Friday, November 11, 2016

Chapter 6.1 - Basic details about Acids

In the previous section, we saw the relation between non-metallic oxides and acids. In this section we will see metallic oxides.
Let us do an experiment:
For this experiment, we need 'quick lime'. Quick lime is the common name for calcium oxide (CaO). It is obtained by heating naturally occuring substances like limestone or sea shells. Limestone and sea shells are calcium carbonates (CaCO3). When it is heated, carbon dioxide escapes, and CaO is left behind. Let us write the equation:
Reactants:
    ♦ Calcium carbonate. One molecule is CaCO3
Products
    ♦ Calcium oxide. One molecule is CaO
    ♦ Carbon dioxide. One molecule is CO2
• So the skeletal equation is: CaCO→ CaO + CO2 . This is a balanced equation.

1. So we have an oxide CaO. An oxide of a metal calcium. So it is a metallic oxide. We are going to do the experiment with it. 
2. Take water in a beaker. Add some quick lime to it and stir well. A reaction takes place. It is between CaO and water. If we touch the sides of the beaker, we can feel heat. This is because the reaction releases heat. In other words, it is an exothermic reaction.  Let us write the equation:
Reactants:
    ♦ Calcium oxide. One molecule is CaO
    ♦ Water. One molecule is H2O
Product
    ♦ Calcium hydroxide. One molecule is Ca(OH)2
• So the skeletal equation is: CaO + H2O → Ca(OH)2. This is a balanced equation.

3. How are we going to use this calcium hydroxide?
• We added quick lime to water. The quick lime was converted into Ca(OH)2 (calcium hydroxide).
• Some water will be used up for the reaction. But there will still be some excess water. Because all the water will not be used up. 
• So the product, which is Ca(OH)2, will dissolve in the excess water. And we get a solution of Ca(OH)2 in water. 
• But all the Ca(OH)2 will not dissolve in water. Some of it will precipitate to the bottom of the beaker. The clear portion above the precipitate is called 'supernate'.

■ We see a separation as a precipitate and a supernate because, there is not enough water for all the Ca(OH)2 to dissolve. That portion of Ca(OH)2, which is unable to dissolve, will form the precipitate. That means, the supernate is a solution of Ca(OH)2 in water. 

Take some of this supernate in a test tube. Add a drop of red litmus to it. We can see that, a blue colour appears in the solution. So we can infer that Ca(OH)2 is alkaline in nature. Let us analyse what we have done:
• We took quick lime. It is an oxide of calcium, which is a metal. So quick lime is a metallic oxide.
• This metallic oxide, when added to water, gave an alkali. We can say this: Metallic oxides are generally alkaline in nature.


■ So, if we are given a list of oxides, we can classify them as acidic or alkaline. Given below is a list:
SO3NO2, CaO, K2OP2O5, Na2OCO2, MgO
Let us classify them into acidic and alkaline:

Acidic oxides Alkaline oxides
SO3 CaO 
P2O5 K2O
CO2 Na2O
NO2 MgO
The above classification is based on the following two facts:
■ Non-metallic oxides are acidic in nature
■ Metallic oxides are alkaline in nature

Following are some of the properties of acids and alkalies:
■ Acids:
• Have sour taste
• Changes blue litmus to red
• Produces carbon dioxide gas on reacting with carbonates
• Produces hydrogen gas on reacting with metals like Fe, Mg etc.,
■ Alkalies:
• Have bitter taste
• Changes red litmus to blue
• Slippery to touch

Let us explore some more about acids:
Acids can be classified as Mineral acids and Organic acids
■ Mineral acids are also known as inorganic acids. They are prepared from one or more inorganic compounds, and are stronger than organic acids. Most common mineral acids are Sulphuric acid, Nitric acid and Hydrochloric acid. Mineral acids should not be tasted.
■ Organic acids are those acids found in naturally occuring organic compounds. They are generally weak acids. The sour taste of some of the food items is due to the presence of organic acids. Some examples are given below:
• Lime – Citric acid
• Tamarind – Tartaric acid
• Tomato – Oxalic acid
• Sour milk – Lactic acid

• Vinegar – Acetic acid


The common factor in acids

Let us write the chemical formula of some commonly known acids:

Name of acid Chemical formula
Hydrochloric acid HCl
Nitric acid
HNO3
Acetic acid CH3COOH
Carbonic acid H2CO3
Sulphuric acid H2SO4
• From the above list, we find that hydrogen is present in all the acids. So hydrogen is a common factor in acids.

Hydrogen is the component present in all acids and is responsible for the common properties of acids. 

Let us see a reaction in which an acid is a reactant:
Hydrochloric acid reacts with iron to give ferric chloride and hydrogen. Let us write the equation:
Reactants:
    ♦ Iron. One molecule is Fe
    ♦ Dilute Hydrochloric acid. One molecule is HCl.
Products:
    ♦ Ferric chloride. One molecule is FeCl2.
    ♦ Carbon dioxide. One molecule is CO2.
• So skeletal equation is:
Fe + HCl → FeCl2 + H2. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: Fe + HCl → FeCl2 + H2
Step 2: Fe + 2HCl → FeCl2 + H2

Reactants Products
Fe H Cl Fe H Cl
Step 1 1 1 1 1 2 2
Step 2 1 2 2 1 2 2
■ So the balanced equation isFe + 2HCl → FeCl2 + H2
We can say this:
The acid HCl, reacts with the metal iron to liberate hydrogen. In fact, it is a common property of all acids:
■ All acids react with metals to liberate hydrogen
We saw another example in the preparation of hydrogen in the lab. Details here.

We will now discuss about acids in detail:
We know that in HCl:
• There is a covalent bonding between H and Cl.
• Cl is more electronegative, and so attracts the shared pair of electrons towards itself
• So the hydrogen has a partial positive charge, and chlorine has a partial negative charge. 
• We have seen the above details here
• When HCl is mixed with water, the bond between H and Cl breaks. The hydrogen will have to leave with out taking it's electron.
• This is because, Cl is more electronegative, and so, will keep both the shared electrons
• The H atom thus becomes H+ ion and Cl becomes Cl-
• The Hions will go towards the water molecules. They will attach with the water molecules. 
• The water molecules will then become ions because of the positive charge brought by the Hions. 
• The new ion is written as H3O+. It is called the hydronium ion
• These hydronium ions are responsible for the characteristics of acids.

Consider the following scenario:
We have a situation in which we want the effects of an acid. We have a bottle of HCl. If we keep the HCl in the bottle itself, or pour it into another suitable container, there is no effect. To see the effects of acid, the acid must come in contact with other materials.

Note that it is very important to handle acids carefully. All safety measures should be taken. Some of the compulsory safety measures are:
• Wearing lab coat or lab apron
• Wearing safety goggles for protection of eyes
• Using acid resistant gloves
Experiments should be performed only under the guidance of authorised professionals

• We have seen the effect when HCl comes into contact with water: The hydronium ions (H3O+)are formed, and that gives the acidic effect.
• If more H+ ions are liberated, more hydronium ions will be formed, and there will be greater acidic effect
■ If one molecule of an acid gives one H+ ion, It is a monobasic acid
Eg: HCl → H+ + Cl-.
HNO→ H+ + NO3-
■ If one molecule of an acid gives two H+ ions, It is a dibasic acid
Eg: H2SO4 → 2H+ + SO42-
It may be noted that, the two H+ ions are obtained as a result of two steps as shown below:
Step 1: H2SO4 dissociates into H+ and HSO4-
Step 2: HSO4- dissociates into H+ and SO42-
Step 1 gives one H+ and one HSO4-. But after step 2, the HSO4- is no longer present. It has dissociated into H+ and SO42-
So we get a total of two H+ ions

Another example: 
H2CO3 → 2H+ + CO32-.
Here also, the two H+ ions are obtained as a result of two steps as shown below:
Step 1: H2CO3 dissociates into H+ and HCO3-
Step 2: HCO3- dissociates into H+ and CO32-.
So we get a total of two H+ ions
■ If one molecule of an acid gives three H+ ions, It is a tribasic acid
Eg: H3PO4 → 3H+ + PO43-
Here, the three H+ ions are obtained as a result of three steps as shown below:
Step 1: H3PO4 dissociates into H+ and H2PO4-
Step 2: H2PO4dissociates into H+ and  HPO42-.
Step 3: HPO42- dissociates into H+ and  PO43-.
So we get a total of three H+ ions

We have seen some of the important details about acids. In the next section, we will discuss about alkalies. 

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