralization reaction, but the "only kind" of neutralization reaction.

There are many uses of neutralization reactions which are acid-alkali reactions. A very common use is antacid tablets. These are designed to neutralize excess stomach acid (HCl) which may be causing discomfort in the stomach or lower esophagus. Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur. (http://www.wddty.com/UtilityPages/Print.aspx?nodeId=-3363800369331166395)

Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. (http://www.practicalchemistry.org/experiments/intermediate/acids-alkalis-and-salts/neutralisation-curing-acidity,103,EX.html) Fertilizers which improve plant growth are made by neutralizing sulfuric acid (H2SO4) or nitric acid (HNO3) with ammonia gas (NH3) making ammonium sulfate or ammonium nitrate. These are salts utilized in the fertilizer. (http://www.docbrown.info, 2000-2008, p. 3)

If you have ever soothed an ant bite or bee sting with a paste of baking soda, you would have been neutralizing the formic acid produced by the ant or bee with the alkali, sodium hydrogen carbonate.

Shampoos and conditioners make use of the fact that hair bleaching and highlighting agents contain alkaline substances, which damage hair. To improve the hair’s appearance and shine, a slightly acidic shampoo can be used to reverse the damage. (Glencoe Chemistry, 2000, p. 501) From Milady’s Standard Cosmetology: Cosmetology, we learn “The neutralizing shampoos and normalizing lotions used to neutralize hydroxide hair relaxers work by creating an acid-alkali neutralization reaction.” (2002, p. 180)

Industrially, a by-product of the burning of coal, sulfur dioxide gas may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the calcium sulfite and any remaining unreacted sulfur dioxide. (Zumdahl, 2000, p. 226, 228)

ee also

*Base
*Alkali
*Acid
*Acid-base reaction theories


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