SPP2601 MINOR TEST ASSESSMENT 3 2026
DUE 12 AUGUST 2026
QUESTION 1: Adsorption Study
1.1 State and discuss at least five (5) limitations of Langmuir Adsorption Equation
The Langmuir adsorption isotherm, useful for describing many adsorption systems
1. Assumption of homogeneous surface (Study Guide, p. 123)
The Langmuir model assumes that all adsorption sites on the surface are identical and
have equal energy. In reality, solid surfaces are heterogeneous with sites of varying
energy levels. This leads to deviations from the ideal Langmuir behaviour, particularly at
low and high concentrations.
2. Monolayer adsorption only (Study Guide, p. 123)
The model assumes that adsorption is limited to a single molecular layer on the surface.
However, in practice, multilayer adsorption occurs, especially at higher pressures or
concentrations. This limitation makes the Langmuir isotherm unsuitable for systems
where multilayer formation is significant (Study Guide, p. 128-129).
3. No interaction between adsorbed molecules (Study Guide, p. 123)
The Langmuir equation assumes that adsorbed molecules do not interact with each
other. In reality, lateral interactions between adsorbate molecules on the surface often
occur, affecting the adsorption behaviour. These interactions can be attractive or
repulsive, causing deviations from ideal behaviour.
, 4. Immobile adsorption (Study Guide, p. 123)
The model assumes that molecules adsorb at fixed sites and do not migrate across the
surface. However, surface diffusion is common in many adsorption systems, particularly
at higher temperatures where molecules have sufficient thermal energy to move along
the surface.
5. Ideal assumptions about energy (Study Guide, p. 123)
The Langmuir model assumes that the heat of adsorption is constant and independent
of surface coverage. In reality, the heat of adsorption typically decreases as surface
coverage increases due to variations in site energy and adsorbate-adsorbate
interactions.
6. Reversibility assumption
The Langmuir isotherm assumes adsorption is completely reversible. However, many
adsorption processes exhibit hysteresis or irreversible behaviour, particularly with
chemisorption or when pore structure effects are significant.
7. Inapplicability to porous materials (Study Guide, p. 123)
The model was originally developed for flat, uniform surfaces and does not adequately
account for pore structure effects, capillary condensation, or the influence of pore
geometry on adsorption behaviour.
8. Temperature independence limitation
The Langmuir constants (K and qmax) are temperature-dependent, and the model does
DUE 12 AUGUST 2026
QUESTION 1: Adsorption Study
1.1 State and discuss at least five (5) limitations of Langmuir Adsorption Equation
The Langmuir adsorption isotherm, useful for describing many adsorption systems
1. Assumption of homogeneous surface (Study Guide, p. 123)
The Langmuir model assumes that all adsorption sites on the surface are identical and
have equal energy. In reality, solid surfaces are heterogeneous with sites of varying
energy levels. This leads to deviations from the ideal Langmuir behaviour, particularly at
low and high concentrations.
2. Monolayer adsorption only (Study Guide, p. 123)
The model assumes that adsorption is limited to a single molecular layer on the surface.
However, in practice, multilayer adsorption occurs, especially at higher pressures or
concentrations. This limitation makes the Langmuir isotherm unsuitable for systems
where multilayer formation is significant (Study Guide, p. 128-129).
3. No interaction between adsorbed molecules (Study Guide, p. 123)
The Langmuir equation assumes that adsorbed molecules do not interact with each
other. In reality, lateral interactions between adsorbate molecules on the surface often
occur, affecting the adsorption behaviour. These interactions can be attractive or
repulsive, causing deviations from ideal behaviour.
, 4. Immobile adsorption (Study Guide, p. 123)
The model assumes that molecules adsorb at fixed sites and do not migrate across the
surface. However, surface diffusion is common in many adsorption systems, particularly
at higher temperatures where molecules have sufficient thermal energy to move along
the surface.
5. Ideal assumptions about energy (Study Guide, p. 123)
The Langmuir model assumes that the heat of adsorption is constant and independent
of surface coverage. In reality, the heat of adsorption typically decreases as surface
coverage increases due to variations in site energy and adsorbate-adsorbate
interactions.
6. Reversibility assumption
The Langmuir isotherm assumes adsorption is completely reversible. However, many
adsorption processes exhibit hysteresis or irreversible behaviour, particularly with
chemisorption or when pore structure effects are significant.
7. Inapplicability to porous materials (Study Guide, p. 123)
The model was originally developed for flat, uniform surfaces and does not adequately
account for pore structure effects, capillary condensation, or the influence of pore
geometry on adsorption behaviour.
8. Temperature independence limitation
The Langmuir constants (K and qmax) are temperature-dependent, and the model does
