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

Temperature rise Temperature drop

Exothermic

Endothermic

Temperature rise Temperature drop

Exothermic

Reactants

Products

Energy

Heat given out

Reactants

Products

Energy

Heat taken in

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 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 given out

Reactants

Products

Energy

Heat taken in

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 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

Products

CO2 + 2H2O

Energy Energy

taken in

Reactants

CH4 + 2O2

Energy given out

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 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

Products

CO2 + 2H2O

Energy

Energy taken in = + 2640 kJ/mole

Reactants

CH4 + 2O2

Energy given out = - 3338 kJ/mole

Energy change

= - 698 kJ/mole

Calculating energy changes

• Energy in (bonds broken)

• Energy out (bonds made)

Energy difference = energy in – energy out