Whenever the word ‘salt’ is used by people they usually refer to common salt or table salt with the general formula NaCl and written chemically as sodium chloride. This is the edible salt I have spent a whole week talking about, including chemical structure and its importance in history. (Week 9) In Chemistry, the word salt is used more generally and includes the product formed whenever an acid and a base ( that we studied last week) react with each other. The reaction is called a neutralization reaction, since the original properties of the acid or the base are neutralized and a new substance, a salt, and water are formed:
HCl + NaOH → NaCl + H2O
HCl + KOH → KCl + H2O
2HCl + Ca(OH)2 → CaCl2 + 2H2O
HNO3 + NaOH → NaNO3 +H2O
HNO3 + KOH → KNO3 + H2O
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
H2SO4 + 2NaOH → Na2SO4 + 2H2O
H2SO4 + 2KOH → K2SO4 +2H2O
H2SO4 + Ca(OH)2 → CaSO4 +2H2O
As you can see there are several salts that can be formed when an acid and a base react. The salts are sodium chloride, potassium chloride, calcium chloride, sodium nitrate, potassium nitrate, calcium nitrate, sodium sulfate, potassium sulfate, calcium sulfate, respectively. So with three bases and three acids we have a total of 9 salts! The other product is always water. So now we know that another property of an acid is that it can react with a base to form salt and water. Of course another property of a base is that it can react with an acid to form salt and water.
Last week, we talked about H or hydrogen ion as characteristic of all acids; in actuality a hydrogen ion is just a bare proton and does not exist by itself in an aqueous (in water) solution. It exists as a hydronium ion shown below:
So, when hydrogen chloride HCl gas reacts with water to form hydrochloric acid, which is a liquid, it forms the aqueous hydronium ion and the chloride ion:
HCl (g) + H2O (l) → H3O+(aq) + Cl–(aq)
According to the Br∅nsted Lowry Theory, the proton donor is the acid, HCl and the proton acceptor is the base, H3O+, the hydronium ion. It is important to rewrite all the above equations in the ionic form as well as remember that since the reaction can only take place in the the presence of water molecules so the hydrogen ion is not just a proton, but a hyrodinium ion.(Ref. 1)
Old fire extinguishers were made using baking soda and acid to generate carbon dioxide. Baking soda is sodium bi-carbonate, NaHCO3, a salt, and the reaction takes place when the bottle containing acid is broken during use. (Ref.2) Today we use pressurized carbon dioxide with a propellant. Sometimes water sprinklers are set up in homes without anything else for extinguishing fires. So salts are not used these days in fire extinguishers. (Ref. 3)
But salts play an important part in industry, medicine and everyday life. Lives are saved by UNICEF when they give people a sodium chloride and sugar solution to drink when dehydrated and suffering from heat exhaustion, thereby supplying oral re-hydration therapy. (Ref. 4) We already know that sodium carbonate is used in the glass-making industry. Epsom salts or magnesium sulfate is used for minor home remedies and treatments.(Fig.5) Calcium sulfate is used in food preparation, especially in making soy milk coagulate (turn solid or semi-solid) to tofu, as a calcium supplement and for minor joint aches and pains, as well as a fertilizer.(Fig.6) Needless to say, there are several more salts not mentioned, but you now have a taste of the applications of different salts in different fields.
Activities for Middle School Teachers:
We have not even considered acids like phosphoric acid, iodic acid and bromic acid. Add to that, common bases with barium, Ba, and lithium, Li and the list of salts increases. To calculate the number of salts that can exist, list all the possible acids and bases and use the multiplication principle from mathematics to calculate the total possible salts.
Starting with a single acid like hydrochloric acid, HCl (aq), if you replace the hydogen ion, (or hydronium ion in aqueous solutions) by metal ions, check how many salts you can create! (Ref.1)
Last week we talked about moles and molarity. Molarity is really the concentration of the acid/ base or any chemical compound, usually in water or what is known as an aqueous solution. Knowing the moles present from an equation, you should be able to calculate the products in grams if the reactants are also given in grams. It will be a simple proportionality problem. Try it.
Some uses of a few salts is given above; do a study on the uses of salts in industry, medicine, manufacturing of goods.
Nuggets of Information:
Several theories have been put forward to explain acids and bases. Besides explaining their physical properties, in 1884, Svante Arrhenius defined acids and bases as:
An Arrhenius acid solution contains an excess of H+ ions.
An Arrhenius Base solution contains an excess of OH– ions.
Then in 1923, Br∅nsted Lowry (a Danish and an English chemist) put forward their definitions indicated above. Finally Lewis made it even more general by saying that an acid accepts electrons and a base donates electrons. (Ref.1)
Hydroxides of some metals like zinc, Zn, aluminium, Al, and Chromium,Cr are amphoteric. This means they can act as both acids and bases. When treated with a strong acid, they act like bases and when treated with a strong base, they will react as an acid (Ref.1):
Zn(OH)2 (s) + 2HCl (aq) → ZnCl2 (aq) + 2H2O (l)
Zn(OH)2 (s) + 2NaOH (aq) → Na2Zn(OH)4 (aq)
What is the ‘cool fizz’ we feel when we drink soft drinks? Chemists say that the tingling on our tongues is caused by chemisthesis. This sensation does not involve taste or odor receptors, but is due to a chemically induced sensation. Protons, H+ are released when an enzyme called carbonic anydrase reacts with carbon dioxide. The nerve-endings are acidified by the protons and hence there is tingling.(Ref.1)
As seen above, there are probably more salts than acids and bases. Most of the rocks and minerals found in nature are some form of a salt. Metal mining involves finding metals like iron, Fe, gold, Au, and other metals as salts. Salts are usually crystalline and have high melting and boiling points. (Ref.1)
All these compounds are ionic compounds and exist as charged particles. Just like acids and bases, salts are electrolytes. Electrolytes are substances that conduct electricity in an aqueous solution. (Ref.1)
Some of the formations in caves called stalactites and stalagmites are nothing but salts of calcium carbonate. Salts like carbonates and bicarbonates react with acids to release carbon dioxide, along with an ionic salt and water:
2HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + H2O (l) + CO2 (g)
HCl (aq) + NaHCO3 (aq) → NaCl (aq) + H2O (l) + CO2 (g)
Last week we only considered strong acids and bases. These are also strong electrolytes , which means that they are almost 100% ionized in an aqueous solution. But there are also weak inorganic acids and bases that do not dissociate completely to H+, or more exactly, H3O+,or OH–. Examples of weak acids are nitrous acid, carbonic acid and weak bases include ammonia., also called weak electrolytes. These acids and base do not get ionized completely. If you connect an electric bulb to a strong electrolyte like sodium choride, hydrochloric acid or sodium hydroxide, the bulb will glow brightly. With weak electrolytes, the bulb will glow dimly.(Ref.1)
References:
- Hein, Morris & Arena, Susan, Foundations of College Chemistry,pp.375 -79(John Wiley and sons, 2007)
- http://www.answers.com/Q/How_do_soda-acid_fire_extinguishers_work
- http://www.scientificamerican.com/article/what-chemicals-are-used-i/
- http://www.csd-i.org/oral-rehydration-techniques/
- http://www.care2.com/greenliving/use-epsom-salts-13-wonderful-ways.html
- http://www.answers.com/Q/What_are_some_uses_of_calcium_sulfate
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