Endothermic and exothermic reactions

Endothermic and exothermic reactions Objectives: To explain the difference between an exothermic and an endothermic reaction Outcomes: All students will classify reactions observed in previous lesson as either exothermic or endothermic Most students will explain the terms exothermic and endothermic reaction in terms of bond making and bond breaking Some students will perform calculations to deduce whether a reaction is endo or exo thermic

Endothermic Exothermic Temperature rise Temperature drop

Endothermic Exothermic Temperature rise Temperature drop Reactants Products Energy Heat given out Products Energy Heat taken in Reactants

What happens in a chemical reaction? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H Bonds between the reactants have to be broken: 4 x C-H bonds 2 x O=O bonds This requires energy to be put into the system ie this stage is endothermic Bonds between products have to be made: 2 x C=O 4 x H-O Energy is given out when bonds are made ie this stage is exothermic

Endothermic- breaking bonds Exothermic- making bonds Reactants Products Energy Heat taken in Reactants Products Energy Heat given out

So: why is a reaction exothermic or endothermic overall? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H Bonds between the reactants have to be broken: 4 x C-H bonds 2 x O=O bonds This requires energy to be put into the system ie this stage is endothermic Bonds between products have to be made: 2 x C=O 4 x H-O Energy is given out when bonds are made ie this stage is exothermic It depends on the individual bond energies

Energy Energy given out Energy taken in Reactants CH4 + 2O2 Products CO2 + 2H2O

Exothermic or endothermic overall? CH4 + 2O2→ CO2 + 2H2O H H–C–H O=O O=O → O=C=O H–O–H H–O–H Bonds between the reactants have to be broken (energy in): 4 x C-H bonds 2 x O=O bonds = (4 x 412 kJ/mole) = 1648 kJ/mole = (2 x 496 kJ/mole) = 992 kJ/mole Total energy put in = 1648 + 992 = 2640 kJ/mole Bonds between products have to be made (energy out): 2 x C=O 4 x H-O = (2 x 743 kJ/mole) = 1486 kJ/mole =(4 x 463 kJ/mole) = 1852kJ/mole Total given out = 1486 + 1852 = 3338kJ/mole Overall energy change: + 2640 kJ/mole – 3338 kJ/mole = -698 kJ/mole

Energy given out = - 3338 kJ/mole Energy taken in = + 2640 kJ/mole Energy Reactants CH4 + 2O2 Energy change = - 698 kJ/mole Products CO2 + 2H2O

Calculating energy changes Energy in (bonds broken) Energy out (bonds made) Energy difference = energy in – energy out