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© Boardworks Ltd 2003 Exothermic and Endothermic Reactions.

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Presentation on theme: "© Boardworks Ltd 2003 Exothermic and Endothermic Reactions."— Presentation transcript:

1 © Boardworks Ltd 2003 Exothermic and Endothermic Reactions

2 © Boardworks Ltd 2003 Exothermic Reactions Examples include: –Burning reactions including the combustion of fuels. –Detonation of explosives. –Reaction of acids with metals. Thermit reaction Magnesium reacting with acid Exothermic reactions increase in temperature.

3 © Boardworks Ltd 2003 Magnesium + Hydrochloric acid Gets hot 25 o C 45 o C magnesium Hydrochloric acid Heat energy given out Exothermic Reactions

4 © Boardworks Ltd 2003 If heat is given out this energy must have come from chemical energy in the starting materials (reactants). Reactants convert chemical energy to heat energy. The temperature rises. 25 o C 45 o C Exothermic Reactions

5 © Boardworks Ltd 2003 Energy / kJ) Progress of reaction (time) Explaining Exothermic Reactions reactants Reactants have more chemical energy. Some of this is lost as heat which spreads out into the room. products Products now have less chemical energy than reactants.

6 © Boardworks Ltd 2003 Energy / kJ Progress of reaction reactants products  H=negative  H is how much energy is given out  H is given a negative sign for exothermic reactions Energy of reactants and products in Exothermic Reactions

7 © Boardworks Ltd 2003 Exothermic reactions give out energy. There is a temperature rise and  H is negative. Definition of an Exothermic Reaction products Energy / kJ) Progress of reaction reactants  H is negative

8 © Boardworks Ltd 2003 Cools Heat energy Is used up during the reaction so the contents in tube feel cold Starts 25°C Cools to 5°C Ammonium nitrate Water Endothermic reactions cause a decrease in temperature. Endothermic Reactions

9 © Boardworks Ltd 2003 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. Reactants convert heat energy into chemical energy as they change into products. The temperature drops. 25 o C 5 o C Explaining Endothermic Reactions

10 © Boardworks Ltd 2003 products Energy / kJ) Progress of reaction reactants  H=+ Energy of Reactants and Products in an Endothermic Process  H is given a positive sign for endothermic reactions This is how much energy is taken in

11 © Boardworks Ltd 2003 Endothermic reactions take in energy. There is a temperature drop and  H is positive. Definition of Endothermic Reactions  H=+ products Energy / kJ Progress of reaction reactants

12 © Boardworks Ltd 2003

13 Are these endothermic or exothermic? 1.A red glow spread throughout the mixture and the temperature rose. 2.The mixture bubbled vigorously but the temperature dropped 15 0 C. 3.Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space. 4.The decaying grass in the compost maker was considerably above the outside temperature. exo endo exo Activity

14 © Boardworks Ltd 2003 New bonds form Old bonds start to break Activation energy Chemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken. activation energyThis means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur. Reactants Activation energy needed to break existing bonds Energy is given out as new bonds form

15 © Boardworks Ltd 2003 Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started. Energy in chemicals Reaction Activation energy Break old bonds Energy taken in Form new bonds Energy given out Explaining Activation Energy

16 © Boardworks Ltd 2003 Energy / kJ) Progress of reaction reactants products  H= - Activation Energy and Exothermic Reactions Activation energy

17 © Boardworks Ltd 2003 Energy / kJ) Progress of reaction reactants products  H=+ Activation energy Activation Energy and Endothermic Reactions

18 © Boardworks Ltd 2003 Use this diagram to explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps. Energy / kJ) Progress of reaction Petrol + oxygen Carbon dioxide + water ActivationE nergy Activity

19 © Boardworks Ltd 2003 The 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. Energy / kJ) Progress of reaction Petrol + oxygen Carbon dioxide + water ActivationE nergy Answer

20 © Boardworks Ltd 2003

21 Catalysts 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.

22 © Boardworks Ltd 2003 Effect of a Catalyst on Activation Energy

23 © Boardworks Ltd 2003 Inside 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. 1.Copy the energy profile for the uncatalysed reaction and draw in new lines showing how the presence of a catalyst will alter the profile. 2NO 2 N 2 + 2O 2


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