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THE MECHANISM OF CHEMICAL REACTIONS

THE MECHANISM OF CHEMICAL REACTIONS. The steps that atoms go through as their arrangement changes from reactants to products is known as the MECHANISM of the reaction. On the microscopic level a number of steps are essential before a reaction will occur.

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THE MECHANISM OF CHEMICAL REACTIONS

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  1. THE MECHANISM OF CHEMICAL REACTIONS

  2. The steps that atoms go through as their arrangement changes from reactants to products is known as the MECHANISM of the reaction. On the microscopic level a number of steps are essential before a reaction will occur. The model used to explain reaction mechanism is THE COLLISION THEORY.

  3. The Collision Theory • reacting atoms, molecules or ions must collide with each other. • the particles must have sufficient energy and must be oriented correctly. • bonds in the original molecules must break and new bonds must form. • electrons must re-arranged in order to form new bonds.

  4. Amount of energy and orientation of colliding particles leads to some collisions being effective and others not.

  5. Particles with sufficient energy: This Boltzmann curve shows that there is only a small fraction of molecules that have enough energy for the reaction

  6. THE ACTIVATED COMPLEX All reactions need energy to start. The reacting atoms must pass over an “energy hill” to change from reactants to products. The transition state that atoms must pass through is called the ACTIVATED COMPLEX. The activated complex is an unstable structure where bonds are forming and breaking at the same time.

  7. ENERGY DIAGRAM showing the changes in the potential energy of the reacting substances during the reaction. In an exothermic reaction the energy of products is less than the energy of the reactants.

  8. In an endothermic reaction the energy of products is greater than the energy of the reactants.

  9. Know these definitions: ACTIVATION ENERGY – the difference in energy between the reactants and the activated complex. Ea is measured in kJ/mol. Ea is also referred to as the minimum amount of energy required for a molecule to react.

  10. HEAT OF REACTION – difference in energy between reactants and products. ΔH = energy products – energy reactants ΔH is measured in kJ/mol ΔH > 0 for endothermic reactions ΔH < 0 for exothermic reactions

  11. Comparing exothermic and endothermic reactions:

  12. What are you expected to do? • state the general steps in the collision theory. • state the factors necessary for an effective collision. • describe and explain energy changes in a chemical reaction. • interpret graphs showing energy changes in chemical reactions.

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