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Exothermic & Endothermic reactions

Endoth­ermic reactions

Endoth­ermic reactions are reactions that take in energy from the surrou­ndings leading to the decrease in the temper­ature of the surrou­ndings.
Endoth­ermic reactions absorb or take-in energy.
Bond breaking is an endoth­ermic reaction because energy must be taken-in to break bonds.
Understand that the word endo means inside. You can use this to grasp the concept that exothermic reactions take-in energy or simply absorb energy inside a body or something like that.
Examples of Endoth­ermic reactions include;
1. Photos­ynt­hesis
2. Electr­olysis
3. NaCO3 + CH3COOH
4. Melting & boiling

Exothermic Reactions

Exothermic reactions are reactions which transfer thermal energy to the surrou­ndings which causes the temper­ature of the surrou­ndings to increase.
Exothermic reactions release heat to the suroun­dings.
In this type of reaction, the energy of the reactants is more or larger than that of the products. This is why on a reaction diagram the reactant is on a high level and the product is on a lower level.
Bond making is an exothermic reaction because energy (heat energy, to be exact) is released or given out when bonds are made.
Understand that the word exo means external or outside , this will enable you to understand that energy is given out in an exothermic reaction.
Examples of Exothermic reactions include:
1. Combustion reactions
2. Most Oxidation reactions
3. Respir­ation
4. Neutra­lis­ation reactions ie. acid+b­ase­=sa­lt+­water
5. freezing & conden­sation

Terms in Chemical energetics

Activation energy E
Enthalpy change H
Activation energy is the minimum amount of energy required for colliding materials to react.
Enthalpy change is the transfer of thermal energy.
The enthalpy change value of an exothermic reaction is usually a negative number.

The enthalpy change value for an endoth­ermic reaction is usually a positive number.


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