1. Learn
  2. Quick Notes
  3. List
  4. Energy Changes in Chemical Reactions
Back

Heat Change in Reactions

 
3.1 Heat Change in Reactions
 
Classification of Chemical Reaction
Exothermic Reaction Endothermic Reaction
Chemical reactions that release heat to the surroundings. Chemical reactions that absorb heat from the surroundings.
 
Chemical Reaction
Exothermic Reaction
  • Definition: Chemical reaction that releases heat to the surrounding.
  • Heat released to the surroundings causes the surrounding temperature to rise.
  • The container becomes hot.
  • Examples;
    • Respiration
    • Combustion of fuel
    • Decomposition of organic compound
    • Iron rusting
    • The reaction of reactive metal (potassium and sodium) with water
    • Neutralisation reaction
    • The reaction of an acid with metal
    • The reaction of an acid with metal carbonate
    • Dissolving sodium hydroxide in water
    • Addition of concentrated acid into water
    • Metal oxidation
    • Formation of ammonia in the Haber process
  • The energy of the reactant is higher than the energy of the product.

 

Endothermic Reaction
  • Definition: Chemical reaction that absorbs heat from the surrounding.
  • Heat absorbed from the surroundings causes the temperature to drop.
  • The container becomes cold.
  • .The example of endothermic reaction is as stated below:

The image is a slide with the title ‘Example of Endothermic Reaction.’ It lists the following examples: - Photosynthesis. - Dissolving ammonium salt in water. - The process of cooking food. - Decomposition of metal carbonate. - Decomposition of metal nitrate. The slide has a blue background with white text and is branded with the ‘Pandai’ logo at the bottom right corner. There are decorative elements in red and blue at the corners of the slide.

  • The energy of the reactant is lower than the energy of the product.
 

 

 
Energy Level Diagram
  • During a chemical reaction, heat is absorbed or released.
  • This heat is called heat of reaction, and is given the symbol \(\Delta \text{H}\).
  • The unit for heat of reaction is \(\text{kJ mol}^{−1}\).
  • In a chemical reaction, when heat is released to the surroundings, \(\Delta \text{H}\) is given a negative sign.
  • When heat is absorbed from the surroundings, \(\Delta \text{H}\) is given a positive sign.
  • The energy change in a chemical reaction can be represented using an energy level diagram.
  • The energy level diagram shows the difference in the heat energy content between the reactants and the products.
  • The definition of heat of reaction is as stated below:
Definition of Heat of Reaction, \(\Delta H\)

The heat change of one mole of reactant that reacts or one mole of product that is formed

\(\Delta \text{H}= \text{H}_{\text{product}}-\text{H}_{\text{product}}\)
  • Example of energy level diagram of the exothermic reaction:
Energy Level Diagram of the Exothermic Reaction
This image shows a diagram illustrating an exothermic reaction. The y-axis is labeled ‘Energy,’ and the x-axis represents the reaction progress. The energy level of the reactants is higher than that of the products. The difference in energy (ΔH) is indicated as a negative value, showing that energy is released during the reaction.
 

 

 
  • Example of energy level diagram of the endothermic reaction:
Energy Level Diagram of the Endothermic Reaction
This is a graph showing the energy change in a chemical reaction. The y-axis represents energy, and there are two horizontal lines labeled ‘Energy of reactants’ at the bottom and ‘Energy of products’ at the top. An arrow labeled ‘ΔH = Positive value’ points upward from the reactants to the products, indicating an endothermic reaction where the energy of the products is higher than the energy of the reactants. The background is light blue, and the graph is on a grid with a red label ’Energy’ at the top of the y-axis
 
Energy Change During Breaking and Formation of Bonds
  • During a chemical reaction, the chemical bonds in the reactants are broken, and new bonds are formed in the products.
  • In an exothermic reaction, the heat energy released during the formation of the bonds in the products is greater than the heat energy absorbed to break the bonds in the reactants.
  • In an endothermic reaction, the heat energy absorbed to break the bonds in the reactants is greater than the heat energy released during the formation of the bonds in the products.
Change of Energy Value of\(\Delta \text{H}\) Type of Reaction
Energy absorbed to break bond < Energy released to form a bond Negative Exothermic
Energy absorbed to break bond > Energy released to form a bond Positive Endothermic
 

 

 

 

 

 

 

 

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Heat Change in Reactions

 
3.1 Heat Change in Reactions
 
Classification of Chemical Reaction
Exothermic Reaction Endothermic Reaction
Chemical reactions that release heat to the surroundings. Chemical reactions that absorb heat from the surroundings.
 
Chemical Reaction
Exothermic Reaction
  • Definition: Chemical reaction that releases heat to the surrounding.
  • Heat released to the surroundings causes the surrounding temperature to rise.
  • The container becomes hot.
  • Examples;
    • Respiration
    • Combustion of fuel
    • Decomposition of organic compound
    • Iron rusting
    • The reaction of reactive metal (potassium and sodium) with water
    • Neutralisation reaction
    • The reaction of an acid with metal
    • The reaction of an acid with metal carbonate
    • Dissolving sodium hydroxide in water
    • Addition of concentrated acid into water
    • Metal oxidation
    • Formation of ammonia in the Haber process
  • The energy of the reactant is higher than the energy of the product.

 

Endothermic Reaction
  • Definition: Chemical reaction that absorbs heat from the surrounding.
  • Heat absorbed from the surroundings causes the temperature to drop.
  • The container becomes cold.
  • .The example of endothermic reaction is as stated below:

The image is a slide with the title ‘Example of Endothermic Reaction.’ It lists the following examples: - Photosynthesis. - Dissolving ammonium salt in water. - The process of cooking food. - Decomposition of metal carbonate. - Decomposition of metal nitrate. The slide has a blue background with white text and is branded with the ‘Pandai’ logo at the bottom right corner. There are decorative elements in red and blue at the corners of the slide.

  • The energy of the reactant is lower than the energy of the product.
 

 

 
Energy Level Diagram
  • During a chemical reaction, heat is absorbed or released.
  • This heat is called heat of reaction, and is given the symbol \(\Delta \text{H}\).
  • The unit for heat of reaction is \(\text{kJ mol}^{−1}\).
  • In a chemical reaction, when heat is released to the surroundings, \(\Delta \text{H}\) is given a negative sign.
  • When heat is absorbed from the surroundings, \(\Delta \text{H}\) is given a positive sign.
  • The energy change in a chemical reaction can be represented using an energy level diagram.
  • The energy level diagram shows the difference in the heat energy content between the reactants and the products.
  • The definition of heat of reaction is as stated below:
Definition of Heat of Reaction, \(\Delta H\)

The heat change of one mole of reactant that reacts or one mole of product that is formed

\(\Delta \text{H}= \text{H}_{\text{product}}-\text{H}_{\text{product}}\)
  • Example of energy level diagram of the exothermic reaction:
Energy Level Diagram of the Exothermic Reaction
This image shows a diagram illustrating an exothermic reaction. The y-axis is labeled ‘Energy,’ and the x-axis represents the reaction progress. The energy level of the reactants is higher than that of the products. The difference in energy (ΔH) is indicated as a negative value, showing that energy is released during the reaction.
 

 

 
  • Example of energy level diagram of the endothermic reaction:
Energy Level Diagram of the Endothermic Reaction
This is a graph showing the energy change in a chemical reaction. The y-axis represents energy, and there are two horizontal lines labeled ‘Energy of reactants’ at the bottom and ‘Energy of products’ at the top. An arrow labeled ‘ΔH = Positive value’ points upward from the reactants to the products, indicating an endothermic reaction where the energy of the products is higher than the energy of the reactants. The background is light blue, and the graph is on a grid with a red label ’Energy’ at the top of the y-axis
 
Energy Change During Breaking and Formation of Bonds
  • During a chemical reaction, the chemical bonds in the reactants are broken, and new bonds are formed in the products.
  • In an exothermic reaction, the heat energy released during the formation of the bonds in the products is greater than the heat energy absorbed to break the bonds in the reactants.
  • In an endothermic reaction, the heat energy absorbed to break the bonds in the reactants is greater than the heat energy released during the formation of the bonds in the products.
Change of Energy Value of\(\Delta \text{H}\) Type of Reaction
Energy absorbed to break bond < Energy released to form a bond Negative Exothermic
Energy absorbed to break bond > Energy released to form a bond Positive Endothermic
 

 

 

 

 

 

 

 

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Thermochemistry
Topic : Energy Changes in Chemical Reactions
Form 5 Chemistry
Related notes

Report this note
Online Tuition

Live class daily with celebrity tutors

Learn more
Register for a free Pandai account now
© 2025 Pandai.org All Rights Reserved

Quiz
View video m
Notes
Account