### **What is a Hydrocarbon?** A **hydrocarbon** is an organic compound consisting entirely of **hydrogen (H)** and **carbon (C)** atoms. These molecules form the basis of many fuels, such as gasoline, diesel, natural gas, and more. Hydrocarbons can vary greatly in structure and molecular size, ranging from simple molecules like methane to complex chains or rings. They are generally categorized into: 1. **Alkanes (Saturated hydrocarbons)**: Contain single bonds between carbon atoms. Example: Methane (CH₄), Ethane (C₂H₆). 2. **Alkenes (Unsaturated hydrocarbons)**: Contain one or more double bonds between carbon atoms. Example: Ethene (C₂H₄). 3. **Alkynes**: Contain one or more triple bonds between carbon atoms. Example: Acetylene (C₂H₂). 4. **Aromatic hydrocarbons**: Contain a ring structure (e.g., benzene, C₆H₆). ![[Pasted image 20241015173515.png]] --- ### **Hydrocarbons and Molecular Weight** The **molecular weight** of hydrocarbons is directly related to the number of carbon and hydrogen atoms they contain. As the molecular weight increases, the **boiling points** (condensation temperatures) of hydrocarbons rise, affecting the temperature at which they condense from a gas to a liquid. This principle is essential in **fractional distillation**, a process used in refineries to separate crude oil into different fuel grades based on their condensation temperatures. 1. **Lighter hydrocarbons** (low molecular weight) have **lower boiling points** and condense at lower temperatures. 2. **Heavier hydrocarbons** (high molecular weight) have **higher boiling points** and condense at higher temperatures. ### **Fractional Distillation and Fuel Grades** In oil refineries, crude oil is separated into different **fractions** (fuel grades) based on molecular weight through **fractional distillation**. The temperature gradient in a distillation column allows hydrocarbons to condense at different levels depending on their molecular weight. ![[distillation-of-crude-oil.gif]] Here’s a breakdown of how **hydrocarbon molecular weight** influences fuel grades: 1. **Natural Gas (C₁ to C₄)**: - **Molecular weight**: Very low (CH₄, C₂H₆, C₃H₈, etc.). - **Condensation temperature**: Below room temperature; remains gaseous at ambient conditions. - **Uses**: Heating, electricity generation, and cooking. 2. **Gasoline (C₅ to C₁₂)**: - **Molecular weight**: Low to moderate (e.g., pentane, hexane). - **Boiling range**: 30°C to 200°C. - **Uses**: Fuel for cars and small engines. 3. **Kerosene and Jet Fuel (C₁₀ to C₁₅)**: - **Molecular weight**: Moderate. - **Boiling range**: 150°C to 275°C. - **Uses**: Aviation fuel, heating. 4. **Diesel (C₁₂ to C₂₀)**: - **Molecular weight**: Higher (heavier hydrocarbons). - **Boiling range**: 200°C to 350°C. - **Uses**: Trucks, buses, and large engines. 5. **Heavy Fuel Oil (C₂₀ and above)**: - **Molecular weight**: Very high (long-chain hydrocarbons). - **Boiling range**: 350°C and above. - **Uses**: Ships, power plants, and industrial heating. 6. **Asphalt and Bitumen (C₅₀ and above)**: - **Molecular weight**: Extremely high. - **Boiling range**: Very high temperatures. - **Uses**: Paving roads, roofing materials. --- ### **Condensation Temperatures and Hydrocarbon Grades** - **Light hydrocarbons** (e.g., methane, ethane, propane) have **low boiling points** and remain gases at normal temperatures. - **Medium-weight hydrocarbons** (e.g., octane, dodecane) condense to liquids like gasoline or kerosene at moderate temperatures. - **Heavy hydrocarbons** (e.g., paraffin, tar) condense at **higher temperatures**, forming diesel fuel, lubricants, and even solids like asphalt. --- ### **Lightest and Heaviest Common Hydrocarbons** - **Lightest Hydrocarbon**: **Methane (CH₄)**, with a molecular weight of about **16 g/mol**. It is the primary component of **natural gas** and is a gas at room temperature. - **Heaviest Common Hydrocarbon**: **Asphaltenes** and **bitumen**, which can have molecular weights well over **1000 g/mol**. These are solid or highly viscous and are used in road construction and industrial applications. --- ### **Summary of Key Points** - Hydrocarbons consist of hydrogen and carbon atoms and vary in structure and molecular weight. - **Molecular weight** dictates the **condensation temperature** of hydrocarbons, which in turn determines the **grade of fuel**. - **Lighter hydrocarbons** are used for fuels like natural gas and gasoline, while **heavier hydrocarbons** are used in diesel and fuel oil. - The **lightest hydrocarbon** is **methane**, and some of the **heaviest hydrocarbons** are found in materials like **bitumen** used in asphalt.