1Everything Science www.everythingscience.co.za12. Energy and chemical changeChemistryGrade 11
2Everything Science www.everythingscience.co.zaEnergy changes in chemical reactionsWhen a reaction occurs, bonds in the reactants break and new bonds in the products form.These changes involve energy. When bonds break, energy is absorbed and when new bondsform, energy is released.The bond energy is a measure of bond strength in a chemical bond. It is the amount of energy(in kJ·mol−1) that is needed to break the chemical bond between two atoms.Enthalpy is a measure of the total energy of a chemical system for a given pressure and isgiven the symbol H.
3Everything Science www.everythingscience.co.zaEndothermic and exothermic reactionsIf the energy that is needed to break the bonds is less than the energy that is released whennew bonds form, then the reaction is exothermic. The energy of the products is less thanthe energy of the reactants.An exothermic reaction is one that releases energy in the form of heat or light.Combustion reactions and respiration are both examples of exothermic reactions.If the energy that is needed to break the bonds is more than the energy that is releasedwhen new bonds form, then the reaction is endothermic. The energy of the products isgreater than the energy of the reactants.An endothermic reaction is one that absorbs energy in the form of heat or light.Photosynthesis and the thermal decomposition of limestone are both examples ofendothermic reactions.
4Everything Science www.everythingscience.co.zaHeat of reactionThe difference in energy between the reactants and the product is called the heat of reactionand has the symbol ∆H. ∆H is calculated using:● In an endothermic reaction, ∆H is a positive number (greater than 0).● In an exothermic reaction, ∆H will be negative (less than 0).Δ H=Eprod − EreactType of reaction Exothermic EndothermicEnergy absorbed orreleasedReleased AbsorbedRelative energy ofreactants and productsEnergy of reactants greaterthan energy of productsEnergy of reactants less thanenergy of productsSign of ΔH Negative Positive
5Everything Science www.everythingscience.co.zaEnergy graph for exothermic reactions
6Everything Science www.everythingscience.co.zaEnergy graph for endothermic reactions
7Everything Science www.everythingscience.co.zaActivation energyThe activation energy is the minimum amount of energy that is needed to start a chemicalreaction.The activated complex (or transition state) is the complex that exists as the bonds in theproducts are forming and the bonds in the reactants are breaking. This complex exists for avery short period of time and is found when the energy of the system is at its maximum.The two examples below show how the activated complex is drawn:
8Everything Science www.everythingscience.co.zaEnergy graph for exothermic reactions
9Everything Science www.everythingscience.co.zaEnergy graph for endothermic reactions
10Everything Science www.everythingscience.co.zaFor more practice see:www.everythingscience.co.zaShortcode: ESBQS