What are the differences between adsorption isobars for physisorption and chemisorption ?
Answers
Adsorption isotherm is a graph between the amount adsorbed and the pressure (P) of the adsorbate at constant temperature.
In the above adsorption isotherm, reaches its maximum value at value of equilibrium pressure. represents the saturation pressure and this state is called saturation state.
At high pressure, the value becomes independent of value of P, i.e.
At low pressure, the graph is nearly straight, i.e.
For small values of pressure,
where,
n is positive integer
k is constant depending on the nature of adsorbent and adsorbate.
The above relationship is known as Freundlich adsorption isotherm.
The validity of the equation can be checked by taking logarithm on both sides,
A graph between and log P will give a straight line with slope equal to and intercept equal to log k.
In this graph, some deviation from linearity is shown at high pressures. The relation is thus an approximate one and is suitable at low pressures.
Adsorption isobar
Adsorption isobar is a graph between the amount adsorbed and the temperature (T) of the adsorbate at constant pressure.
Adsorptivity decreases with increase in temperature because the forces of attraction between the two becomes weaker.
The following figure shows the adsorption isobars of physisorption and chemisorption.
The physisorption isobar shows a decrease in throughout with rise in temperature. The chemisorption isobar shows an initial increase in with temperature and then expected decrease, which is because of the heat supplied, acts as activation energy.
Adsorption from solutions
Solutes can be adsorbed from solutions by solid surfaces. For example, when cane juice is heated with animal charcoal, the green colouring matter in it is removed by its adsorption on the surface of charcoal. Activated charcoal can adsorb colouring impurities from the solutions of organic compounds. Similarly, when a dilute solution of acetic acid is boiled with charcoal powder and then filtered, filtrate does not smell acetic acid whereas residue does due to the removal of acetic acid from the solution by its adsorption on the surface of charcoal.
For the adsorption from solutions, the relationship between (mass of solute adsorbed per gram of adsorbent) and equilibrium concentration, c of the solute in the solution is similar to the adsorption isotherm. It is given by,
Taking log on both sides,
A plot of against log c is a straight line with slope equal to and intercept equal to log k.