Essential Concepts in Fuels, Catalysis, and Adsorption Chemistry

Posted on Nov 7, 2025 in Chemistry

Fuel Definition, Classification, and Characteristics

A fuel is any substance that produces heat energy upon combustion.

Fuel Classification

• Solid Fuels: Coal, coke, wood.
• Liquid Fuels: Petrol, diesel, kerosene.
• Gaseous Fuels: LPG (Liquefied Petroleum Gas), CNG (Compressed Natural Gas), producer gas.

Characteristics of an Ideal Fuel

An ideal fuel must possess the following characteristics:

1. High calorific value.
2. Moderate ignition temperature.
3. Low moisture content.
4. Low non-combustible matter (ash content).
5. Moderate combustion velocity.
6. Harmless combustion products (non-toxic, eco-friendly).
7. Low cost and easy availability.
8. Easy storage and transportation.
9. Uniform size (especially for solid fuels).
10. Controllable combustion rate.

Bomb Calorimeter: Principle and Mechanism

Working Principle

A known mass of fuel is burned completely in excess oxygen, and the heat released is absorbed by a known quantity of water and the calorimeter apparatus itself.

The fundamental equation is:

Heat released by fuel = Heat absorbed by water + Heat absorbed by calorimeter

Parts of the Bomb Calorimeter

• Strong steel bomb (pressure vessel).
• Oxygen inlet valve.
• Ignition wire (for initiating combustion).
• Calorimeter vessel (containing water).
• Stirrer (to ensure uniform temperature).
• Thermometer (for precise temperature measurement).

Mechanism and Calculation

Fuel is ignited in oxygen → combustion releases heat → heat is absorbed by water and the apparatus → temperature rise is measured → calorific value is calculated.

The calorific value (H) is calculated using the formula:

$$H = \frac{(W + w)(t_2 – t_1)}{x}$$

Where:

• $W$ = weight of water (in the calorimeter)
• $w$ = water equivalent of the calorimeter
• $x$ = weight of fuel sample
• $t_1$ and $t_2$ = initial and final temperatures, respectively.

Alternative Fuels: Power Alcohol and Biodiesel

Power Alcohol

Power alcohol is a blend of 20–25% ethanol with petrol (gasoline). It improves the antiknock quality of the fuel and generally requires no engine modification. Methanol and ethanol can be used, but ethanol is mainly sourced from biomass (renewable resources).

Biodiesel

Biodiesel is manufactured via the trans-esterification of vegetable oil or animal fat using methanol or ethanol.

This process produces mono-alkyl esters (biodiesel) and glycerol as a byproduct. Biodiesel is a renewable, biodegradable, and eco-friendly fuel alternative.

Physical Versus Chemical Adsorption

Adsorption is classified based on the nature of the forces involved:

Factors Affecting Gas Adsorption on Solids

1. Surface Area: Increasing the surface area of the adsorbent increases the extent of adsorption.
2. Nature of Gas: Gases that are easily liquefiable (have higher critical temperatures) adsorb more readily.
3. Heat of Adsorption: Adsorption is an exothermic process (releases heat).
4. Temperature: Physical adsorption decreases significantly with increasing temperature, while chemisorption generally increases initially.
5. Pressure: Increasing pressure leads to increased adsorption.
6. Thickness of Adsorbed Layer: At low pressures, adsorption is typically monolayer; at high pressures, it can become multimolecular (multilayer).

Adsorption Isotherms

Langmuir Adsorption Isotherm (Theory & Equation)

The Langmuir model describes monolayer adsorption based on several key assumptions:

• Adsorption occurs only in a single layer (monolayer).
• The surface of the adsorbent is uniform (all sites are equivalent).
• There is no interaction between adsorbed molecules on adjacent sites.

Derivation and Equation

At equilibrium, the rate of adsorption equals the rate of desorption:

$$k_a(1 – \theta)P = k_d\theta$$

Where $\theta$ is the fraction of surface covered, $P$ is pressure, and $k_a$ and $k_d$ are rate constants.

Rearranging gives the Langmuir equation for surface coverage:

$$\theta = \frac{k_aP}{k_d + k_aP} = \frac{KP}{1 + KP}$$

If $x$ is the amount adsorbed, the equation is often written as:

$$x = \frac{aP}{1 + bP}$$

At very high pressure ($P$), $x$ approaches a constant value (monolayer saturation).

Freundlich Adsorption Isotherm

The Freundlich isotherm is an empirical relation describing adsorption, particularly useful for multilayer adsorption and non-uniform surfaces:

$$\frac{x}{m} = kP^{1/n}$$

Where:

• $x/m$ = amount of gas adsorbed per gram of adsorbent.
• $P$ = equilibrium pressure.
• $k$ and $n$ = constants specific to the system ($n > 1$).

A plot of $\text{Log}(x/m)$ versus $\text{Log}(P)$ yields a straight line, confirming the empirical relationship.

Battery Classification and Applications

Battery Types

1. Primary Batteries (Non-rechargeable): Chemical reaction is irreversible. Example: Zinc–Manganese Dioxide (Zn–MnO₂) dry cell.
2. Secondary Batteries (Rechargeable): Chemical reaction is reversible by applying an external current. Examples: Lead-acid, Nickel–Cadmium (Ni–Cd), Lithium-ion (Li-ion).
3. Reserve Batteries: Batteries that are inert until activated (e.g., by adding an electrolyte). Used when long, reliable storage is required. Example: LiV₂O₅ cell (used in missiles and defense applications).

Applications

• Automobiles: Lead-acid batteries (starting, lighting, ignition).
• Portable Devices: Lithium-ion batteries (smartphones, laptops).
• Space & Defense: Reserve batteries and specialized secondary cells.
• Energy Storage: Backup systems and large-scale renewable energy storage (e.g., grid storage).

Catalysis: Definition and Classification

Definition of a Catalyst

A catalyst is a substance that alters the rate of a chemical reaction without being consumed in the process.

Classification of Catalytic Reactions

1. Positive Catalysis: The catalyst increases the rate of reaction.
2. Negative Catalysis (Inhibition): The catalyst decreases the rate of reaction.
3. Homogeneous Catalysis: The catalyst and the reactants exist in the same physical phase (e.g., all liquid or all gas).
4. Heterogeneous Catalysis: The catalyst and the reactants exist in different physical phases (e.g., solid catalyst and gaseous reactants).
5. Enzyme Catalysis: Catalysis involving biological molecules (enzymes) in biochemical reactions.

Factors Affecting the Catalysis Process

1. Nature and surface area of the catalyst.
2. Temperature and pressure conditions.
3. Presence of promoters or poisons.
4. Nature of the reactants.
5. Concentration and adsorption ability of reactants on the catalyst surface.

Types of Catalysis: Acid-Base and Autocatalysis

Acid-Base Catalysis

In acid-base catalysis, the catalyst provides H⁺ or OH⁻ ions to promote the reaction mechanism.

• Acid Catalysis: Example: Ester hydrolysis catalyzed by H⁺ ions.
• Base Catalysis: Example: Ester hydrolysis catalyzed by OH⁻ ions.

Autocatalysis

Autocatalysis occurs when one of the reaction products acts as a catalyst for the reaction itself. Consequently, the reaction rate increases as the product concentration builds up.

Example: The oxidation of oxalic acid by potassium permanganate (KMnO₄), where Mn²⁺ ions produced act as the catalyst.

Catalyst Promoters and Poisons

Promoters

Promoters are substances that, when added in small amounts, enhance the catalytic activity of the primary catalyst.

Example: Molybdenum (Mo) acts as a promoter for the Iron (Fe) catalyst used in the Haber process for ammonia synthesis.

Poisons

Poisons are substances that reduce or destroy the efficiency of a catalyst, typically by blocking or permanently deactivating the active sites on the catalyst surface.

Examples: Carbon monoxide (CO) or Arsenic trioxide (As₂O₃) can poison Platinum (Pt) or Iron (Fe) catalysts.