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VCE Chemisrtry Unit 4 AOS 1 © Ms Lanna Derry & Edrolo 2014

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Unit 4, Area of Study 1: Industrial Chemistry

Energy Profile Diagrams & the use of Delta H

Watch this lesson online: https://edrolo.com.au/vce/subjects/chemistry/vce-chemistry/unit-4-area-study-1/energy-profile-diagrams-use-delta-h/key-knowledge/

CHEMISTRY


We do our best to make these slides comprehensive and up-to-date, however there may be errors. We'd appreciate it if you pointed these out to us! 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



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



•  Kinetic energy -‐ movement of particles – Movement of electrons – Vibrations of and rotations around bonds.

– Actual movement in the substance (gases vs liquids and solids)



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.



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.



Energy pro-ile diagram Exothermic reaction

•  We can visualise this change using an energy pro-ile diagram:



•  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.



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:



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



•  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’



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.



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:



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.



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

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