Energy changes

Chemical Energetics 1: All chemical reactions involve an energy change. The transfer of energy, usually heat, into or out of the reaction mixture’. For example: When petrol burns heat is given out. When ammonium nitrate dissolves in water heat is taken in.

Chemical Energetics 2: Instead of writing ‘energy change’ all the time chemists use the symbols:  H Pronounced ‘delta aitch’.  =greek letter ‘delta’ meaning change. H=heat. So,  H means ‘heat change’.

Temperature Changes 1 ,[object Object],[object Object],[object Object]

Temperature Changes 2 ,[object Object],[object Object],[object Object]

Representing Energy Changes: ,[object Object],[object Object],[object Object],See next slide for some examples.

Energy Level Diagrams ,[object Object],This energy level diagram shows a reaction where  H is positive (an endothermic reaction). Reactants Products  H negative, exothermic. energy Reactants Products  H positive, endothermic. energy

Making and Breaking Bonds: ,[object Object],Breaking chemical bonds is always endothermic. All chemical reactions involve bond ‘rearrangements’. Bonds are broken and new ones are formed. ,[object Object],[object Object],[object Object]

Reaction Profiles: ,[object Object],[object Object],Reaction profiles can be used to do numerical calculations on the energy changes that happen in chemical reactions. The next slide shows the energy profile for the reaction between hydrogen and chlorine to form hydrogen chloride.

Reaction Profile for: ,[object Object],energy H-H, Cl-Cl Reactants H, H, Cl, Cl (Atoms) H-Cl, H-Cl Products Energy taken in to break bonds. Energy given out when bonds are made. Overall energy change,  H

Calculating Energy Changes: ,[object Object],‘ Bond energy’ is the amount of energy required to break a bond. This value is always endothermic , ie. has a positive value. When a bond is formed the energy given out is exactly equal to, but of opposite sign, to the energy required to break the bond. Bond making is always an exothermic process . .

An Example Calculation: ,[object Object],H 2 + Cl 2  2HCl to see the reaction profile. In this reaction one H-H bond and one Cl-Cl bond is broken and two H-Cl bonds are formed. The H-H bond energy is 436kJ/mol The Cl-Cl bond energy is 242kJ/mol So, the energy needed to break these bonds is 436 + 242 = 678kJ The H-Cl bond energy is 431kJ So the energy given out when these bonds are formed is 2 x -431 = -862kJ Overall change = 678 – 862 = -184kJ, an exothermic reaction. Next slide shows the reaction profile with these values added.

Reaction Profile for: ,[object Object],energy H-H, Cl-Cl Reactants H, H, Cl, Cl (Atoms) H-Cl, H-Cl Products Energy out = -862kJ Overall energy change,  H = -184kJ Energy in = +678kJ

Chemical reactions take place in two stages: ,[object Object],[object Object],Summary Bond breaking is endothermic, bond making is exothermic. The energy changes in a chemical reaction can be represented using energy level diagrams and reaction profile diagrams. ,[object Object],[object Object]

Energy changes

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