CONTENT LIST
Acids Bases and Salts
UNDERSTANDING THE CHEMICAL PROPERTIES OF ACIDS AND BASES
Properties of acids
- Acids are sour taste in an aqueous medium.
- They turn blue litmus red
- Phenolphthalein turns colorless in acid
- Aqueous solutions of acids are electrolytes mean conduct electricity.
Properties of bases
- Bases are bitter taste in an aqueous medium.
- They turn red litmus blue
- They can conduct electricity in aquous solution.
Indicators are substance that change color when added with acid or base. There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators.
How do Acids and Bases React with Metals?
Metals react with acid & displace hydrogen atoms from the acids & hydrogen gas is formed with another compound called a salt.
Acid + Metal → Salt + Hydrogen gas
(Mg+2HCl⟶MgCl2+H2)
(Mg+H2SO4⟶MgSO4+H2)
(Fe+H2SO4⟶FeSO4+H2)
When dilute sulphuric acid is added on zinc granules, then zinc displaces hydrogen it from the acid and forms zinc sulphate and hydrogen gas.
Zn + H2SO4 → ZnSO4 + H2
Hydrogen is a combustible gas which burns with a popping sound. This can be observed when a burning match stick bought near to the test tube.
How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?
Metal carbonates and Metal Hydrogencarbonates react with acids and produces corresponding metal salt, carbon dioxide and water.
Metal carbonate + Acid → Salt + Carbon dioxide + Water
Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
Example 1:
When sodium carbonate reacts hydrochloric acid forms sodium chloride, carbon dioxide and water.
Na2CO3 + 2HCl⟶2NaCl+CO2 +H2O
Example :
When sodium bicarbonate reacts hydrochloric acid forms sodium chloride, carbon dioxide and water.
NaHCO3 +HCl→NaCl+CO2+H2O
On passing the carbon dioxide gas evolved through lime water,Insoluble milky white precipitate of calcium carbonate and water is produced when calcium hydroxide reacts with carbon dioxide gas & the formed milkiness disappears.
Ca(OH)2+CO2 → CaCO3 + H2O
This excess amount of carbon dioxide is passed through lime water, the formation of calcium bicarbonate takes place.
CaCO3+H2O+CO2→Ca(HCO3)2.
How do Acids and Bases React with each other?
When an acid and a base reacts, they neutralize the acid and base properties & produced salt with water. The H(+) cation of the acid combines with the OH(-) anion of the base to form water &The compound formed by the cation of the base and the anion of the acid is called a salt.
The salt produced after neutralisation can be acidic, basic, or neutral & reaction mixture (product) heats up as a result of neutralisation, which is an exothermic reaction.
Acid+Base−→Salt+Water+heat (exothermic reaction)
Example 1:
When sodium hydroxide reacts hydrochloric acid forms sodium chloride and water.
NaOH+HCl→NaCl+H2O
Reaction of Metallic Oxides with Acids
Acids react with metal oxides and produce a salt and water.
Acid+(metal oxide )Base−→Salt+Water+heat (exothermic reaction)
Example 1:
Calcium is a metal and calcium oxide is a metallic oxide ( basic in nature). When hydrochloric acid reacts with calcium oxide, neutralization reaction takes place and calcium chloride along with water is formed.
2HCl + CaO --−→ CaCl2+H2O
Reaction of a Non-metallic Oxide with Base
Non-metal oxides are acidic . When a base reacts with non-metal oxide (acid), both neutralize each other resulting salt and water are produced.
Base + Non-metal oxide ⟶ Salt + Water
Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water.
On passing the carbon dioxide gas evolved through lime water,Insoluble milky white precipitate of calcium carbonate and water is produced when calcium hydroxide reacts with carbon dioxide gas & the formed milkiness disappears.
Ca(OH)2+CO2 → CaCO3 (salt) + H2O
WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?
All acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids.
Acids produce hydrogen ions, H+ (aq), in solution, which are responsible for their acidic properties.
The electric current is carried through the acidic solution by ions.
What Happens to an Acid or a Base in a Water Solution?
Acids in water solution dissociates H+ ions.
Bases in water solution dissociates OH- ions.
HOW STRONG ARE ACID OR BASE SOLUTIONS?
The solution is considered acidic if the pH value of the solution is less than 7; the solution is neutral if the pH value is around 7; if the pH value is greater than 7, the solution is called basic.
In acidic solotion [H+] > [OH–]
In basic solution [H+] < [OH–]
In pure water(nutaral solution) [H+] = [OH–]
The strength of acids and bases depends on the number of H+ ions and OH– ions produced, respectively.
Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids.
Bases that give rise to more OH- ions are said to be strong bases, and bases that give less OH- ions are said to be weak basess.
The process of dissolving an acid or a base in water is a highly exothermic .
The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns.
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH– ) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
What is pH scale?
pH scale is a commonly used measuring acidity or the basicity of a substance.
Importance of pH in Everyday Life
Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change.
When pH of rain water is less than 5.6, it is called acid rain.When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers.
Plants require a specific pH range for their healthy growth.
It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach. During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids.These antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.
Tooth decay starts when the pH of the mouth is lower than 5.5.
MORE ABOUT SALTS
Salts are ionic compound formed which are formed in the neutralization reaction between an acid and a base.
Salt or common salt is sodium chloride (NaCl) and is derived from reaction between sodium hydroxide and hydrochloric acid. This salt forms when chlorine and sodium ions bind together.
Family of Salts
Salts having the same positive or negative radicals are said to belong to a family. For example, NaCl and Na2SO4 belong to the family of sodium salts.
pH of Salts
Salts of a strong acid and a strong base are neutral with pH value of 7.
Salts of a strong acid and weak base are acidic with pH value less than 7 .
Salts of strong base and weak acid are basic in nature, with pH value more than 7.
Chemicals from Common Salt
Salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food.
Common salt — A raw material for chemicals
Common salt is the raw material for many chemical compound . They are :
Sodium hydroxide
Bleaching powder
Baking soda
Washing soda
Process of makingof Sodium hydroxide(NaOH)
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process.
2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2 (g) + H2(g)
Process of synthesis of Bleaching powder ( CaOCl2)
Chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2
]. Bleaching powder is represented as CaOCl2.
Ca(OH)2 + Cl2 → CaOCl2 + H2O
Bleaching powder is used
(i) for bleaching cotton and linen in the textile industry, for bleaching
wood pulp in paper factories and for bleaching washed clothes
in laundry;
(ii) as an oxidising agent in many chemical industries; and
(iii) to make drinking water free from germs.
Process of making of Baking soda(NaHCO3)
Baking soda is produced using sodium chloride as one of the raw materials.
NaCl + H2O + CO2 + NH4 → NH4Cl + NaHCO3
It is a mild non-corrosive basic salt. The following reaction takes place when it is
heated during cooking –
2NaHCO3→ Na2CO3 + H2O+CO2
Uses of Baking soda
(i)Reduces the acidity in the stomach.
(ii)Acts as an antacid which is used to treat stomach upset and indigestion.
(iii)Used in the process of washing as a water softener.
(iV)Due to the formation of soapy foam, it is used in fire extinguishers.
(V) It is also used in soda-acid fire extinguishers.
Process of making of Washing soda
Another chemical that can be obtained from sodium chloride is Na2CO3.10H2O (washing soda).
Sodium carbonate can be obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda.
(1) Cold and concentrated solution of sodium chloride is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate.
(2) Sodium hydrogen carbonate is separated , dried and heated.On heating sodium hydrogen carbonate decomposes to form sodium carbonate.
(3) Anhydrous sodium carbonate is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation.
2NaHCO3→ Na2CO3 + H2O+CO2
Uses of Washing Soda
(1) To remove permanent hardness of water.
(2) Used in the manufacture of glass,soap,paper.
(3) Used for washing clothes.
(4) To manufacture of borax.
Are the Crystals of Salts really Dry?
The crystals are realy dry because the water present in them doesn't behave like water but gives them a crystalline structure. Crystals of Copper sulphate appears dry but contain water of crystallization.
Chemical formula for hydrated copper sulphate is CuSO4.5H2O.
Gypsum has two water molecules as water of cyrstallisation. It has the chemicalformula CaSO4.2H2O.On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO4 .1/2H2O)). This is called Plaster of Peris.