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Understanding Chemical Reactions

Learn about the concepts of bond breaking and forming in chemical reactions, including the energy changes involved. Explore exothermic and endothermic reactions and understand their energy level diagrams. Discover the difference between physical changes and chemical reactions.

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Understanding Chemical Reactions

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  1. Understanding Chemical Reactions Bonds & Energy

  2. Bond breaking and forming • Chemical reactions involve enthalpy changes as a result of making and breaking bonds.

  3. Breaking and Forming • Breaking bonds requires energy. It is endothermic. • Making new bonds releases energy. It is exothermic.

  4. Exothermic reactions In these reactions, the energy required for bond breaking IS LESS than the energy released by bond making. 2H2 + O2 2H2O

  5. Endothermic Reactions In these reactions, the energy required for bond breaking IS GREATER than the energy released by bond making. This means that to make endothermic reactions take place, extra energy is needed from an external source.

  6. ENERGY AND CHEMICAL REACTIONS Exothermic reactions • These reactions release energy usually in the form of heat. • DH is negative • Examples, combustion reactions, (metal displacement and neutralisation reactions).

  7. ENERGY AND CHEMICAL REACTIONS Endothermic reactions • These reactions take in heat energy from their surroundings. • These cause temperatures to fall. • Evaporation of sweat, photosynthesis.

  8. Predicting endo or exo • Sometimes you can look at a reaction and using your common sense/ chemistry knowledge decide whether it is endo or exo. • (Usually you need data tables giving you information on the strengths of bonds broken and formed (not part of the WACE course)).

  9. Exothermic energy level diagram • ∆H (‘delta H’) is the symbol for the ‘change in energy’. • In an exothermic reaction the products have less energy than the reactants. • ∆H is negative for an exothermic reaction.

  10. Endothermic energy level diagram • In an endothermic reaction the products have more energy than the reactants. • ∆H is positive for an endothermic reaction.

  11. Physical changes involve enthalpy changes but these are smaller than the changes involved in a chemical reaction. • Melting, evaporation and boiling are endothermic process requiring energy. • Freezing and condensation are exothermic processes releasing energy.

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