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In my book the laboratory preparation of dinitrogen is as follows:

$$\ce{NH4Cl(aq) + NaNO2(aq) -> N2(g) + 2H2O(l) + NaCl(aq)}$$

Small amounts of $\ce{NO}$, $\ce{HNO3}$ are also formed in this reaction: these impurities can be removed by passing the gas through aqueous sulfuric acid containing potassium dichromate. $\ce{N2}$ is collected by downward displacement of water.

So my question is how does it actually happen that impurities can be removed by passing the gas through aqueous sulfuric acid containing potassium dichromate? What are the reactions involved? Any video in which the following process is being illustrated will be helpful to understand.

I have understood what does it mean when it is said that $\ce{N2}$ is collected by downward displacement of water.This video https://www.youtube.com/watch?v=Ol-2JN01Cfo illustrates it nicely.

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The collection of nitrogen $(\ce{N2})$ by downward displacement of water works since nitrogen is poorly soluble in water (at room temperature and standard pressure) and does not chemically react with water.

For the same reason, some impurities (aerosols, water-soluble gases and vapours – for example $\ce{HNO3}$) can be removed by passing the stream of nitrogen through a washing bottle that is filled with water.

Nitric oxide $(\ce{NO})$ is only poorly soluble in water and therefore cannot be removed by this method. However, $\ce{NO}$ can be oxidized. Strong oxidizing agents can convert $\ce{NO}$ into $\ce{NO3-}$:

$$\ce{NO + 2H2O <=> NO3- + 4H+ + 3e-}\quad E^\circ = +0.959\ \mathrm V$$

A suitable strong oxidizing agent is acidic dichromate $(\ce{Cr2O7^2-})$:

$$\ce{2Cr^3+ + 7H2O <=> Cr2O7^2- +14H+ + 6e-}\quad E^\circ = +1.38\ \mathrm V$$

The balanced overall reaction is:

$$\ce{2NO + Cr2O7^2- +6H+ <=> 2NO3- + 2Cr^3+ + 3H2O}$$

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