OCR GCSE Chemistry: Energy in Reactions – Exothermic and Endothermic Processes

Energy in Reactions: Exothermic and Endothermic Processes

Introduction

Chemical reactions involve the breaking and forming of bonds, a process that always involves energy changes. Some reactions release energy into the surroundings, making them feel hot, while others absorb energy from the surroundings, making them feel cold. This tutorial will explore these two types of reactions: exothermic and endothermic processes.

Exothermic Reactions

Definition: An exothermic reaction releases energy to the surroundings. This energy is often released as heat, but can also be in the form of light or sound.

Characteristics:

• Energy change: The enthalpy change (?H) is negative.
• Temperature change: The temperature of the surroundings increases.
• Reaction profile: The products have lower energy than the reactants.

Examples:

• Combustion: Burning fuels like wood or gas.
• Neutralisation reaction: The reaction between an acid and a base.
• Explosions: Rapid exothermic reactions that produce a large amount of energy in a short time.

Endothermic Reactions

Definition: An endothermic reaction absorbs energy from the surroundings. This energy is often taken in as heat, making the surroundings feel cold.

Characteristics:

• Energy change: The enthalpy change (?H) is positive.
• Temperature change: The temperature of the surroundings decreases.
• Reaction profile: The products have higher energy than the reactants.

Examples:

• Melting ice: Solid ice absorbs heat to become liquid water.
• Photosynthesis: Plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.
• Decomposition of calcium carbonate: Heating calcium carbonate breaks it down into calcium oxide and carbon dioxide, absorbing heat in the process.

Reaction Profiles

Definition: A reaction profile is a diagram that shows the energy changes that occur during a chemical reaction. It plots the energy of the reactants and products, along with the activation energy.

Key features:

• Reactants: The initial energy state of the reactants.
• Products: The final energy state of the products.
• Activation energy: The minimum energy required for the reaction to occur.
• Enthalpy change (?H): The difference in energy between reactants and products.

Exothermic reaction profile:

       Energy
         ^
         |      
         |  *     
         |   \     
         |    *     
         |     \  
         |______*__> Reaction progress
                ?H  

Endothermic reaction profile:

       Energy
         ^
         |
         |  *     
         |   /     
         |    *     
         |     /  
         |______*__> Reaction progress
               ?H

Calculating Energy Changes

Bond energies: The energy required to break one mole of a specific bond.

Energy change = (Energy required to break bonds in reactants) - (Energy released when bonds form in products)

Example:

Calculate the enthalpy change for the following reaction:

H2(g) + Cl2(g) -> 2HCl(g)

Bond energies:

• H-H = 436 kJ/mol
• Cl-Cl = 242 kJ/mol
• H-Cl = 431 kJ/mol

Calculation:

Energy required to break bonds: (1 x H-H) + (1 x Cl-Cl) = 436 + 242 = 678 kJ/mol
Energy released when bonds form: (2 x H-Cl) = 2 x 431 = 862 kJ/mol
Enthalpy change (?H) = 678 - 862 = -184 kJ/mol

Therefore, the reaction is exothermic with an enthalpy change of -184 kJ/mol.

Practical Applications

• Heating and cooling: Exothermic reactions are used to generate heat, for example in boilers and power plants. Endothermic reactions are used to absorb heat, for example in cold packs.
• Energy storage: Batteries store energy by using chemical reactions, often involving exothermic and endothermic processes.
• Food preservation: Exothermic reactions can be used to sterilize food and kill bacteria, while endothermic reactions can be used to freeze and preserve food.

Conclusion

Understanding the difference between exothermic and endothermic processes is fundamental to understanding energy transfer in chemical reactions. This knowledge is crucial for various practical applications in fields like energy production, food science, and medicine. By studying reaction profiles and calculating energy changes, we can gain a deeper insight into the energy dynamics of chemical reactions and their impact on our world.