2Exothermic Reactions Examples include: Burning reactions including the combustion of fuels.Detonation of explosives.Reaction of acids with metals.Exothermic reactions increase in temperature.Magnesium reacting with acidThermit reaction
3Exothermic Reactions Magnesium + Hydrochloric acid 25o C 45o C HeatenergygivenoutGets hotHydrochloricacid
4Exothermic ReactionsIf heat is given out this energy must have come from chemical energy in the starting materials (reactants).45o C25o CReactants convert chemical energy to heat energy.The temperature rises.
5Explaining Exothermic Reactions Energy / kJ)Progress of reaction (time)reactantsReactants have more chemical energy.Some of this is lost as heat which spreads out into the room.productsProducts now have less chemical energy than reactants.
6Energy of reactants and products in Exothermic ReactionsH is howmuch energyis given outEnergy / kJProgress of reactionreactantsproductsH=negativeH is given a negative sign for exothermic reactions
7Definition of an Exothermic Reaction Exothermic reactions give out energy. There is a temperature rise and H is negative.productsEnergy / kJ)Progress of reactionreactantsH is negative
8Endothermic Reactions Endothermic reactions cause a decrease in temperature.HeatenergyIs used up during the reaction so the contents in tube feel coldAmmoniumnitrateWaterCoolsStarts 25°CCools to 5°C
9Explaining Endothermic Reactions Extra energy is needed in order for endothermic reactions to occur.This comes from the thermal energy of the reaction mixture which consequently gets colder.5o C25o CReactants convert heat energy into chemical energy as they change into products. The temperature drops.
10Energy of Reactants and Products in an Endothermic ProcessThis is howmuch energyis taken inproductsEnergy / kJ)Progress of reactionreactantsH=+H is given a positive sign for endothermic reactions
11Definition of Endothermic Reactions Endothermic reactions take in energy. There is a temperature drop and H is positive.productsEnergy / kJProgress of reactionreactantsH=+
13Are these endothermic or exothermic? ActivityAre these endothermic or exothermic?A red glow spread throughout the mixture and the temperature rose.The mixture bubbled vigorously but the temperature dropped 150C.Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space.The decaying grass in the compost maker was considerably above the outside temperature.exoendoexoexo
14Activation energyEnergy is given out as new bonds formChemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken.This means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur.Activation energy needed to break existing bondsNew bonds formReactantsOld bonds start to break
15Explaining Activation Energy Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started.Energy in chemicalsReactionActivation energyBreak old bondsEnergy taken inForm new bondsEnergy given out
16Activation Energy and Exothermic Reactions reactantsH= -Energy / kJ)productsProgress of reaction
17Activation Energy and Endothermic Reactions productsH=+Energy / kJ)reactantsProgress of reaction
18ActivityUse this diagram to explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps.ActivationEnergyPetrol + oxygenEnergy / kJ)Carbon dioxide + waterProgress of reaction
19AnswerThe reaction is exothermic but requires the Activation energy to be provided before the reaction can get underway.This is necessary to break some of the bonds in the oxygen or petrol before new bonds can start forming.ActivationEnergyPetrol + oxygenEnergy / kJ)Carbon dioxide + waterProgress of reaction
21Catalysts For chemical reactions to occur: Existing bonds have to begin breaking so that new ones can be formed.The molecules have to collide in such a way that the reacting parts of the molecules are brought together.Catalysts can help with either or both of these processes.A catalyst is a substance that speeds up a reaction without getting used up in the process.
23Inside car engines some of the nitrogen and oxygen from the air combine to form poisonous nitrogen oxide. Inside the exhaust system a catalyst encourages decomposition back into nitrogen and oxygen.Copy the energy profile for the uncatalysed reaction and draw in new lines showing how the presence of a catalyst will alter the profile.2NO2N2 + 2O2