Exothermic and endothermic reaction

Learning objectives:

To learn about energy changes in exothermic and endothermic reactions how the arrangement of electrons 

Exothermic reactions

When a chemical reaction occurs, energy s either taken in or given out

Exothermic reactions transfer energy to the surrounding in the form of heat. This is usually indicated by a rise in temperature on a thermometer.
Most common exothermic reactions are combustion, oxidation or neutralization reactions
In terms of bonds, energy is releases when new bonds form, bond making is an exothermic process.

Endothermic reactions

These reactions will take in energy from the surroundings, therefore when measuring with a thermometer the temperature will go down. Examples of endothermic reactions are electrolysis and thermal decomposition reactions
In terms of bonds, energy is absorbed to break bonds so bond breaking is an exothermic process.

Energy diagrams

These diagrams show the different levels of energy in the reactants and the products. The bigger the difference the bigger the energy change. Energy diagrams help to illustrate whether a reaction is endothermic or exothermic

This is illustrated below

In an endothermic reaction, energy is absorbed so the products will always have more energy than the reactants, therefore the H is always positive.

Bond energy calculations

You can calculate the bond energy by measuring the total energy of the reactants and the total energy in the products; to calculate the change in energy you simply follow this equation

Worked example

H –H +Cl-Cl 2 x (H-Cl)

Energy of reactants = 436 + 243 = 679
Energy of products = 2 x 432 = 864

Energy change = reactants – products = 679 – 864 = -185

This shows that the reaction was exothermic

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