CHEMICAL REACTIONS AND ENERGY CHANGES, EXOTHERMIC REACTIONS

 

CHEMICAL REACTIONS AND ENERGY CHANGES:

             During chemical reactions bond rearangement takes place. Bonds in the reactants must be broken first before new bonds in the products are formed. Bond breaking absorbeds energy whereas bond forming releases energy. Usually the total energy required to break the  bonds is not the same as the energy released when the new bonds are formed. In some reactions energy absorbed from the surrounding is more and in some other reactions energy released by the system to the surrounding is more. Hence, chemical reactions are accompanied by energy exchange between the reacting system and the surrounding. This energy change of a chemical reactions is called enthalpy change of reaction and is represented by H.

 

EXOTHERMIC REACTIONS:

Reactants

Reaction pathway

- h

Products

 

An exothermic reaction gives out energy to the surrounding. This happens because more energy is released during bond formation compared to the energy absorbed during bond. Breaking. The enthalpy of the exothermic products is lower than the enthalpy of the reactants. The temperature of the reactions mixture increases. Mathematically.                             H = H                   - H                     (Product)                       (Reactants)                 The value of enthalpy is negative for exothermic reactions i.e;  H. Its units are kilojoules per mole         (kj mole-1).

ENDOTHERMIC REACTIONS:

 

Products.

+ H

Reactants

Reaction pathway

            An endothermic reaction takes in energy from the surroundings. This happens because more energy is required to break the bonds as compared to the energy released when bonds are formed. The enthalpy of the products is higher than the enthalpy of the reactants. The temperature of the reaction mixture decreases i.e;  it become cold.

Mathematically;                                                                                            

                                                                                                                                 

             H = H                              - H

                    (Product)                       (Reactants)  

          

The value of enthalpy change is positive for endothermic reactions i.e; + H. Its units are kilojoules per mole (kj mole^-1).

 

EXAMPLE:

            Exothermic reactions: Include oxidation reactions such as combustion of fuels, respiration. Reaction of metal with acids;

Mg + 2HCL₂ + H₂+ energy

           

On the other hand endothermic reactions require heat energy. The energy input may come from a flame,  electricity, sunlight ot the surroundings. Examples of endothermic reactions include the decomposition of limestone by heating.

            CaCO₃ + energy                                 CaO + CO₂

 

Photosynthesis, the energy is supplied to the reaction by sunlight.

            6CO₂ + 6H₂O + energy                                   C₆ H₁₂ O₆ + 6O₂    

            Dissolving ammonium chloride in water, energy is transferred from the surroundings. As a result solution become cold, which is used in packs to relief pain during injury.

 

            NH₄  + H₂O + energy                                NH  + Cl