Question
What is adsorption isotherm? Explain about Freundlich adsorption isotherm.
✓
Answer
1. Adsorption isotherms represents the variation of adsorption at constant temperature.
Adsorption isoterm can be studied quantitatively.
2. A plot between the amount of adsorbate adsorbed and pressure or concentration of adsorbate at constant temperature is called adsorption isotherms.
3. Freundlich adsorption isotherm. According to Freundlich $\frac{ x }{ m }= kP ^{1 / n }$
Where $x$ is the amount of adsorbate (or) adsorbed on ' $m$ ' $gm$ of adsorbent at a pressure of P. $k$ and $n$ are constants. Value of' $n$ ' is always less than unity.
4. This equation is applicable for adsorption of gases on solid surfaces. The Same equation becomes $\frac{ x }{ m }= kc ^{1 / n }$ when used for adsorption in solutions with ' $c ^{\prime}$ as concentration.
5. These equation quantitatively predict the effect of pressure (or concentration) on the adsorption of gases (or adsorbates) at constant temperature.
6. Taking log on both sides of equation
$\frac{ x }{ m }= kP ^{1 / n }$
$\log \frac{x}{m}=\log k +\frac{1}{n} \log P$
7. Hence the intercept represents the value of log $k$ and the slope $\frac{b}{q}$ gives $\frac{1}{n}$
8. This equation explains the increase of with increase in pressure. But experimental values shows the deviation at low pressure.
9. Limitations:
1. This equation is purely empirical and valid over a limited pressure range.
2. The value of $k$ and $n$ also found vary with temperatures. No theoretical explanations were given.
Adsorption isoterm can be studied quantitatively.
2. A plot between the amount of adsorbate adsorbed and pressure or concentration of adsorbate at constant temperature is called adsorption isotherms.
3. Freundlich adsorption isotherm. According to Freundlich $\frac{ x }{ m }= kP ^{1 / n }$
Where $x$ is the amount of adsorbate (or) adsorbed on ' $m$ ' $gm$ of adsorbent at a pressure of P. $k$ and $n$ are constants. Value of' $n$ ' is always less than unity.
4. This equation is applicable for adsorption of gases on solid surfaces. The Same equation becomes $\frac{ x }{ m }= kc ^{1 / n }$ when used for adsorption in solutions with ' $c ^{\prime}$ as concentration.
5. These equation quantitatively predict the effect of pressure (or concentration) on the adsorption of gases (or adsorbates) at constant temperature.
6. Taking log on both sides of equation
$\frac{ x }{ m }= kP ^{1 / n }$
$\log \frac{x}{m}=\log k +\frac{1}{n} \log P$
7. Hence the intercept represents the value of log $k$ and the slope $\frac{b}{q}$ gives $\frac{1}{n}$
8. This equation explains the increase of with increase in pressure. But experimental values shows the deviation at low pressure.
9. Limitations:
1. This equation is purely empirical and valid over a limited pressure range.
2. The value of $k$ and $n$ also found vary with temperatures. No theoretical explanations were given.
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