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KS4 Chemistry Energy Transfer.

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Presentation on theme: "KS4 Chemistry Energy Transfer."— Presentation transcript:

1 KS4 Chemistry Energy Transfer

2 Energy Transfer Exothermic reactions Endothermic reactions
Contents Energy Transfer Exothermic reactions Endothermic reactions Bonds and energy Activation energy Summary activities

3 Exothermic or endothermic?
Are these endothermic or exothermic? A red glow spread throughout the mixture and the temperature rose. The mixture bubbled vigorously but the temperature dropped 15°C. 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. exo endo exo exo

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

5 Exothermic reaction: Mg + HCl
magnesium + hydrochloric acid 25° C 45° C magnesium Heat energy given out Gets hot hydrochloric acid

6 Exothermic reactions: source of energy
If heat is given out this energy must have come from chemical energy in the starting materials (reactants). 45° C 25o C Reactants convert chemical energy to heat energy. The temperature rises.

7 Exothermic reactions: energy changes
Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. 25° C 45o 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.

8 Exothermic energy level diagram
Energy / kJ) Progress of reaction (time) 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.

9 Exothermic reactions and ΔH
H (delta H) is how much energy is given out Energy / kJ Progress of reaction reactants products H=negative H is negative because the products have less energy than the reactants.

10 Definition of an exothermic reaction
Exothermic reactions give out energy. There is a temperature rise and H is negative. products Energy / kJ) Progress of reaction reactants H is negative

11 Exothermic reactions summary

12 Energy Transfer Exothermic reactions Endothermic reactions
Contents Energy Transfer Exothermic reactions Endothermic reactions Bonds and energy Activation energy Summary activities

13 Endothermic reactions
Endothermic reactions cause a decrease in temperature. 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.

14 Endothermic reaction: NH4NO3 + H2O
Endothermic reactions cause a decrease in temperature. ammonium nitrate water Heat energy taken in as the mixture returns back to room temp. Cools Starts 25° C Cools to 5° C Returns to 25° C

15 Endothermic reactions: source of energy
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. 5° C 25o C Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

16 Endothermic reactions: energy changes
The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. 25° C 5o C 25o C The reactants gain energy. 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.

17 Endothermic reactions and ΔH
This is how much energy is taken in products Energy / kJ) Progress of reaction reactants H=+ This is positive because the products have more energy than the reactants.

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

19 Endothermic reactions summary

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

21 True or false?

22 Energy Transfer Exothermic reactions Endothermic reactions
Contents Energy Transfer Exothermic reactions Endothermic reactions Bonds and energy Activation energy Summary activities

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

24 Energy given out as bonds form between atoms
Making chemical bonds 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. Heat given out Energy in chemicals Energy given out Energy given out as bonds form between atoms

25 Bonds and endothermic reactions
In most chemical reactions some existing bonds are broken (endothermic) But new bonds are made (exothermic) Energy in chemicals reactants products Energy given out as new bonds form Energy taken in as old bonds break +H Overall endothermic in this case

26 Bonds and exothermic reactions
Again some existing bonds are broken (endothermic) And new bonds are formed (exothermic) Energy in chemicals reactants products Energy taken in as old bonds break Energy given out as new bonds form -H Overall exothermic – in this case

27 Summary of bond changes
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. Energy in chemicals reactants products -H Bonds break Bond forming +H Bonds form Exo Endo

28 Burning methane -H This is an exothermic reaction: C H O H O C H O C
Progress of reaction Energy in chemicals O C H Bond Breaking Bond Forming O H C O C H -H

29 Energy Transfer Exothermic reactions Endothermic reactions
Contents Energy Transfer Exothermic reactions Endothermic reactions Bonds and energy Activation energy Summary activities

30 Activation energy Why do all reactions need some energy to start with?

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

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

33 Petrol + oxygen 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. activation energy petrol + oxygen Energy / kJ) carbon dioxide + water Progress of reaction

34 Petrol + oxygen: activation energy
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. activation energy petrol + oxygen Energy / kJ) carbon dioxide + water Progress of reaction

35 Exothermic reactions:
Summary 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: Bond breaking is endothermic. Bond forming is exothermic. Reactions require an activation energy to help start the bond breaking process.

36 Energy Transfer Exothermic reactions Endothermic reactions
Contents Energy Transfer Exothermic reactions Endothermic reactions Bonds and energy Activation energy Summary activities

37 Glossary activation energy – The amount of energy needed for a reaction to begin. bond – A strong force that joins atoms or ions together in molecules and giant lattices. delta H – The amount of energy change during a reaction. endothermic – A type of reaction that requires energy. exothermic – A type of reaction that generates energy.

38 Anagrams

39 Exothermic or endothermic?

40 Activity 1 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?

41 Activity 1 answer 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. products Energy / kJ Progress of reaction reactants

42 Activity 2 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 O H Bond Energy (kJ) H-O 464 O-O 146 O=O 498 O H O O H O H

43 Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ
Activity 2 answer Bond Energy (kJ) H-O 464 O-O 146 O=O 498 O H O H Bond breaking. (endothermic) 4(O-H) + 2(O-O) = = +2148kJ Bond forming: (exothermic) 4(O-H) + 1(O=O) = = -2354kJ H = – 2354 = -206kJ (Exothermic) O O H O H

44 Summary on activation energy
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 (Ea or Eact) Once an exothermic reaction is underway it can provide its own activation energy (from the bond forming stage) and so sustains the reaction.

45 Multiple-choice quiz


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