Question

Experiment 7: Thin Layer Chromatography

1. What is the name of the technique you will be using in experiment No.7 lab?
   1. Thin Layer Chromatography
   2. Mass Spectrometry
   3. Thick Liquid Calorimetry
   4. Liquid Chamber Chromatography

2. TLC plates separate molecules on the basis of:
   1. Polarity
   2. Color
   3. Size
   4. Bond orientation

3. The mixture is dissolved in a fluid called the ___________ phase, which carries it through   a structure holding another material called the __________ phase.

4. The polarity of a bond between two elements can be best determined by

1. The difference in electronegativity between the elements
2. The difference in first ionization energy between the elements
3. The number of electrons shared in the bond
4. The difference in atomic radius between the elements

5. EXPLAIN the phrase “like dissolves like.”

Answer 1

1. The name of the technique is Thin Layer Chromatography as the name of the experiment suggests. Thin layer chromatography also written as TLC is a technique that works on the principle of varying solubility of different compounds in different solvent/compounds and is used in qualitative and quantitative analysis to separate different compounds in a non volatile mixture.

Hence option a) Thin layer chromatography is correct.

2. TLC uses the fact that like dissolves like.​​​ This means that polar compounds will dissolve/ adsorb polar compounds and vice versa. Hence TLC separates the molecules on the basis of their polarity.

The TLC plate consists of silica adsorbent which is highly polar. Hence more polar compounds in the solution will stick more on the TLC plate and move slowly while as less polar compounds will stick less on the TLC plate and move faster and this way a separation of compounds is achieved.

Hence option a) polarity is correct.

3. TLC uses two kinds of phases, one phase is the fluid which is called the mobile phase in which the mixture (which is to be separated) is dissolved. As this fluid moves through the structure, that is why it is called mobile phase.

The structure through which the mobile phase moves is called stationary phase. It doesn't move hence called as stationary phase. This stationary phase adsorbs the compounds present in the mobile phase on the basis of their polarity, as discussed above.

Therefore the given statement goes as follows: The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase.

4. Electronegativity is a measure of the affinity towards electrons. More the electronegativity, more will be the affinity of the element to get electrons.

Hence when two elements of different electronegativities are in a bond with each other, the element with more electronegativity pulls the electrons towards itself, thus causing the electrons to be shifted more towards that element. This shift is basically termed as polarity.

Hence it is the electronegativity that determines the polarity of a bond between teo elements.

Therefore option a) difference in the electronegativity is correct

5. The phrase Like dissolves Like is already discussed above in part 2). It says that polar solvents will dissolve polar solutes and vice versa. For example, water is a polar solvent, hence it dissolves NaCl which is a polar solute. On the other hand, oil is non polar, therefore does not dissolve in water.