Practical Organic Chemistry
7.0 Solved Example (Subjective)
7.0 Solved Example (Subjective)
Example 1: Gelatin is generally added to ice creams.
Solution: Ice cream is an emulsion of milk or cream in water (oil in water). Gelatin is added in the preparation of ice cream to act as emulsifier i.e., it helps in the formation of a stable emulsion.
Example 2: The addition of ferric hydroxide solution to arsenious sulphide solution results in the precipitation of both.
Solution: This is possible only in case equimolar solutions (solutions with a same number of moles) are mixed. Since the ferric hydroxide sol carries positive charge and arsenic oxide sol is negatively charged on mixing they will get their charge neutralized and will be coagulated.
Example 3: Lyophilic solar are called reversible colloids.
Solution: Lyophilic sols are generally known as reversible colloids. In fact, if the dispersed phase is removed completely the colloidal solution can be formed again by mixing the dispersed phase (residue) left in a fresh sample of the dispersion medium. for example, if a colloidal sol of starch in water is dried completely, it can be reformed by mixing the residue with fresh water.
Example 4: Artificial rain can be caused by spraying charged dust particles over colloids.
Solution: Clouds represent the colloidal solutions of water drops in the air (liquid in gas type). These drops are expected to carry some charge (positive or negative). In order to neutralize the charge on these, charged dust particles carrying opposite charge are sprayed over a certain layer of cloud. These will neutralize the charge on water droplets resulting in their coagulation. The bigger water drops can no longer be retained by the atmosphere and will result in the artificial rain.
Example 5: Ferric hydroxide sol is more readily coagulated by $N{a_3}P{O_4}$ in comparison to $KCl.$
Solution: Ferric hydroxide sol is positively charged and to cause its coagulation, ions carrying negative charge are needed. Since ions have higher negative charge than Cl– ions, therefore, $N{a_3}P{O_4}$ coagulates the ferric hydroxide sol most efficiently.
Example 6: Delta is generally formed when the river meets the ocean.
Solution: River water is generally muddy and carries along with colloidal dust particles which are charged in nature. Seawater contains a large number of electrolytes. When the river comes in contact with the sea water, the colloidal particles get their charge neutralized by the oppositely charged ions present in seawater and are coagulated. This ultimately results in a hard solid mass known as a delta.
Example 7: The layer of fat in the pans used for preparing soaps can be removed by adding boiling washing soda solution. How will you account for it?
Solution: Washing soda $\left( {N{a_2}C{O_3}} \right)$ gets hydrolysed to form $NaOH$ as follows:\[N{a_2}C{O_3} + {H_2}O\xrightarrow{{(boil)}}2NaOH + {H_2}O + C{O_2}\]
$NaOH$ reacts with fat which is a triglyceride to form soap and glycerol by saponification reaction. The soap thus formed helps in cleaning the pan.
Example 8: $Sn{O_2}$ forms a positively charged colloidal sol in the acidic medium and negatively charged sol in the basic medium. Explain.
Solution: $Sn{O_2}$ is of amphoteric in nature which means that it can react with both acids and bases. With an acid such as $HCl,$ if forms $SnC{l_4}.$ The $S{n^{4 + }}$ ions are adsorbed on the surface of $Sn{O_2}$ to give a positively charged colloidal sol.\[Sn{O_2} + 4HCl\xrightarrow{{}}SnC{l_4} + 2{H_2}O\]\[Sn{O_2} + S{n^{4 + }}\xrightarrow{{}}\mathop {[Sn{O_2}]S{n^{4 + }}}\limits_{Positively\;ch\arg ed} \] Similarly with base like $NaOH,$ if forms sodium stannate $\left( {N{a_2}Sn{O_3}} \right).$ The stannate ions get adsorbed on the surface of $Sn{O_2}$ to give negatively charged colloidal sol.\[Sn{O_2} + 2NaOH\xrightarrow{{}}N{a_2}Sn{O_3}\]\[Sn{O_2} + SnO_3^{2-}\xrightarrow{{}}\mathop {[Sn{O_2}]SnO_3^{2--}}\limits_{negatively\;ch\arg ed} \]
Example 9: In fine weather, we normally see red colour of setting sun. Can you co-relate this with the colloids?
Solution: We all know that dust particles are present in the atmosphere. When light emitted by the setting sun passes through the blanket of dust, the blue part of the light is scattered away from our eyes and at the same time, the red colour is seen. Thus, the setting sun has a red colour.
Example 10: Which out of the following solutions having the same concentration will be most effective in causing the coagulation of the arsenic sulphide sol which is yellowing colour: $KCl,$ $MgC{l_2},$ $AlC{l_3}$ or $N{a_3}P{O_4}$?
Solution: Arsenic sulphide $\left( {A{s_2}{S_3}} \right)$ sol has a negative charge on it. To cause it coagulation or precipitation the active ions must be positively charged. According to hardy-Schulze rule, greater the magnitude of the positive charge on the ion more will be its coagulating power. Thus, $AlC{l_3}$ containing $A{l^{3 + }}$ ions will be the most effective in causing the coagulation of the sol.
Example 11: The coagulation values of the electrolytes $AlC{l_3}$ and $NaCl$ for ${A{s_2}{S_3}}$ sol are $0.093$ and $52$ respectively. How many times $AlC{l_3}$ has greater coagulating power than $NaCl$?
Solution: It may be noted that the coagulating powers of the electrolytes are inversely proportional to their coagulating values. Thus, $$\frac{{{\text{Coagulatign power of AlC}}{{\text{l}}_{\text{3}}}}}{{{\text{Coagulating power of NaCl}}}} = \frac{{52}}{{0.093}} = 559$$ Thus, coagulating power of $AlC{l_3}$ is $559$ times more than that of $NaCl.$
Example 12: Draw a flow diagram for the separation and recovery in almost quantitative yield of a mixture of the water $–$ insoluble compounds $PhCHO,$ $PhNM{e_2},$ $PhCl,$ $p - Me{C_6}{H_4}OH$ and $PhCOOH.$
Solution:
