2. After completing this topic you should be able to : The activated complex is the unstable intermediate formed at the peak of the potential energy diagram. The activation energy is the minimum energy required by colliding particles to form an activated complex. An activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction. Learn to describe the relationship between temperature, kinetic energy and activation energy.

3. Activation Energy Svante August Arrhenius (1859 –1927) In 1889 Arrhenius explained the fact that most reactions require added heat energy to proceed by formulating the concept of activation energy, an energy barrier that must be overcome before two molecules will react. https://www.youtube.com/watch?v=fuFjXO-hqDc https://www.youtube.com/watch?v=gigpMI-xnbs A 10 o C rise in temperature will approximately double the rate of the reaction Oxalic acid and Potassium Permaganate (relative rate vs temperature)

4. Potential energy diagrams give useful information about the energy profile of a reaction. The activation energy is the minimum kinetic energy required by colliding molecules for a reaction to occur. In the diagrams shown above the activation energy appears like a ‘energy barrier’ which reactants must get over to become products. Activation energy and reaction pathway Higher Chemistry Eric Alan and John Harris

5. Activation energy and reaction pathway Activation Energy is the additional P.E. which has to be attained by colliding molecules to form an activated complex. Activated complex is the unstable arrangement of atoms formed at the maximum of the potential energy barrier. 1 2 3 Activated complex Breaking bonds Making bonds Activation Energy E A P.E. 1 2 3 Reaction Path

6. As a reaction proceeds from reactants to products, an intermediate stage is reached at the top of the activation barrier at which a highly energetic species called an activated complex is formed. A + B → X → C+D Activation energy and reaction pathway Higher Chemistry Eric Alan and John Harris

7. This unstable activated complex only exist for a short period of time. From the peak of the energy barrier it can lose energy in one of two ways i.e. to the stable products or to form the reactants again. The higher the E a the higher the barrier and the more energy required to start the reaction. Activation energy and reaction pathway A + B → X → C+D Higher Chemistry Eric Alan and John Harris

8. Endothermic reactions take in thermal energy from their surroundings.  H = +ve EaEa +  H P.E EaEa -  H-  H Exothermic reactions give out thermal energy  H = -ve Activation energy and reaction pathway R R P P

9. 1. Mark E a and ∆H on the PE diagrams and then calculate the value of each for the forward reaction. A Ea = 50 KJmol -1 Ea ∆ H = -10 kJmol -1 ∆H B Ea = 30 KJmol -1 Ea ∆ H = -40 kJmol -1 ∆H C Ea = 40 KJmol -1 Ea ∆ H = +20 kJmol -1 ∆H Higher Chemistry Eric Alan and John Harris

10. Activation energy and reaction pathway 2. Mark E a and ∆H on the PE diagrams and then calculate the value of each for the reverse reaction. A Ea = 60 KJmol -1 Ea ∆ H = +10 kJmol -1 ∆H B Ea = 70 KJmol -1 Ea ∆ H = +40 kJmol -1 ∆H C Ea = 20 KJmol -1 Ea ∆ H = -20 kJmol -1 ∆H Higher Chemistry Eric Alan and John Harris

11. Activation energy and reaction pathway Ea Calculate Ea for the forward reaction Ea = 210 – 20 = 190kJ Higher Chemistry Eric Alan and John Harris

12. Temperature and kinetic energy What do we mean by temperature and heat (thermal energy) ? The thermal energy of a system is a measure of both the potential and kinetic energy within the system. The temperature is a measure of how ‘hot’ a system is. The temperature is a measure of the average kinetic energy in a system..

13. Activation Energy Svante August Arrhenius (1859 –1927) In 1889 Arrhenius explained the fact that most reactions require added heat energy to proceed by formulating the concept of activation energy, an energy barrier that must be overcome before two molecules will react. https://www.youtube.com/watch?v=fuFjXO-hqDc https://www.youtube.com/watch?v=gigpMI-xnbs A 10 o C rise in temperature will approximately double the rate of the reaction Oxalic acid and Potassium Permaganate (relative rate vs temperature)

14. Temperature and energy

15. Energy Distribution Total number of collisions with sufficient K.E. energy is the area under the graph to the right of the E a. Number of collisions which result in new products being formed. No of molecules Kinetic energy EaEa

16. Temperature and Activation energy No of molecules Energy associated with molecules T1T1 T2T2 Activation energy E A The grey area represents the number of particles with energy in excess of the E A. These particles have enough energy to cause successful collisions At the higher temperature T 2 the number of particles with energy in excess of E A is greater. The blue area now represents particles with energy in excess of E A. The blue area is larger than the grey so at temperature T 2 more particles have enough energy to cause successful collisions. Increasing the temperature means a greater number of molecules have energies in excess of E A. Even a small rise in temperature causes a large increase in the number of particles with energy above E A. Therefore a greater proportion of collisions will be successful.

17. Energy distribution E a does not change but the number of successful collisions increases. No of molecules E a does not change but the number of successful collisions increases significantly, so rate increases. Increased temperature No of molecules Kinetic energy EaEa Increased concentration EaEa Kinetic energy

18. E a does not change but the number of successful collisions increases significantly, so rate increases. Increased temperature No of molecules Kinetic energy EaEa

19. 1.Before collision Reactants 2. In full collision Activated complex Partially broken reactant bonds and partially formed product bonds 3. After collision Products Energy changes Reactants Products If the reactants have enough combined K.E. to overcome E a, their K.E. is converted into the energy needed to form the activation complex. Activation energy and reaction pathway