Surface Chemistry flash cards

1. 1.What is adsorption? How does it differ from absorption?

   Adsorption is the accumulation of a substance (adsorbate) at the surface of a solid/liquid (adsorbent), giving a higher concentration at the surface than in the bulk. Absorption is uniform penetration/distribution of a substance throughout the bulk. Adsorption is a surface phenomenon; absorption is a bulk phenomenon.

   Hint: Surface only vs throughout the bulk.

2. 2.Define the terms adsorbate and adsorbent.

   Adsorbate = the substance that gets accumulated/held on the surface. Adsorbent = the substance on whose surface adsorption occurs (e.g. charcoal, silica gel, alumina).

   Hint: Adsorbate sticks; adsorbent holds.

3. 3.What is sorption?

   When adsorption and absorption occur simultaneously, the process is called sorption. Example: dyes on cotton, or hydrogen gas on palladium (adsorbed on surface and absorbed into bulk).

   Hint: Both surface + bulk at once.

4. 4.Why is adsorption always an exothermic process? Give the sign convention of $\Delta H$.

   During adsorption, residual surface forces get satisfied and surface energy decreases, releasing heat, so $\Delta H < 0$ (enthalpy of adsorption is negative). Adsorption also decreases randomness of the gas, so $\Delta S < 0$.

   Hint: Surface energy falls; heat released.

5. 5.Using $\Delta G = \Delta H - T\Delta S$, explain why adsorption becomes non-spontaneous at high temperature.

   For adsorption $\Delta H < 0$ and $\Delta S < 0$. Spontaneity needs $\Delta G < 0$. As $T$ increases, the positive $-T\Delta S$ term grows and eventually makes $\Delta G > 0$, so adsorption stops being spontaneous.

   Hint: $-T\Delta S$ term dominates as $T$ rises.

6. 6.Distinguish physisorption and chemisorption by the nature of forces involved.

   Physisorption: adsorbate held by weak van der Waals forces. Chemisorption: adsorbate held by strong chemical (covalent) bonds formed with the adsorbent surface.

   Hint: van der Waals vs chemical bond.

7. 7.Compare the enthalpy of adsorption for physisorption vs chemisorption.

   Physisorption: low, about $20{-}40\ \text{kJ mol}^{-1}$. Chemisorption: high, about $80{-}240\ \text{kJ mol}^{-1}$.

   Hint: ~20-40 vs ~80-240 kJ/mol.

8. 8.Is physisorption specific or non-specific? What about chemisorption?

   Physisorption is non-specific — any gas is adsorbed on any surface to some extent (more for easily liquefiable gases). Chemisorption is highly specific — occurs only if chemical bonding is possible between adsorbate and adsorbent.

   Hint: van der Waals are universal; bonds are selective.

9. 9.How does reversibility differ between physisorption and chemisorption?

   Physisorption is readily reversible (weak forces; adsorbate can be desorbed by lowering pressure or raising temperature). Chemisorption is generally irreversible (strong chemical bonds).

   Hint: Weak = reversible; bonded = not.

10. 10.Physisorption forms how many molecular layers vs chemisorption?

    Physisorption can be multilayer (many layers) under favourable conditions. Chemisorption is restricted to a unimolecular (monolayer) because it needs direct bonding to the surface.

    Hint: Many layers vs single layer.

11. 11.How does temperature affect physisorption?

    Physisorption decreases with increase in temperature (exothermic; like condensation, favoured at low $T$). It typically occurs near the boiling point of the adsorbate.

    Hint: Low $T$ favoured.

12. 12.Why does chemisorption first increase then decrease with temperature?

    Chemisorption needs activation energy, so it increases with temperature initially (more molecules cross the energy barrier). After the optimum, further heating causes desorption, so it decreases. This gives a maximum in the plot.

    Hint: Activation energy up, then desorption.

13. 13.How does surface area affect the extent of adsorption?

    Extent of adsorption increases with surface area of the adsorbent. That is why finely divided/porous adsorbents (activated charcoal, silica gel, colloidal metals) are excellent adsorbents.

    Hint: More surface = more adsorption.

14. 14.Which is more temperature-dependent in activation energy: physisorption or chemisorption?

    Physisorption has no appreciable activation energy (spontaneous, fast). Chemisorption requires high activation energy (bond formation), hence it is sometimes called activated adsorption.

    Hint: Activated adsorption = chemisorption.

15. 15.Which type of adsorption can transform into the other as temperature rises, and how?

    Physisorption of a gas (e.g. $H_2$ on nickel) at low temperature can change into chemisorption at higher temperature, because the added energy allows chemical bond formation with the surface.

    Hint: Physisorption → chemisorption on heating.

16. 16.Which gases are adsorbed to a greater extent by physisorption and why?

    Easily liquefiable gases (high critical temperature) like $NH_3$, $HCl$, $Cl_2$, $SO_2$ are adsorbed more, because greater van der Waals forces make them easier to condense on the surface. $H_2$, $N_2$, $O_2$ (low critical $T$) are adsorbed less.

    Hint: Higher critical temperature → more adsorbed.

17. 17.State the Freundlich adsorption isotherm equation for a gas on a solid.

    $\dfrac{x}{m} = k\,p^{1/n}$, where $x/m$ is mass of gas adsorbed per gram of adsorbent, $p$ is pressure, and $k,n$ are constants ($n>1$) depending on adsorbent, gas and temperature.

    Hint: $x/m = k p^{1/n}$.

18. 18.Write the logarithmic (linear) form of the Freundlich isotherm and describe its plot.

    $\log\dfrac{x}{m} = \log k + \dfrac{1}{n}\log p$. A plot of $\log(x/m)$ vs $\log p$ is a straight line with slope $1/n$ and intercept $\log k$.

    Hint: Straight line: slope $1/n$, intercept $\log k$.

19. 19.In the Freundlich isotherm, what does the value of $1/n$ physically indicate? Range of $1/n$?

    $1/n$ lies between $0$ and $1$. If $1/n = 0$, adsorption is independent of pressure ($x/m$ constant). If $1/n = 1$, adsorption is directly proportional to pressure. Real cases lie between these limits.

    Hint: $0 \le 1/n \le 1$.

20. 20.Describe how $x/m$ varies with pressure in the three regions of the adsorption isotherm.

    At low pressure: $x/m \propto p^{1}$ (linear). At intermediate pressure: $x/m \propto p^{1/n}$ (fractional power, Freundlich). At high pressure: $x/m$ becomes independent of pressure ($p^{0}$, saturation).

    Hint: $p^1 \to p^{1/n} \to p^0$.

21. 21.What is the main limitation of the Freundlich adsorption isotherm?

    It is a purely empirical relation and fails at high pressure — experimental data deviate because the isotherm does not account for surface saturation (monolayer completion).

    Hint: Empirical; breaks down at high $p$.

22. 22.State the key assumptions of the Langmuir adsorption isotherm.

    (1) Adsorption forms a single monolayer. (2) The surface has a fixed number of identical adsorption sites. (3) No interaction between adsorbed molecules. (4) Dynamic equilibrium between adsorption and desorption.

    Hint: Monolayer + fixed sites + equilibrium.

23. 23.Write the Langmuir isotherm equation and its high/low pressure limits.

    $\dfrac{x}{m} = \dfrac{a\,p}{1 + b\,p}$ (or $\theta = \dfrac{Kp}{1+Kp}$). At low $p$: $x/m \propto p$. At high $p$: $x/m$ = constant (saturation, monolayer complete).

    Hint: $\theta = Kp/(1+Kp)$.

24. 24.What quantity is measured to compare adsorption power, and what is the enthalpy of adsorption's role?

    Extent of adsorption is measured as $x/m$ (mass adsorbed per unit adsorbent). Enthalpy (heat) of adsorption reflects the strength of adsorbate-adsorbent interaction; larger magnitude indicates stronger (chemisorption) binding.

    Hint: $x/m$ and magnitude of $\Delta H_{ads}$.

25. 25.Give three important applications of adsorption.

    (1) Gas masks (activated charcoal adsorbs toxic gases). (2) Silica/alumina gel as desiccants (adsorb moisture). (3) Chromatography, decolourisation of solutions, heterogeneous catalysis, creating vacuum, and froth flotation of ores.

    Hint: Gas masks, desiccants, chromatography.

26. 26.What is a catalyst and what is catalysis?

    A catalyst is a substance that alters (usually increases) the rate of a reaction without itself being consumed. The phenomenon is called catalysis. A positive catalyst speeds up; a negative catalyst (inhibitor) slows down.

    Hint: Changes rate, unconsumed.

27. 27.How does a catalyst increase reaction rate in terms of activation energy and $\Delta G$?

    A catalyst provides an alternative path with lower activation energy, so more molecules can react. It does not change $\Delta H$, $\Delta G$, or the equilibrium position — it only helps equilibrium be reached faster.

    Hint: Lowers $E_a$; $\Delta G$ unchanged.

28. 28.Does a catalyst change the equilibrium constant or position of equilibrium?

    No. A catalyst speeds up both forward and backward reactions equally, so it helps equilibrium be attained faster but does not change $K$ or the equilibrium amounts of products.

    Hint: Faster equilibrium, same $K$.

29. 29.Distinguish homogeneous and heterogeneous catalysis.

    Homogeneous: catalyst and reactants are in the same phase (e.g. all gases or all in solution). Heterogeneous: catalyst is in a different phase from the reactants (usually a solid catalyst with gaseous/liquid reactants).

    Hint: Same phase vs different phase.

30. 30.Give an example of homogeneous catalysis.

    Lead chamber process: oxidation of $SO_2$ to $SO_3$ using $NO$ gas as catalyst (all gaseous). Also, ester hydrolysis catalysed by mineral acid $H^+$ (all in solution).

    Hint: $NO$ in lead chamber; acid hydrolysis of esters.

Open the interactive deck for the other 67 cards, with self-grading so the ones you keep missing come back.