CHEMISTRY. Unit 4, Area of Study 1: Industrial Chemistry. Energy Profile Diagrams & the use of Delta H

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1 Watch this lesson online: CHEMISTRY Unit 4, Area of Study 1: Industrial Chemistry Energy Profile Diagrams & the use of Delta H 1

2 Key Knowledge energy pro-ile diagrams and the use of ΔH notation including: activation energy; alternative reaction pathways for catalysed reactions; and deduction of ΔH for an overall reaction given energy pro-iles or ΔH of two related reactions 2

3 Chemical energy The chemical energy of a substance is made up of its potential energy and kinetic energy. Potential energy attraction between nuclei and electrons Repulsion between electrons Repulsion between nuclei 3

4 Kinetic energy - movement of particles Movement of electrons Vibrations of and rotations around bonds. Actual movement in the substance (gases vs liquids and solids) 4

5 Enthalpy The chemical energy of a substance cannot be directly measured, but it can be determined theoretically. It is known as enthalpy or heat content and has the symbol H. However, a change in enthalpy, ΔH, for a chemical reaction can be determined. 5

6 Measuring enthalpy The change in temperature, ΔT, gives an indication of change in enthalpy. If temperature increases during the course of a reaction, then heat energy is being released to the surroundings by the reaction So enthalpy of the products is less than enthalpy of reactants. 6

7 Energy pro-ile diagram Exothermic reaction We can visualise this change using an energy pro-ile diagram: 7

8 If temperature decreases during the course of a reaction, then heat energy is being absorbed from the surroundings by the reaction So enthalpy of the products is greater than enthalpy of reactants. 8

9 Energy pro-ile diagram endothermic reaction To visualise the decrease in enthalpy when a temperature decrease occurs we can also use an energy pro-ile diagram: 9

10 De-inition of ΔH ΔH = H(products) H(reactants) So if the temp increases, H(products) < H(reactants), So ΔH is negative This is an exothermic reaction If the temp decreases H(products) > H(reactants), So ΔH is positive This is an endothermic reaction 10

11 We can recognise an exothermic reaction by the increase in temperature of the surroundings eg Combustion reactions, neutralization reactions Endothermic reactions are much less common. The temperature of the surroundings decreases eg chemical cold packs 11

12 Activation energy The activation energy is the amount of energy required to break the bonds of the reactants. It is the amount of energy needed to start a reaction. eg a spark to start a -ire, or to burn fuel in an engine. 12

13 Reversing the reaction If we were to turn products back into reactants, then the activation energy could still be determined from the energy pro-ile diagram: 13

14 Catalysts and activation energy A catalyst increases the rate of a reaction. It does this by creating an alternative energy pathway for the reaction. This pathway has a lower activation energy than the original reaction. 14

15 Sample exam question energy pro-ile diagrams The reaction A + B C involves a two step process. A + B X: ΔH positive and X C; ΔH negative Which one of the diagrams below best represents the energy changes during the course of the reaction? VCAA June 2007 Q19 15