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Exothermic and Endothermic Reactions Linking Energy Profile Diagrams to Thermometer Readings.

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Presentation on theme: "Exothermic and Endothermic Reactions Linking Energy Profile Diagrams to Thermometer Readings."— Presentation transcript:

1 Exothermic and Endothermic Reactions Linking Energy Profile Diagrams to Thermometer Readings

2 Candidates should be able to: (a) describe the meaning of enthalpy change in terms of exothermic (ΔH negative) and endothermic (ΔH positive) reactions (b) *represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation energies (see 6.1(c)) (c) describe bond breaking as an endothermic process and bond making as an exothermic process (d) *explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds (e) describe combustion of fuels as exothermic, e.g. wood; coal; oil; natural gas; hydrogen (f) describe hydrogen, derived from water or hydrocarbons, as a potential fuel for use in future, reacting with oxygen to generate electricity directly in a fuel cell (details of the construction and operation of a fuel cell are not required) (g) name natural gas, mainly methane, and petroleum as sources of energy

3 T HE C ONCEPT O F E NTHALPY C HANGE We can measure the change in energy, we call this change the enthalpy change (or heat of reaction). In chemical reactions, we can measure changes in terms of energy. We cannot measure the total amount of energy in the beginning, we also cannot measure the total amount of energy in the end.

4 Exothermic Reaction 25 o C Initial temperature Reactants+ Products

5 Exothermic Reaction 25 o C

6 Exothermic Reaction temperature time 25C 32C Reactants give out energy to the surroundings

7 Exothermic Reaction Reactants give out energy to the surroundings Temperature of surroundings increase Energy of reactants decrease Energy of reactants Reactants ΔH is negative Temperature (surroundings) 25C 32C Products

8 Energy Levels Exothermic reactions Energy is given out The products have less energy than the reactants Combustion and neutralisation are exothermic

9 Energy Level Diagrams Exothermic reactions energy course of reaction

10 Energy Level Diagrams Exothermic reactions energy course of reaction reactants

11 Energy Level Diagrams Exothermic reactions energy course of reaction reactants products

12 Energy Level Diagrams Exothermic reactions energy course of reaction reactants products energy given out H is negative

13 Endothermic Reaction 25 o C Initial temperature Reactants+ Products

14 Endothermic Reaction 25 o C

15 Endothermic Reaction temperature time 19C 25C Reactants take in energy from the surroundings

16 Endothermic Reaction Reactants take in energy from the surroundings Temperature of surroundings decrease Energy of reactants increases Energy of reactants Reactants ΔH is positive Temperature (surroundings) 19C 25C Products

17 Energy Level Diagrams Endothermic reactions Energy is taken in The products have more energy than the reactants The energy is taken in from the surroundings

18 Energy Level Diagrams Endothermic reactions energy

19 Energy Level Diagrams Endothermic reactions energy course of reaction

20 Energy Level Diagrams Endothermic reactions energy course of reaction reactants

21 Energy Level Diagrams Endothermic reactions energy course of reaction reactants products

22 Energy Level Diagrams Endothermic reactions energy course of reaction energy taken in H is positive reactants products

23 Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings H is negativeH is positive Products have less energy than reactants Products have more energy than reactants

24 E XOTHERMIC & E NDOTHERMIC P ROCESSES 1.Solid Liquid Gas 2.Dissolving of ammonium chloride or ammonium sulphate crystals in water 3.Thermal decomposition CaCO 3 CaO + CO 2 Endothermic processes 1.Gas Liquid Solid 2.Combustion of fuels (e.g. wood; coal; natural gas) 3.Neutralisation reactions ( acid + base) Exothermic processes Examples:

25 H How much energy is given out or taken in? Energy is needed to break chemical bonds Energy is given out when bonds are made H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

26 Bond energies The energy needed to break a chemical bond Different chemical bonds have different bond energies Chemical bondBond energy, kJ/mole HH436 O=O498 OH464

27 Bond energies 2H 2 + O 2 2H 2 O Chemical bond Bond energy, kJ/mole HH436 O=O498 OH464 HH OO HH Stage 1: Bond breaking +436kJ +498 kJ HH OO H H Stage 2: Bond forming HH OO H H -464kJ

28 Working out H Summary The energy values have units of kJ/mole In the exam, you will be given the energy values and all the bonds to make or break Energy goes in to break bonds Energy goes out when bonds are made


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