Question on Avogadro's Law

0.5 moles of a gas occupied 3 liters at certain temperature and pressure. If 0.25 moles of the same gas was added to it how much volume will it occupy at the same temperature and pressure?

Question on Atomic Structure

If electrons in a sample of hydrogen atoms were excited to the 5th excited state how many different spectral lines will be observed when all of them return to the ground state?

Curdling of Milk

What happens when we add few drops of lime juice when the milk is about to boil over? It curldes, isn’t it? Why does this happen?

Milk is a colloid. It is a lyophobic colloid to be more specific. See, colloids are of two types, the lyophilic ones and the lyophobic ones. The lyophilic ones are those from which the dispersed phase (solute) and the dispersion medium (solvent) can be separated and also can be re-mixed into the original colloid. The lyophobic ones on the other hand can be separated by creating an imbalance in its pH but can’t be re-mixed into the original colloid. Once it is separated, it is separated. There is no going back.

When we add lemon juice to the hot milk it decreases the pH of milk and the dissolved particles (or the compounds that milk is made of) come out of the solvent in chunks. If we try mixing these chunks back with the solvent we will not be able to do so.

The Chemistry of Ozone

Scientists were observing rotten egg smell when an electric discharge was passed through air. It was later attributed that the rotten egg smell was due to the formation of a gas and it was named ozone. The name ozone derives from ozein (ὄζειν), the Greek word for smell.

Preparation of Ozone

Ozone is prepared by passing an electric discharge through dry oxygen. The device in which the preparation is done is known as the ozonizer. The following reactions take place.

O₂ + Energy ® O + O

O₂ + O ® O₃

The procedure usually invoves passing of dry oxygen from one end of the apparatus and obtaining ozonized oxygen from the other end. This mixture of ozone and oxygen is then passed into a zone of cool temperature where the temperature is around -115°C. At this temperature ozone condenses into a liquid and thus can be separated from oxygen.

Chemical Reactions of Ozone

Ozone acts as a good oxidising agent and its standard oxidation potential is +2.07V.

The preparation of ozone from oxygen is endothermic implying that the reverse reaction should be exothermic. Ozone easily decomposes to give dioxygen (O₂) and nascent oxygen (O).

Following are some of the reactions illustrating the oxidising property of ozone;

2FeSO₄ + H₂SO₄ + O₃ ® Fe₂(SO₄)₃ + H₂O + O₂

PbS + 4O₃ ® PbSO₄ + 4O₂

NaNO₂ + O₃ ® NaNO₃ + O₂

Na₂SO₃ + O₃ ® Na₂SO₄ + O₂

S + H₂O + O₃ ® H₂SO₄ + O₂

2As + 3H₂O + 5O₃ ® 2H₃AsO₄ + 5 O₂

2P + 3H₂O + 5O₃ ® 2H₃PO₄ + 5 O₂

Neutral KI is converted into I₂ and alkaline KI is converted into KIO₃ and KIO₄ according to the following reactions;

2KI + H₂O + O₃ ® 2KOH + I₂ + O₂

KI + 3O₃ ® KIO₃ + 3O₂

KI + 4O₃ ® KIO₄ + 4O₂

Moist I₂ and dry I₂ are converted into HIO₃ and I₄O₉ respectively according to the following reactions;

5O₃ + H₂O + I₂ ® 5O₂ + HIO₃

9O₃ + I₂ ® 9O₂ + I₄O₉

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide and in aqueous phase produced H₂SO₄ as well.

H₂S + O₃ ® SO₂ + H₂O

H₂S + O₃ ® S + O₂ + H₂O

3H₂S + 4O₃ ® 3H₂SO₄

When mercury reacts with O₃ it forms Hg₂O and starts sticking to the sides of the test tube. Because of this mercury loses its meniscus. This is called as the Tailing of Mercury.

2Hg + O₃ ® Hg₂O + O₂

Another major use of ozone is in the ozonolysis of unsaturated organic compounds. The reaction can be shown as below;

The structure of Ozone is given below. It exists as resonating structures.

Avogadro's Law

Avogadro’s Law states that, “Equal volume of all gases at same the pressure and temperature contain the same number of molecules_.”

Mathematically Avogadro’s Law can be written as follows;

V a n

or

\[ \frac{V}{n} = K \]

Where V is the volume of the gas and n is the number of moles of the gas and K is a Constant.

According to Ideal Gas Law,

PV = nRT

\[ V = \frac {nRT}{P} \]

\[ \frac {V}{n} = \frac {RT}{P} \]

\( \frac{RT}{P} \) is the constant where R is the universal gas constant, T is the temperature on Kelvin Scale and P is pressure.

Avogadro’s Law can also be expressed as follows;

\[ \frac{V_1}{n_1} = \frac {V_2}{n_2} \]

The above expression is very useful for comparing the same substance under two different sets of conditions.

The Laughing Gas

Nitrous Oxide is one of the important oxides of nitrogen which is commonly known as the Laughing Gas. Its molecular formula is N₂O. It is a colourless gas but has a pleasant smell. It is heavier than air and is fairly soluble in cold water and is insoluble in hot water.

When inhaled in prescribed amounts it produces euphoric effect; however when inhaled more than the prescribed amount it could even be fatal. Because of this euphoric laugh it induces it is used as an anaesthetic.

Nitrous Oxide can be assumed to be the anhydride of hyponitrous acid (H₂N₂O₂). But, when dissolved in water N₂O does not produce H₂N₂O₂. It is a neutral oxide and hence is neutral to litmus.

Nitrous Oxide is prepared by the reaction between NH₄SO₄ and NaNO₃ at a temperature below 240°C. NH₄SO₄ and NaNO₃ react together to form NH₄NO₃ which on heating decomposes to produce N₂O and H₂O. 

The reactions are as follows;

NH₄SO₄ + NaNO₃ ® NaSO₄ + NH₄NO₃

NH₄NO₃ ® N₂O + H₂O

N₂O when heated above 500°C decomposes into N₂ and O₂. Thus it supports combustion of Sulphur and Phosphorus turning them into SO₂ and P₂O₅ respectively. It also converts Cu into CuO. While converting these elements into their oxides, N₂O gets itself converted into N₂.

The structure of N₂O is as follows;