Short Answer
Enthalpy change (ΔH) is the amount of heat energy absorbed or released during a chemical reaction at constant pressure. It tells us whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). A negative ΔH means heat is released, while a positive ΔH means heat is absorbed.
For example, burning fuel has a negative ΔH because it releases heat, whereas melting ice has a positive ΔH because it absorbs heat. Enthalpy change helps us understand how energy flows in chemical and physical processes.
Detailed Explanation :
Enthalpy Change ΔH
Enthalpy change, represented by the symbol ΔH, is a measure of the heat energy gained or lost during a chemical reaction that occurs at constant pressure. Every reaction involves breaking old bonds and forming new ones. These processes require or release energy. The difference between the energy contained in the reactants and the products is known as the enthalpy change.
Enthalpy change helps us understand whether a reaction produces heat or requires heat to proceed. It is one of the most important concepts in thermochemistry because it explains the energy behavior of chemical systems and helps predict reaction conditions, feasibility, and stability.
Meaning of ΔH
The symbol ΔH represents:
- Δ (delta): Change
- H (enthalpy): Heat content of a system
Therefore, ΔH means the change in heat content when a reaction occurs.
ΔH = H(products) – H(reactants)
This equation shows how much energy is released or absorbed.
Types of Enthalpy Change
Enthalpy change can be:
- Negative ΔH (Exothermic Reaction)
- Heat is released to surroundings
- Products have less energy than reactants
- Temperature of surroundings increases
Example: Combustion of fuels
CH₄ + 2O₂ → CO₂ + 2H₂O + heat
- Positive ΔH (Endothermic Reaction)
- Heat is absorbed from surroundings
- Products have more energy than reactants
- Temperature of surroundings decreases
Example: Photosynthesis
6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂
These two categories help classify reactions based on energy exchange.
How Enthalpy Change Occurs
Chemical reactions involve two energy processes:
- Breaking bonds → requires energy
- Forming bonds → releases energy
If more energy is released while forming new bonds, ΔH is negative.
If more energy is required to break bonds, ΔH is positive.
This difference explains how the system absorbs or releases heat.
Examples of Enthalpy Change in Daily Life
Enthalpy change is a part of many everyday activities:
- Cooking Food
Heat is absorbed (positive ΔH) to boil, fry, or steam food.
- Burning Fuels
Wood, LPG, and petrol release heat (negative ΔH), making them useful for cooking and heating.
- Melting and Freezing
Melting absorbs heat (positive ΔH).
Freezing releases heat (negative ΔH).
- Dissolving Substances
Some salts absorb heat when dissolved (cold packs).
Others release heat (hot packs).
- Respiration
Glucose breakdown releases heat, helping maintain body temperature.
These examples show that enthalpy change affects many natural and human activities.
Energy Diagram for Enthalpy Change
Energy diagrams help visualize ΔH:
- Exothermic reaction: Products are lower in energy than reactants; ΔH is negative.
- Endothermic reaction: Products are higher in energy than reactants; ΔH is positive.
The height difference between the reactants and products shows the magnitude of ΔH.
Factors Affecting Enthalpy Change
Several factors influence ΔH:
- Temperature: Higher temperature may affect energy required for bond breaking.
- Physical state of reactants and products: Solid, liquid, or gas states affect heat content.
- Concentration: In solutions, concentration changes energy values.
- Pressure: Important for reactions involving gases.
Understanding these factors helps chemists control reaction conditions.
Importance of Enthalpy Change in Chemistry
Enthalpy change is important because it helps:
- Predict reaction behavior
- Determine whether a reaction needs heating or produces heat
- Design industrial processes like manufacturing fertilizers, fuels, and chemicals
- Understand natural processes like photosynthesis and respiration
- Control energy usage in everyday life
It is also used in engineering, environmental studies, and material science.
Standard Enthalpy Change
Sometimes ΔH is measured under standard conditions (25°C, 1 atm pressure, 1 mol concentration). This is called standard enthalpy change (ΔH°). It allows comparison of different reactions under the same conditions.
Examples include:
- Standard enthalpy of formation
- Standard enthalpy of combustion
- Standard enthalpy of neutralization
These values help predict reaction feasibility and energy requirements.
Conclusion
Enthalpy change (ΔH) is the amount of heat absorbed or released during a reaction at constant pressure. A negative ΔH indicates an exothermic reaction, while a positive ΔH indicates an endothermic reaction. ΔH helps explain energy flow in chemical systems, predict reaction behavior, and control industrial and natural processes. Understanding enthalpy change is essential for studying thermochemistry and real-world energy applications.