Exothermic and Endothermic Reactions Linking Energy Profile Diagrams to Thermometer Readings

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

THE CONCEPT OF ENTHALPY CHANGE 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. We can measure the change in energy, we call this change the enthalpy change (or heat of reaction).

Exothermic Reaction Reactants + 25oC Products 25oC Initial temperature

Exothermic Reaction 25oC

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

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

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

Energy Level Diagrams Exothermic reactions energy course of reaction

Energy Level Diagrams Exothermic reactions energy reactants course of reaction

Energy Level Diagrams Exothermic reactions energy reactants products course of reaction

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

Endothermic Reaction Reactants + 25oC Products 25oC Initial temperature

Endothermic Reaction 25oC

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

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

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

Energy Level Diagrams Endothermic reactions energy

Energy Level Diagrams Endothermic reactions energy course of reaction

Energy Level Diagrams Endothermic reactions energy reactants course of reaction

Energy Level Diagrams Endothermic reactions energy products reactants course of reaction

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

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

Endothermic processes EXOTHERMIC & ENDOTHERMIC PROCESSES Examples: Exothermic processes Gas  Liquid  Solid Combustion of fuels (e.g. wood; coal; natural gas) Neutralisation reactions ( acid + base) Endothermic processes Solid  Liquid  Gas Dissolving of ammonium chloride or ammonium sulphate crystals in water Thermal decomposition CaCO3  CaO + CO2

∆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

Bond energies The energy needed to break a chemical bond Different chemical bonds have different bond energies Chemical bond Bond energy, kJ/mole H―H 436 O=O 498 O―H 464

Bond energies 2H2 + O2  2H2O H O H O H O O H Chemical bond Bond energy, kJ/mole H―H 436 O=O 498 O―H 464 Stage 1: Bond breaking H O H O +436kJ +498 kJ +436kJ Stage 2: Bond forming -464kJ H O O H

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