KS4: Energy Transfer in Reactions

1. KS4: Energy Transfer in Reactions

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

3. Activity Say whether these processes are exothermic. Charcoal burning A candle burning. A kettle boiling Ice melting A firework exploding yes yes no no yes You have to put heat in for boiling and melting. You get heat out from all the other processes

4. Magnesium + Hydrochloric acid 45o C 25o C magnesium Hydrochloric acid Exothermic Reactions Heat energy given out Gets hot

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

6. Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. Exothermic Reactions 45o C 25o C Chemical energy becomes heat energy. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. It follows that reaction products have less chemical energy than the reactants had to start with.

7. Reactants have more chemical energy. reactants Some of this is lost as heat which spreads out into the room. Energy / kJ) Products now have less chemical energy than reactants. products Progress of reaction (time) Energy Level Diagram for an Exothermic Reaction

8. H is how much energy is given out reactants H is negative because the products have less energy than the reactants. H=negative Energy / kJ products Progress of reaction Energy Level Diagram for an Exothermic Reaction 2. Energy Level Diagram for an Exothermic Reaction

9. Energy / kJ) Progress of reaction reactants products Exothermic Reaction - Definition Exothermic reactions give out energy. There is a temperature rise and H is negative. His negative

10. Activity

11. Endothermic chemical reactions are relatively rare. A few reactions that give off gases are highly endothermic - get very cold. Dissolving salts in water is another process that is often endothermic. Endothermic Reactions Endothermic reactions cause a decrease in temperature.

12. Heat energy taken in as the mixture returns back to room temp. Ammonium nitrate Water Endothermic Reactions Endothermic reactions cause a decrease in temperature. Cools Starts 25°C Cools to 5°C Returns to 25°C

13. 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. Endothermic Reactions 5o C 25o C Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

14. The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. Endothermic Reactions The reactants gain energy. 25o C 5o C 25o C This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy.

15. This is how much energy is taken in products This is positive because the products have more energy than the reactants. H=+ Energy / kJ) reactants Progress of reaction Energy Level Diagram for an Endothermic Process

16. Energy / kJ Progress of reaction products reactants Endothermic Reaction Definition Endothermic reactions take in energy. There is a temperature drop and H is positive. H=+

18. Are 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. Activity exo endo exo exo

19. Sketch the two energy diagrams and label exothermic and endothermic as appropriate. Activity Energy / kJ Energy / kJ) reactants H=- Progress of reaction Progress of reaction products H=+ products reactants

20. Most chemicals will decompose (break up) if we heat them strongly enough. This indicates that breaking chemical bonds requires energy – is an endothermic process. Energy in chemicals Energy needed Breaking chemical bonds Heat taken in Energy needed to overcome the bonding between the atoms

21. It is reasonable to assume that bond making will be the opposite of bond breaking Energy will be given out in an exothermic process when bonds are formed. Energy in chemicals Energy given out Making chemical bonds Heat given out Energy given out as bonds form between atoms

22. In most chemical reactions some existing bonds are broken (endothermic) Energy given out as new bonds form Energy taken in as old bonds break Overall endothermic in this case Energy in chemicals H products reactants Changes to chemical bonds Endothermic Reactions • But new bonds are made (exothermic)

23. Again some existing bonds are broken (endothermic) Energy taken in as old bonds break Overall exothermic – in this case Energy given out as new bonds form Energy in chemicals H reactants products Changes to chemical bonds Exothermic Reactions • And new bonds are formed (exothermic)

24. Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic. Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic. Exo Endo Bonds break Bond forming Bonds form Energy in chemicals Energy in chemicals Bonds break products H H reactants reactants products Summary – Bond Changes

25. The formation of nitrogen (IV) oxide (formula NO2) from reaction of nitrogen with oxygen in car engines has a H value of +33.2kJ per mol of nitrogen oxide. Write a word equation for the reaction. Write a chemical equation for the reaction. Is H positive or negative? Is the reaction exothermic or endothermic? Draw an simple energy diagram for the reaction (not showing bond breaking and forming.) Which involves the biggest energy change: bond breaking or bond forming? Activity

26. Nitrogen + oxygen  nitrogen(IV)oxide N2 + 2O2 2NO2. H positive (+33.2kJ/mol). The reaction is endothermic. Energy diagram Bond breaking involves the biggest energy change. Answer products Energy / kJ reactants Progress of reaction

27. This is an exothermic reaction C H H H H O O O O Bond Breaking Bond Forming H O Energy in chemicals O C O O O H H H H H O C O O H H Progress of reaction Burning Methane H

28. Hydrogen peroxide decomposes as shown: Calculate energy for bond breaking. Calculate the energy from bond making What is the value of H for the reaction shown Activity H H O O O O H H O O H H H H O O

29. Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ Bond forming: (exothermic) 4(O-H) + 1(O=O) =1856+498 = -2354kJ H = +2148 – 2354 = -206kJ (Exothermic) Answer H H O O O O H H O O H H H H O O

30. Most chemical reactions, including exothermic reactions, seem to need an input of energy to get the reaction started. This fits completely with what we have already explained: Before new bonds can be formed we need to break at least some existing chemical bonds. This requires an energy input –known as the activation energy (Eaor Eact) Once an exothermic reaction is underway it can provide its own activation energy (from the bond forming stage) and so sustains the reaction. Activation Energy.

31. Activation Energy and Exothermic Reactions Ea= + Activation energy reactants H= - Energy / kJ) products Progress of reaction

32. Activation Energy and Endothermic Reactions Ea= + Activation energy products H=+ Energy / kJ) reactants Progress of reaction

33. Copy the energy diagram and use it to help you explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps. Activity ActivationEnergy Petrol + oxygen Energy / kJ) Carbon dioxide + water Progress of reaction

34. 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. Answer ActivationEnergy Petrol + oxygen Energy / kJ) Carbon dioxide + water Progress of reaction

35. Copy the summary using the words from the box to fill in the gaps: Activity • Exothermic reactions are _____. • Reactions that get cold are called _____. • Bond forming is an _______ process. • Endothermic reactions have a _____ H. • In exothermic reactions the chemicals ___ chemical energy. common endothermic exothermic positive lose continued

36. Copy the summary using the words from the box to fill in the gaps: Activity • The energy needed to start off a reaction is called the ______ energy • In endothermic reactions bond breaking requires ___ energy than is produced by bond forming. • Bond breaking is an _________ process. activation more endothermic

37. Exothermic reactions: Are common, Give out heat. Have a negative H. Bond forming gives out more energy than bond breaking consumes. Have reactants that contain more chemical energy than the products. Endothermic reactions – are the opposite! Bond breaking is endothermic. Bond forming is exothermic. Reactions require an activation energy to help start the bond breaking process. Summary

38. Which of the following is an endothermic process? Burning wood Reacting an acid and alkali Reacting magnesium with acid Dissolving ammonium nitrate in water

39. Which of the following is true for an exothermic process? Energy / kJ Progress of reaction products reactants • The reactants lose energy to the surroundings • The reaction gets cold • The energy diagram will be as shown • The reaction will have a H that is positive (+).

40. Which of the following is true when chemical bonds are broken? • The process is exothermic • Energy is given out • A physical change is occurring • The reaction will have a H that is positive(+).

41. Which of the following is true for an exothermic reaction? • Bond breaking involves a bigger energy change than bond making • Bond making involves a bigger energy change than bond breaking • Bond making involves the same energy change as bond breaking

42. Which of the following is the activation energy: X Y Z