IGCSE Exothermic and Endothermic Reactions 2024 to 2028 Exams Free

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IGCSE Chemistry Revision Notes

Topic:Electrochemistry

Sub Topic: 5.1 Exothermic and endothermic reactions

Syllabus Objectives:

CORE:

State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
Interpret reaction pathway diagrams showing exothermic and endothermic reactions

EXTENDED:

State that the transfer of thermal energy during a reaction is called the enthalpy change, ?H, of the reaction. ?H is negative for exothermic reactions and positive for endothermic reactions
Define activation energy, Ea , as the minimum energy that colliding particles must have to react
Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include:
(a) reactants
(b) products
(c) enthalpy change of the reaction, ?H (d) activation energy, Ea
State that bond breaking is an endothermic process and bond making is an exothermic process and explain the enthalpy change of a reaction in terms of bond breaking and bond making
Calculate the enthalpy change of a reaction using bond energies

Free IGCSE Chemistry Notes Endothermic and Exothermic reactions 2024/2025/2026/2027/2028 Exams pdf

Definition: Exothermic reactions release thermal energy to the surroundings, causing an increase in the temperature of the surroundings.
Examples:
- Burning of substances
- Nuclear fission
- Mixing of water and acid
- Rusting of iron
Energy Dynamics:
- Energy is released during the formation of new bonds, which results in more energy being released than absorbed.

Definition: Endothermic reactions absorb thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
Examples:
- Thermal decomposition
- Photosynthesis
- Action of light on silver bromide
- Electrolysis
Energy Dynamics:
- Energy is absorbed to break bonds in the reactants, resulting in more energy being absorbed than released.

Bond Dynamics:
- Bond Breaking: Requires energy (endothermic process).
- Bond Formation: Releases energy (exothermic process).
Overall Reaction:
- If the energy released during bond formation is greater than the energy absorbed during bond breaking, the reaction is exothermic.
- If the energy absorbed during bond breaking is greater than the energy released during bond formation, the reaction is endothermic.
Example Calculation:
- Reactants: Breaking 1 H-H bond (+436 kJ/mol) and 1 Cl-Cl bond (+242 kJ/mol) totals +678 kJ/mol.
- Products: Forming 2 H-Cl bonds releases 2 x 431 kJ/mol = -862 kJ/mol.
- Net Energy Change: +678 kJ/mol - 862 kJ/mol = -184 kJ/mol, indicating an exothermic reaction.

Definition: The enthalpy change is the transfer of thermal energy during a reaction.
Signs of ?H:
- Negative (?H < 0>
- Positive (?H > 0) for endothermic reactions.

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