Endothermic Change

An endothermic change is a chemical reaction or physical process that absorbs thermal energy from the surroundings, causing the temperature of the surroundings to decrease ($\Delta H > 0$) ¹.

Reaction Energy Pathway

In an endothermic process, products possess higher chemical energy than reactants because energy is taken in from the environment ¹:

Energy ^
       |             Activation Energy (Ea)
       |                 .-'-.
       |                     \
       |                      [========] Products
       |       Delta H (heat   ^
       |       absorbed)       |
       |  [========] ----------'
       |   Reactants
       +---------------------------------------------> Reaction Progress

Bond Energy Balance

In chemical reactions:

• Bond Breaking: Endothermic (absorbs energy).
• Bond Making: Exothermic (releases energy).

A reaction is endothermic when the energy needed to break existing bonds in the reactants is greater than the energy released forming new bonds in the products ¹.

Common IGCSE Examples

1. Chemical Reactions:
   • Thermal Decomposition: Breaking down compounds by heat (e.g. $\text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2$) ¹.
   • Photosynthesis: Plants absorbing light energy ($6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$).
   • Sherbet Reaction: Citric acid reacting with sodium hydrogencarbonate.
2. Physical Phase Changes:
   • Melting: Absorbs latent heat of fusion to loosen lattice bonds ².
   • Boiling & Evaporation: Absorbs latent heat of vaporization to overcome intermolecular forces ².

Related

• Exothermic Change - Reverse process transferring heat to the surroundings.
• Melting - Endothermic solid-to-liquid transition.
• Boiling - Endothermic liquid-to-gas bulk transition.
• Evaporation - Endothermic surface phase change causing cooling.
• Intermolecular Force - Attractive forces overcome by heat absorption.
• Chemistry - Science area investigating energetic transformations.