Surface Chemistry: Definition, Applications and Sample Questions

The study of occurrences on the surface of substances is defined as Surface Chemistry. All types of surface occurrences are dealt with in surface chemistry. The subject finds its applications in industry, analytical work and daily life situations. Adsorption, Catalysis and Colloids such as Emulsions and Gels are important aspects of surface chemistry. 

Key Terms: Surface chemistry, Catalysis, Colloids, Zeolites, Homogeneous catalysis, Emulsions, Gels, Corrosion, Crystallization, Adsorption, Heterogeneous Catalysis, Dissolution

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Surface Chemistry

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Surface Chemistry is a chemical phenomena that occurs at the interface of any two surfaces such as liquid-gas, solid-vacuum, solid-gas, solid-liquid etc. There is no interface between the surface of gases due to complete miscibility. Surface chemistry even finds its applications in surface engineering. Some of the phenomenons occurring at the interfaces are Corrosion, Crystallization, Adsorption, Heterogeneous Catalysis, Dissolution etc.

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Adsorption

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The gathering of various species in higher concentration on the surface of some substance due to some intermolecular force is called adsorption. Few examples like H₂, N₂, O₂ get adsorbed on the surface of activated charcoal. The material on the surface on which the process of adsorption takes place is known as adsorbent whereas the molecular species which accumulate on the surface are known as adsorbate. On the adsorption of one mole of gas, the amount of energy liberated is known as Enthalpy of Adsorption. 

Types of Adsorption

Adsorption can be categorized into two types depending on the type of force of interaction between adsorbent and adsorbate. Mentioned below are the two types:

• Physical Adsorption or Physisorption: Physical adsorption is a multilayered procedure in which a weak van der waals force is involved in between adsorbent and adsorbate. It is a reversible process with low energy of activation and is majorly affected by nature of adsorbate, temperature & pressure. It has low enthalpy of adsorption with value up to 20-40 KJ/mol 
• Chemical Adsorption or Chemisorptions: Chemical adsorption is a single layered process which takes place due to the acting of strong chemical forces in between adsorbent & adsorbate. It is an irreversible process with high energy of activation and is majorly affected by nature of temperature of adsorbate and surface area of the adsorbent. It has high enthalpy of adsorption with values ranging from 40-400 KJ/mol.

Adsorption Isotherms

Adsorption isotherm can be defined as the variation in gaseous amounts which is adsorbed by the adsorbent at constant temperature with constant pressure by a curve. Below mentioned are the two important adsorption isotherms:

1. Freundlich Adsorption Isotherm: This isotherm states the empirical relationship between the gaseous quantities that is adsorbed by a unit mass of solid adsorbent at a particular temperature and pressure can be mathematically expressed as:

xm=k.p ^1/n (n>1)

where, x = mass of the adsorbed gas on solid adsorbent

p = pressure

m = mass of the solid adsorbent

n and k = constants that depends on nature of the gas at some specific temperature

and nature of adsorbent

The logarithmic equation of Freundlich Adsorption Isotherm can be expressed as:

log log x/m= log log k + (1/n) log p

If 1/n = 1, x/m p, the pressure varies directly with the adsorption

If 1/n = 0, x/m is constant, the adsorption is independent of pressure

2. Langmuir Adsorption Isotherm: According to this isotherm:

• The layer adsorbed is all over uniformly distributed
• There is no interaction in between the absorbed molecules that are adjacent
• An adsorbed layer of gas on solid adsorbent is generally a molecule thicker.

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Catalysis

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Catalysis is a process in which the rate of chemical reaction increases in addition to a substance which is not a part of the chemical reaction. Very small amounts of catalyst are used to increase the process of reaction. For example:

2 H₂O_{2 (potassium Permanganate)}→ 2H₂O + O₂

Types of Catalysis

 Catalysis is generally of two types:

• Homogenous Catalysis: The presence of a catalyst in the same phase as of reactants in the chemical reaction is homogeneous catalysis. For example, hydrolysis of sugar.
• Heterogeneous Catalysis: The presence of catalysts whose phases are different from the reactants in a chemical reaction. For example, manufacturing of ammonia from hydrogen & nitrogen using Haber’s process, dehydrogenation of ethanol etc.

Enzyme Catalysis

The reactions taking place in human beings, plants & animals for maintaining life processes are responsible for the catalysis by enzymes. Such enzymes are termed as biochemical catalysis and the criterion is known as biochemical catalysis. These enzymes are specific in nature and have optimum pH. They are highly efficient which possess the capability to increase their activities in presence of coenzymes or activators. Some examples of enzyme catalysis are conversion of glucose into ethyl alcohol, inversion of cane sugar, conversion of maltose into glucose etc.

Mechanism of Enzyme Catalysis

The biochemical catalysts are termed as Enzymes. The mechanism associated with enzyme catalysis is called lock & key mechanism which is completed in below mentioned steps:

• Step 1: Enzyme substrate complex formation
• Step 2: Dissociation of the enzyme –substrate complex resulting in product formation

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Colloids

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Colloids are the heterogeneous mixtures of two matter in which minute particles of one matter (dispersed phase) are dispersed into another (dispersion medium). Particles of the dispersed phase cannot be seen through naked eyes. Here is a list of few examples of colloids along with their dispersed phase & dispersion medium:

Properties of Colloids

Colloids are seemed to have various properties as mentioned below:

• Particles of colloids are extremely small, ranging between 1-1000 nanometres.
• Colloids are heterogeneous mixtures.
• Particles of colloids cannot be separated through filtration, but can be separated through centrifugation
• Brownian movement is seen in colloidal particles
• Colloids show tyndall effect
• Particles of colloid don’t settle down if left undisturbed for a few times. 

Lyophilic and Lyophobic Solutions

Based on the nature of interaction in between the dispersion medium & dispersed phase, we can divide the colloids into two types:

• Lyophilic Colloids or Sols: Lyophilic generally means ‘liquid loving’ which means there is a strong attraction in between the dispersion medium and dispersed phase. For example, egg albumin sol, starch dissolving in water.
• Lyophobic colloids or Sols: ‘Liquid hating’ which means that there is no or very less attraction in between the dispersion medium and dispersed phase. Adding a little bit of electrolyte can easily separate the dispersed phase and dispersion medium. For example, ferric hydroxide sol.

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Emulsions

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In physical chemistry, the mixtures of two or more liquids which are generally immiscible are termed as Emulsion. Emulsion is used in cases where the dispersion medium and dispersed phase are both liquid. For example, Aerosols (Fog, Clouds etc), Latex, Milk, Mayonnaise etc are the emulsions.

Types of Emulsion

Emulsions are of two types as mentioned below:

• Simple Emulsions: These are formed by dispersing water in oil. Example - Butter, Cold Cream or oil in water.
• Complex Emulsions: These are both oil in water and water in oil emulsions can be seen and hence known as multiple emulsions. These are stabilized by surfactants. Examples: water suspended in droplets of oil suspended in water.

Emulsification 

The criteria of formation of emulsion are known as Emulsification. We can elaborate by stating the mixture of one immiscible liquid into another. Example, Oil-Water cutting fluid emulsion. 

To increase the stability in emulsions, emulsifiers are added. For example, milk has natural emulsion casein.

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Zeolites

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Zeolites are hydrated aluminosilicate minerals which are made from tetra hydra of alumina and silica. They are 3-D crystal structure minerals that trap water inside them. 

Features of Zeolite Catalysis

• Zeolites are commonly used as catalysts in pharmaceutical productions.
• Zeolites are used in catalytic crackers for breaking hydrocarbon molecules in diesel, kerosene, waxes and other kinds of petroleum.
• Zeolites are called shape-selective catalysts.
• They are reusable catalysts.

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Applications of Surface Chemistry

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Surface chemistry works based on various phenomena to work on some principle like – Chromatography, Colloid Formation, Catalysis & Electrode Reactions. A major role is also played by surface chemistry in several chemical processes:

• It is used in surface and interface of microchips in the computers in electronic industries
• It is responsible for the enzymatic reactions in various biological interfaces which are found in the cell membranes & cell walls
• It is used in automobile exhausts as heterogeneous catalysts which are found in catalytic converters to clean emissions.

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Things to Remember

• The connection between the extent of adsorption & pressure of gas at some constant temperature is called adsorption isotherm
• Extent of adsorption (gas on solid) is completely dependent on nature of gas, nature of solid, pressure & temperature of gas & surface area of solid
• Emulsions are stabilized using emulsifiers and are used in toothpastes, pigments, dyes, paints and syrups, etc.
• Adsorption is used to reduce the poisonous gases like carbon monoxide, methane etc. with the help of activated charcoal like in gas masks; in controlling humidity; remove hardness of water, etc.
• Alcosol is the sol in which the medium (liquid) is alcohol.
• Colloidal Solutions shows Tyndall Effect when a beam of light is passed through it.
• Addition of NaCl to hydrated ferric oxide sol results in neutralization of charge on ferric sol particles and coagulation takes place.

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Previous year Questions

1. In a reversible reaction, the energy of activation of the forward reaction is 50 kcal. The energy of activation for the reverse reaction will be….[NEET 1996]
2. The plot of concentration of the reactant versus time for a reaction is a straight line with a negative slope. This reaction follows….[NEET 1996]
3. The experimental data for the reaction 2A+B2⟶2AB is The rate equation for the above data is...[NEET 1997]
4. Which of the following relation correctly represents the consumption & formation of reactants and products :...[NEET 2000]
5. Rate law for the reaction can be written as…....[NEET 2012]
6. The rate of appearance of bromine (Br₂) is related to rate of disappearance of bromide ions as following…..[NEET 2009]
7. The value of −d[H2]dt would be -..[NEET 2009]
8. The rate of of appearance of SO3 is….….[KCET 2018]
9. The rate of the reaction with respect to the initial rate when each solution is diluted with an equal volume of water will be…...[KCET 2019]
10. A radioactive isotope has a half life of 10 days. If today 125 mg is left over, what was its original weight 40 days earlier ?...[KCET 2005]
11. The ratio of the reaction velocities will be :...[KCET 2004]
12. How long will the reaction take to be 84% complete ?...[KCET 2012]
13. When temperature is increased from 30∘C to 100∘C, rate of the reaction increases by….[KCET 2003]
14. The half-life period of a order reaction is 60 minutes What percentage will be left over after 240 minutes ?...[KCET 2016]
15. For certain first order reaction m, 75% of the reaction complete in 30 min. How much time it require to complete 99.9% of the reaction?..[KCET 2001]
16. For a reaction 12A→2B rate of disappearance of A is related to rate of appearance of B by the expression..[KCET 2017]
17. Half-life period of a first order reaction is 10min. Starting with initial concentration 12M, the rate after 20min is​...[KCET 2015]
18. Which of these does not influence the rate of reaction?...[KCET 2005]
19. Which one of the following is a second order reaction ?..​[KCET 2007]