Exothermic and Endothermic Reactions - SS1 Chemistry Lesson Note

Chemical reactions involve the breaking and forming of chemical bonds, resulting in the rearrangement of atoms to create new substances. During these reactions, energy changes occur, which can be classified as either exothermic or endothermic. We will explore the concepts of exothermic and endothermic reactions, their characteristics, examples, and the significance of energy changes in chemical processes.

 

Exothermic Reactions:

Exothermic reactions are chemical reactions that release energy to the surroundings. In these reactions, the energy of the products is lower than the energy of the reactants. The excess energy is released as heat, light, or sound. Key characteristics of exothermic reactions include:

 

1. Energy Release: Exothermic reactions release energy in the form of heat, often resulting in a rise in temperature in the surrounding environment.

 

1. Negative ΔH: Exothermic reactions have a negative change in enthalpy (ΔH), indicating that the overall reaction releases energy.

 

1. Spontaneity: Exothermic reactions tend to be spontaneous as they release energy and move towards a more stable state.

 

Examples of exothermic reactions include combustion reactions, such as the burning of fuels like wood or gasoline, where heat and light are released. Other examples include neutralisation reactions, such as the reaction between an acid and a base to form water and a salt, and some oxidation reactions.

 

Endothermic Reactions:

Endothermic reactions are chemical reactions that absorb energy from the surroundings. In these reactions, the energy of the products is higher than the energy of the reactants. The reaction requires an external source of energy to proceed. Key characteristics of endothermic reactions include:

 

1. Energy Absorption: Endothermic reactions absorb energy from the surroundings, resulting in a decrease in temperature.

 

1. Positive ΔH: Endothermic reactions have a positive change in enthalpy (ΔH), indicating that the overall reaction requires energy input.

 

1. Non-Spontaneity: Endothermic reactions are typically non-spontaneous as they require an external energy source to occur.

 

Examples of endothermic reactions include photosynthesis, where plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen. Another example is the process of evaporation, where liquid water absorbs heat energy from the surroundings to convert into water vapour.

 

Energy and Reaction Profiles:

The energy changes in exothermic and endothermic reactions can be visualised using reaction profiles. A reaction profile represents the energy changes during a reaction from the starting materials (reactants) to the final products. In an exothermic reaction, the energy of the reactants is higher than the energy of the products, resulting in a downward slope on the reaction profile. In contrast, an endothermic reaction shows an upward slope, indicating that the energy of the reactants is lower than the energy of the products.

 

Significance of Energy Changes:

Understanding exothermic and endothermic reactions is crucial in various scientific and practical contexts:

 

1. Thermodynamics: The study of energy changes in chemical reactions is central to the field of thermodynamics, which deals with energy transformations and the spontaneity of reactions.

 

1. Energy Transfer: Exothermic and endothermic reactions demonstrate the transfer of energy between the system (the reacting substances) and the surroundings.

 

1. Reaction Control: The understanding of exothermic and endothermic reactions allows scientists and engineers to control and optimise reaction conditions for desired outcomes. For example, in industrial processes, controlling the heat released or absorbed is crucial for safety, efficiency, and product quality.

 

1. Environmental Implications: Exothermic reactions, such as combustion, play a role in energy production but also have environmental implications due to the release of heat and pollutants. Endothermic processes, such as photosynthesis, are vital for carbon dioxide absorption and oxygen production in the environment.