Potential Energy Diagrams

1. Kinetics and Equilibrium [plus] Potential Energy Diagrams

2. A HYDROGEN BALLOON EXPLODES WHEN HEATED WITH A FLAME. We can DRAW the energy in a POTENTIAL ENERGY DIAGRAM

3. The reaction of a hydrogen balloon exploding is: hydrogen + oxygen ---> water + energy This is a VERY exothermic reaction. according to Table I: H is - 483.6 kJ

4. This reaction is FAST. As soon as the hydrogen took in enough energy (heat from flame) the combustion took off and happened instantly. The reactants required some energy to react (hydrogen + oxygen + some energy), then Water is formed, and the bonds formed by the formation of water from hydrogen and oxygen will release a LOT OF ENERGY when they form. The products are water + LOTS of energy.

5. Potential Energy Diagrams In KINETICS we will be drawing and looking at graphs showing the energy levels of the reactants, the products, and what happens energy wise to start reactions. These graphs are called potential energy diagrams. Graphs have axis labels (of course) and titles (of course, again).

6. DON’T BE AFRAID. Everything is hard the first time you see it, but it will grow on you like a fungus.

7. Here’s a Potential Energy Diagram! Let’s decode this little picture and figure out what is shows us... The curve represents the energy levels of the reactants, the reaction and finally the products. The ABC and D arrows also measure things. The reaction coordinate shows the direction of the reaction, the start is on the left, the end is on the right side of the graph.

8. Hereit is again! A The REACTANTS get together at the start of the reaction and have a certain potential energy, which in this graph is “A”. The measure of this energy is USUALLY in kJ/mole or kilo Joules per mole.

9. Can’t get rid of this picture! D The PRODUCTS have a potential energy when they are finally produced, which is “D”. The products are formed at THE END OF THE REACTION, on the right hand part of this curve.

10. B STILL HERE?!! In order to make this reaction occur, the reactants have to collide at the right orientation and with the right amount of energy. The amount of energy it takes to start this reaction is “B”. That’s the ACTIVATION ENERGY

11. I LOVE KINETICS! The difference between the starting potential energy at left, and the ending potential energy at right (which on this graph is “A” minus “D”, is the change in energy, we call this the H.

12. SINCE the potential energy of the reactants is greater than the potential energy of the products, where did that energy go? The energy WAS RELEASED! That’s what EXOTHERMIC means. It’s a -H

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14. Potential Energy Diagram for an ENDOTHERMIC Reaction

15. (a) is the ACTIVATION ENERGY. (c) is this ΔH which is + The products end up with MORE ENERGY than the reactants started with, therefore this energy MUST HAVE BEEN ABSORBED (ENDOthermic).

16. Collision Theory • Collision Theory states that reactions happen when atoms and molecules collide with each other with proper energy and proper orientation. • 4 Factors affect how well collisions will happen. They are:Concentration of Reactants, Temperature, Surface area, and Catalysts. • The first three increase the effectiveness of the collisions, leading to more and faster reactions. • Catalysts decrease the ACTIVATION ENERGY required to start the reaction.

17. A catalyst provides AN ALTERNATIVE reaction pathway, with a LOWER ACTIVATION ENERGY than an “un-catalyzed” reaction. Once a reaction happens you can see how the potential energy of the reactants compares to the potential energy of the products. The difference is called the ΔH. If the ΔH is positive, the reaction is endothermic (the products have MORE energy than the reactants. If the ΔH is negative, the reaction is exothermic (the products have LESS energy than the reactants. Table I shows the ΔH for MANY chemical reactions.

18. While you watch the bunny (!?),think about this::: Some reactions get into a DYNAMIC EQUILIBRIUM. That means that the forward rate of reaction equals the reverse rate of reaction. Like the bunny on the swing, in a sort of balance, changing all the time, butthe same, all the time.

19. LeChatelier’s Principle When a chemical system at dynamic equilibrium, is stressed, the system will shift to relieve that stress, and a new dynamic equilibrium will form at the new conditions. or If a chemical system at equilibrium experiences a change in concentration, temperature, or total pressure; the equilibrium will shift in order to minimize that change. MonsieurLeChatelier The Flag of MonsieurLeChatelier

20. Entropy Entropy is the measure of disorder in a system. A lot of disorder means HIGH ENTROPY. When things are orderly, there is LOW ENTROPY. My main man ICE CUBE (in the picture here) shows LOW ENTROPY, since water molecules in ICE are frozen in space and do not move much at all. The glass of liquid water is in fact much more disordered than ice is, so it has higher entropy than ice cubes do.

21. STEAM from a teapot is shown in this amazing picture. Gases are much more disordered than liquids or solids, so steam has the HIGHEST ENTROPY, liquid water has medium entropy, and ICE CUBES have the lowest entropy.

22. What happens in NATURE Sounds like a biology question, so I will pass wiggle on this one, but… The Universe tends to go towards highest entropy, or most disorder. Big rocks break into smaller ones, not the reverse. Solids turn to liquids, then to gases. Energy is dissipated and spread out. Chemical reactions tend to be one way (unless YOU put the energy back into a reverse reaction to make it go backwards. The UNIVERSE tends towards greater entropy & lower energy.

23. ENTROPY Solids < Liquids < GasesSolids have the LEAST entropy (the most order) Gases have the MOST entropy (the least order) Liquids are medium entropy, medium order.