hi everyone welcome to igcs study body where you can revise chemistry topics from the Cambridge igcc syllabus if you are enjoying our video so far please don't forget to hit the like button and subscribe to our channel in this video you are going to learn topic five chemical energetics ch chemical reactions can be categorized as either exothermic or endothermic based on their heat exchange with the surroundings an exothermic reaction transfers thermal or heat energy to the surroundings leading to an increase in the temperature of the surroundings an endothermic reaction takes in thermal or heat energy from the surroundings leading to a decrease in the temperature of the surroundings so exothermic reactions release heat energy while endothermic reactions absorb heat energy a reaction pathway diagram also known as an energy level diagram shows the energies of both the reactants and final products in chemical reactions an exothermic reaction occurs when the total chemical energy of the reactants is more than that of the products resulting in the transfer of this energy difference to the surroundings in the form of heat an endothermic reaction occurs when the chemical energy of the reactants is less less than that of the products causing the transfer of this energy difference from the surroundings to the chemicals involved so heat is absorbed from the surroundings let's examine the exothermic reaction pathway diagram the Y AIS represents the energy level where the energy of the reactants is higher than that of the products the initial Spike or bump in the curve represents the activation energy which is the energy required to start the reaction following this the energy level of the products is lower than that of the reactants indicating that the difference in energy is released into the surroundings in in the case of an endothermic reaction pathway the energy of the products is higher than that of the reactants the curve represents the activation energy following this the energy level of the products is higher than that of the reactants indicating that additional energy is absorbed from the surroundings to form the products activation energy or E Suba is the minimum energy that colliding particles must have in order to react the transfer of thermal energy during a reaction is called enthalpy change symbol Delta h of the reaction Delta H is negative for exothermic reactions and positive for endothermic reactions let's learn how to calculate the enthalpy change of a reaction using Bond energies first we Write the balanced equation for the reaction then find the energy in this is the total Bond energies for all the bonds in the reactants Now find the energy out this is the total Bond energies for all the bonds in the products finally calculate the enthalpy change enthalpy change is equal to energy taken in take away energy given out therefore if the energy in is less than the energy out the enthalpy or energy change is negative so it's an exothermic reaction and if the energy in is greater than the energy out the enthalpy change is positive so it is an endothermic reaction Bond making is an exothermic process and bond breaking is an endothermic process energy is released when bonds are made in products and energy is taken in to break bonds apart in the reactants whether a reaction absorbs or releases energy depends on how much energy it takes to break old bonds compared to how much is released when new bonds form so if the energy needed to break bonds is less than the energy released then it's an exothermic reaction with a negative enthalpy change value if the energy needed to break bonds is greater than the energy released then it's an endothermic reaction with a positive enthalpy change value now please don't get confused with these energy levels and the energy levels on a reaction pathway diagram in a reaction pathway diagram we are talking about the total chemical energy of the reactants and the products but here we are talking about how much much energy a reaction absorbs to break bonds versus how much energy it releases when new bonds form finally let's look at an example showing how to calculate the enthalpy change of a reaction using Bond energies methane reacts with chlorine gas to produce methy chloride and hydrogen chloride this is the equation for the this reaction use the bond energies given to calculate the enthalpy change Delta H for the reaction and show that the reaction is exothermic bond energy is the amount of energy in KJ per mole which must be supplied to break one mole of the bond so this table shows the bond Energies the displayed formula equation for the reaction helps to identify the type and number of bonds although methane has four CH bonds the main chemical change involves Breaking the Bond between carbon and hydrogen in methane to form methy chloride ch3cl and hydrogen chloride HCL since each methane molecule contributes one carbon hydrogen bond to the reaction we count only one Bond broken for each CH4 molecule involved in the reaction so first we find the energy in the Total Bond energies for all the bonds in the reactants that is 1 CH Bond plus one cl cl bond that adds up to a a total of 654 KJ per mole then find the energy out the Total Bond energies for all the bonds in the products that is 1 CCL Bond plus 1 HCL bond that totals 769 KJ per mole then we calculate the enthalpy change which is energy taken in take away energy given out so 654 subtract 769 gives Min - 115 KJ per mole since the enthalpy changed Delta H is negative it shows that the reaction is exothermic that concludes topic five chemical energetics are you enjoying our videos are they helping you here's a way you can show your appreciation and support our continued efforts you may use YouTube super thanks to send us thanks hope this video helped you please share your thoughts and suggestions in the comments section thank you for watching and please don't forget to subscribe to igcc study 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