First, let us recall about what acids and bases are. Acids are substances which ionize in water to produce a hydrogen ion (H+). Take hydrochloric acid for example.
HCl (aq) à H+ (aq) + Cl- (aq)
A base is any metal oxide or metal hydroxide. Alkalis are soluble bases. Alkalis ionize when they dissolve in water to form hydroxide ions (OH-). Take potassium hydroxide for example.
KOH (aq) à K+ (aq) + OH- (aq)
We also recall the various reactions with acids and bases:
1. Acid + Metal (more reactive than hydrogen) à Metal salt + Hydrogen gas
Take the reaction between sulfuric acid and magnesium as an example:
H2SO4 (aq) + Mg (s) à MgSO4 (aq) + H2 (g)
2. Acid + Base à Salt + Water
Take the reaction between nitric acid and copper (II) oxide as an example:
2HNO3 (aq) + CuO (s) à Cu(NO3)2 (aq) + H2O (l)
3. Acid + Carbonate compound à Salt + Carbon dioxide gas + Water
Take the reaction between hydrochloric acid and sodium bicarbonate as an example:
HCl (aq) + NaHCO3 (aq) à NaCl (aq) + CO2 (g) + H2O (l)
4. Alkali + Ammonium compound à Salt + Ammonia gas + Water
Take the reaction between sodium hydroxide and ammonium chloride as an example:
NaOH (aq) + NH4Cl (aq) à NaCl (aq) + NH3 (g) + H2O (l)
There we have it, just 4 types of reactions. Yet, these 4 types of reactions give way to a whole lot of applications.
One such application is using the Kipp’s apparatus. The Kipp’s apparatus, also known as the Kipp’s gas generator, is laboratory equipment used for generating gases such as carbon dioxide, hydrogen, hydrogen sulfide and other gases. A Kipp’s gas generator is made of 3 connected glass bowls sitting one on top of the other. The top glass bowl is removable and has a long tube that reaches through the middle bowl and into the bottom bowl. To produce hydrogen sulfide gas, the top bowl is removed and pieces of iron (II) sulfide are placed in the centre bowl of the Kipp’s apparatus. The top bowl is then replaced. Hydrochloric acid is poured into the top bowl. The acid flows down the long tube into the bottom bowl. The bottom bowl is connected to the middle bowl, thus when the bottom bowl is filled, the acid begins to fill the middle bowl and covers the iron (II) sulfide. Hydrogen sulfide is produced in the reaction between iron (II) sulfide and hydrochloric acid:
FeS (s) + HCl (aq) à FeCl2 (aq) + H2S (g)
Once gas production begins, a stoppered valve in the centre bowl allows the chemist to control how much gas escapes. When the valve is opened, the hydrogen sulfide gas can be collected. When it is closed, gas pressure builds up in the apparatus and forces the acid out of the centre bowl and stops gas production. The gas production can be restarted by releasing the built-up gas. Hydrogen sulfide is often used in analytical chemistry. Carbon dioxide and hydrogen gas can also be produced by placing a carbonate compound or metal respectively into the middle bowl instead of iron (II) sulfide.
Another use of acids is in making fire extinguishers. One kind of fire extinguisher is the water extinguisher – what an irony. It works by using high pressure to blast water at the fire. The water extinguisher has a glass vial that contains sulfuric acid. The extinguisher’s body contains baking soda solution. When the operator pushes the plunger down to break the vial, the two chemicals react, and water and carbon dioxide are produced:
H2SO4 (aq) + 2NaHCO3 (aq) à Na2SO4 (aq) + H2O (l) + CO2 (g)
As the gas pressure (carbon dioxide) inside the extinguisher increases, it pushes a jet of water out of the extinguisher’s nozzle.
Another use for acids (more specifically sulfuric acid) is in car batteries. Car batteries are a type called a lead-acid storage battery. In a lead-acid battery, the negative electrode is a plate made out of the metal lead. The positive electrode is made of lead (IV) dioxide. The electrolyte is sulfuric acid which ionizes in water:
H2SO4 (aq) à 2H+ (aq) + SO42- (aq)
At the negative electrode, the sulfate ions (SO42-) oxidize lead to produce electrons:
Pb (s) + SO42- (aq) à PbSO4 (s) + 2e-
At the positive electrode, lead (IV) dioxide reacts with electrons from the negative electrode, together with hydrogen and sulfate ions, and is reduced to lead (II) sulfate and water:
Pb4+ (aq) + 2O2+ (aq) + 4H+ (aq) + SO42- (aq) + 2e- à PbSO4 (s) H2O (l)
By combining these two half equations, we get:
Pb (s) + 2SO42- (aq) + Pb4+ (aq) 2O2+ (aq) + 4H+ (aq) à 2PbSO4 (s) + 2H2O
This is a redox reaction as there is a transfer of electrons from lead atoms to lead (IV) ions. This is a transfer of electrons produces electricity.
Perhaps one of the well-known uses of bases would be using calcium hydroxide to test for the presence of carbon dioxide. Below shows the reaction between calcium hydroxide and carbon dioxide:
Ca(OH)2 (aq) + CO2 (g) à CaCO3 (s) + H2O (l)
Calcium carbonate (CaCO3) is insoluble in water and precipitates out, which turns the calcium hydroxide “cloudy”. Ammonia is a base (more specifically an alkali) and has many uses. Ammonia (NH3) might not appear like an alkali because it does not contain the hydroxide ion. However, when mixed in water, in forms ammonium hydroxide:
NH3 (g) + H2O (l) ↔ NH4OH (aq)
One might notice the double arrow. That means that while some ammonia molecules are being converted to ammonium hydroxide, a few of the ammonium hydroxide molecules are being converted back to ammonia molecules. Hence, ammonia is a weak alkali. However, ammonia especially when concentrated is nonetheless corrosive. Ammonia evaporates easily and if concentrated vapors enter one’s lungs, it might burn through his lung cavity. Despite its dangers, ammonia is a well known cleaner.
Ammonia is also used to make nitric acid. It sounds contradicting, using an alkali to make an acid, however it is possible. Ammonia is first made to react with oxygen in the presence of metal catalyst to form nitric oxide:
4NH3 (g) + 5O2 (g) à 4NO (g) + 6H2O (l)
The nitric oxide is then oxidized to form nitrogen dioxide:
4NO (g) + 2O2 (g) à 4NO2 (g)
The nitrogen dioxide is bubbled through water with oxygen gas to form nitric acid:
4NO2 (g) + 2H2O (l) + O2 (g) à 4HNO3 (aq)
Nitric acid is an extremely useful acid. It can be used to make fertilizers such as ammonium nitrate which is rich in nitrogen. Plants need nitrogen to grow. Ammonia reacts with nitric acid to produce ammonium nitrate:
NH4OH (aq) + HNO3 (aq) à NH4NO3 (aq) + H2O (l)
It seems the acids and bases, while dangerous, can be very useful. It all depends on how much we understand about acids and bases. For example, someone who lacks acid and base knowledge might just add water to concentrated acid and cause his face to get burned. Knowing a bit about acid-base chemistry can help us understand the benefits and dangers of these chemicals. Who knows, one day acids and bases might even be the solution to global warming!
References
1. Lew, K. (2009). Acids and bases. New York, USA: Infobase Publishing.
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