ENTHALPY

OBJECTIVES  Exothermic changes cause heat to be released to the surroundings  Endothermic changes cause absorption of heat from the surroundings.  A potential energy diagram can be used to show the energy pathway for a reaction.  The enthalpy change is the energy difference between products and reactants.

Enthalpy Diagrams Exothermic reactions energy course of reaction

Energy Level Diagrams Exothermic reactions energy course of reaction reactants

Energy Level Diagrams Exothermic reactions energy course of reaction reactants products

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

Energy Level Diagrams Endothermic reactions energy course of reaction

Energy Level Diagrams Endothermic reactions energy course of reaction reactants

Energy Level Diagrams Endothermic reactions energy course of reaction reactants products

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

Summary Table Exothermic reactions Endothermic reactions

Summary Table Exothermic reactions Endothermic reactions Energy is given out to the surroundings Energy is taken in from the surroundings

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

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

∆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

C H H HH O O O O

C HHHHOOOO Energy absorbed to break bonds

C HHHHOOOO

C HHHHOOOO Energy released when new bonds are made

Overall energy change = Energy released on making new bonds - Energy taken in to break old bonds

Energy Level Diagrams Exothermic reactions energy course of reaction methane + oxygen carbon dioxide + water

Reaction mixture Test tube

Heat given out to surroundings EXOTHERMIC

Heat taken in from surroundings ENDOTHERMIC

TASK  Draw energy level diagrams for the following reactions: 1.Photosynthesis 2.Calcium with sulphuric acid 3.Dissolving ammonium nitrate