Exothermic+and+Endothermic+reactions

Some reactions absorb heat and others evolve heat as a result of the difference in the energy required to break or make the bonds in the reaction.

//** Enthalpy **// is the energy contained in a chemical bond that can be converted into heat. Its symbol is H. for example the enthalpy of H 2 shows the energy contained in the H-H bond, this energy would be released when the bond is broken. Since we can’t measure the enthalpy of a substance we measure the //**enthalpy change (ΔH)**// when a reaction occurs. (this is because different amounts of energy is needed to break the old bonds from the amounts of energy released when new bonds are formed.)

Enthalpy change is calculated as follows:  **ΔH = H p – H r ** where H p is the enthalpy of the product and H r is the enthalpy of the reactant

//Exothermic reaction // An exothermic reaction is one in which heat is given out (more energy is released from breaking bonds than was needed to break old bonds). When an exothermic reaction happens heat is released to the surroundings and therefore the temperature of the surroundings increases. Combustion reactions and neutralisation reactions are examples of exothermic reactions. //For example: // **Combustion reaction ** C 8 H 18 (//l// ) + 25/2 O 2 ---> 8CO 2 (g) + 9 H 2 O(//l// ) ΔH= -5512 KJ mol -1

HCl (aq) + NaOH (aq) ---> NaCl (aq) + H 2 O (l) ΔH= -57.3 KJ mol -1
 * Neutralisation reaction **

Since the enthalpy of the product is less than enthalpy of the reactants (Hp>Hr), the value of ΔH for an exothermic reaction is always negative. As can be seen in the above examples the enthalpy change of the reaction is negative since heat is lost during the reaction. The enthalpy of the reactants and products in a reaction can be represented using an enthalpy level diagram. Below is the enthalpy <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">level diagram for an exothermic reaction: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As can be seen the products have a lower enthalpy than the reactants. As a result of this lower enthalpy the products are more stable than the reactants in an exothermic reaction. //

//<span style="font-family: Tahoma,sans-serif;">Endothermic reaction //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">An endothermic reaction is one in which heat is absorbed (more energy is taken in from breaking old bonds than is released from making new bonds)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">When an endothermic reaction occurs, heat is absorbed from the surroundings, thus the temperature of the surroundings decreases.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Since the enthalpy of the products is greater than the enthalpy of the reactants (Hp<Hr) in an endothermic reaction, the value of ΔH for an endothermic reaction is always positive.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Below is the enthalpy level diagram for an endothermic reaction: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As can be seen here, the reactants have a lower enthalpy than the products. Due to having a lower enthalpy, the reactants in an exothermic reaction are more stable than the products. //


 * By the end of this lesson you should be able to: **
 * ** Define the terms, exothermic reaction, endothermic reaction and enthalpy change of a reaction. **
 * ** State combustion and neutralization reactions as examples of exothermic reactions. **
 * ** Know the relative stabilities of reactants and products from an enthalpy level diagram. **
 * ** Be able to know the temperature change and enthalpy change value for an exothermic and an endothermic reaction. **


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