2Energy Transfer Exothermic reactions Endothermic reactions ContentsEnergy TransferExothermic reactionsEndothermic reactionsBonds and energyActivation energySummary activities
3Exothermic 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.exoendoexoexo
4Examples 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 acidThermit reaction
6Exothermic reactions: source of energy If heat is given out this energy must have come from chemical energy in the starting materials (reactants).45° C25o CReactants convert chemical energy to heat energy.The temperature rises.
7Exothermic reactions: energy changes Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature.25° C45o CChemical 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.
8Exothermic energy level diagram 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.
9Exothermic reactions and ΔH H (delta H) is how much energyis given outEnergy / kJProgress of reactionreactantsproductsH=negativeH is negativebecause the products have less energy than the reactants.
10Definition 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
12Energy Transfer Exothermic reactions Endothermic reactions ContentsEnergy TransferExothermic reactionsEndothermic reactionsBonds and energyActivation energySummary activities
13Endothermic 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.
14Endothermic reaction: NH4NO3 + H2O Endothermic reactions cause a decrease in temperature.ammoniumnitratewaterHeatenergytakenin as the mixture returns back to room temp.CoolsStarts 25° CCools to 5° CReturns to 25° C
15Endothermic 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° C25o CReactants convert heat energy into chemical energy as they change into products. The temperature drops.
16Endothermic reactions: energy changes The cold reaction products start to gain heat from the surroundings and eventually return to room temperature.25° C5o C25o CThe 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.
17Endothermic reactions and ΔH This is howmuch energyis taken inproductsEnergy / kJ)Progress of reactionreactantsH=+This is positivebecause the products have more energy than the reactants.
18Definition of an endothermic reaction Endothermic reactions take in energy. There is a temperature drop and H is positive.productsEnergy / kJProgress of reactionreactantsH=+
20Energy diagramsSketch the two energy diagrams and label exothermic and endothermic as appropriate.productsEnergy / kJ)Progress of reactionreactantsH = -H = +productsEnergy / kJProgress of reactionreactants
22Energy Transfer Exothermic reactions Endothermic reactions ContentsEnergy TransferExothermic reactionsEndothermic reactionsBonds and energyActivation energySummary activities
23Breaking 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 inEnergy in chemicalsEnergy neededEnergy needed to overcome the bonding between the atoms
24Energy given out as bonds form between atoms Making chemical bondsIt is reasonable to assume that bond making will be the opposite of bond breakingEnergy will be given out in an exothermic process when bonds are formed.Heat given outEnergy in chemicalsEnergy given outEnergy given out as bonds form between atoms
25Bonds and endothermic reactions In most chemical reactions some existing bonds are broken (endothermic)But new bonds are made (exothermic)Energy in chemicalsreactantsproductsEnergy given out as new bonds formEnergy taken in as old bonds break+HOverall endothermic in this case
26Bonds and exothermic reactions Again some existing bonds are broken (endothermic)And new bonds are formed (exothermic)Energy in chemicalsreactantsproductsEnergy taken in as old bonds breakEnergy given out as new bonds form-HOverall exothermic – in this case
27Summary 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 chemicalsreactantsproducts-HBonds breakBond forming+HBonds formExoEndo
28Burning methane -H This is an exothermic reaction: C H O H O C H O C Progress of reactionEnergy in chemicalsOCHBondBreakingBondFormingOHCOCH-H
29Energy Transfer Exothermic reactions Endothermic reactions ContentsEnergy TransferExothermic reactionsEndothermic reactionsBonds and energyActivation energySummary activities
30Activation energyWhy do all reactions need some energy to start with?
31Exothermic reactions and Ea Activation energyreactantsH= -Energy / kJ)productsProgress of reaction
32Endothermic reactions and Ea ActivationenergyproductsH = +Energy / kJ)reactantsProgress of reaction
33Petrol + oxygenCopy 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 energypetrol + oxygenEnergy / kJ)carbon dioxide + waterProgress of reaction
34Petrol + 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 energypetrol + oxygenEnergy / kJ)carbon dioxide + waterProgress of reaction
35Exothermic reactions: SummaryExothermic 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.
36Energy Transfer Exothermic reactions Endothermic reactions ContentsEnergy TransferExothermic reactionsEndothermic reactionsBonds and energyActivation energySummary activities
37Glossaryactivation 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.
40Activity 1The 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?
41Activity 1 answer Nitrogen + oxygen nitrogen(IV)oxide N2 + 2O2 2NO2.H positive (+33.2kJ/mol).The reaction is endothermic.Energy diagramBond breaking involvesthe biggest energy change.productsEnergy / kJProgress of reactionreactants
42Activity 2 Hydrogen peroxide decomposes as shown: Calculate energy for bond breaking.Calculate the energy from bond makingWhat is the value of H for the reaction shownOHBondEnergy (kJ)H-O464O-O146O=O498OHOOHOH
44Summary 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.