Kinetics Class #1

1. Kinetics Class #1 OB: intro to kinetics and equilibrium chemistry. Factors that affect the rate of chemical equations, and some Potential Energy Diagrams (graphs) that show the energy of chemical reactions.

2. Some reactions are fast, like the very first one I demonstrated on the first day. Remember the synthesis of water? Hydrogen gas + oxygen gas + a touch of heat water + lots of energy Some are rather slow, remember the decomposition of hydrogen peroxide? Hydrogen peroxide water and oxygen gas That second one was SOOOOOOO SLOOOOWWWWWW it took a catalyst to make it happen!

3. This part of our course is called kinetics and equilibrium, and it concerns itself with the rate of reactions (fast or slow), changing the rates of reactions, and also, how some reactions are “balanced”. These balanced reactions go forward or in reverse, at steady rates, which means, dynamic equilibrium. We’ll also meet a cool French chemist, Monsieur LeChatelier, who will teach us how to “push” chemical reactions in a direction that makes us happy. There he is now! Note that fine mustache!

4. First we will look over the 4 factors that affect the rate of reaction, but you don’t really know what rate of reaction means yet, so let’s tell a story… Let’s talk about driving from our school to Johnson City High School. It’s 7.07 miles according to mapquest.com If you drive there in 20 minutes, you are driving 7.07 miles in 0.33 hours. That works to be about 21.1 miles per hour. The time it takes is related to, but not the same as your rate of speed. If you drive there in just ten minutes, you are driving 7.07 miles in about 0.17 hours, or about 41.6 miles per hour! Same distance, small change in time, crazy rate change. Time does not equal the rate. Not in driving, not in chemistry either.

5. The 4 factors that affect the rate of a chemical reaction (NOT the time it takes) • 1. Increase in Temperature – Nearly always hotter will mean that the reaction will happen faster • Increase reactant surface area – which allows the reactants to react faster • Increase the concentration of the reactants – more stuff, more chance for a reaction to happen • Adding a catalyst • The first three of these will work because of ONE reason, the catalyst works a different way. • All of these four ways will increase the rate of a chemical reaction. • Rate of reaction does NOT equal the time it takes!

6. The first three factors are all related to making the particles that are in the reaction move faster. Why would more particle motion make for a faster reaction? What actually happens at the invisible atomic level during a chemical reaction? Let’s use the hydrogen gas plus oxygen gas synthesize into water to fine tune our thoughts on this. What happens???

7. The first three factors are all related to making the particles that are in the reaction move faster. Why would more particle motion make for a faster reaction? What actually happens at the invisible atomic level during a chemical reaction? Let’s use the hydrogen gas plus oxygen gas synthesize into water to fine tune our thoughts on this. What happens??? When the two gases are released together, say by popping the balloon, no reaction happens. Why? The molecules can get together, but not with enough energy. To react, particles must collide with other particles with both enough energy and proper orientation. It’s like looking for love…

8. This guy is looking to fall in love, but no matter how he tries, he can’t seem to bump into the love of his life. No collisions, no chance at love. When particles don’t collide, they don’t react. These two fish (two molecules) missed each other, and any chance for romance. When molecules don’t collide, they cannot react together.

9. And even if you collide, with the proper orientation, if you bump too hard, you can’t react, you bounce off of each other before you have the chance!

10. In order to react, particles must collide with sufficient energy, but not too much, in the proper orientation. If this happens, a reaction can happen. • In order for chemical reactions to occur, sufficient particles must collide with proper orientation, and proper energy, to start and sustain a reaction. • Any factors that increase the likelihood of collisions will increase the rate of a chemical reaction. • Let’s review those 4 factors right here, do they increase the likelihood of collisions?? • 1 Increase in Temperature – Nearly always hotter will mean that the reaction willhappen faster • Increase reactant surface area – which allows the reactants to react faster • Increase the concentration of the reactants – more stuff, more chance for a reaction to happen • Adding a catalyst • That’s 3 yes votes in a row, but #4 is a big no here!

11. Let’s review those 4 factors right here, do they increase the likelihood of collisions?? • 1 Increase in Temperature – Nearly always hotter will mean that the reaction willhappen faster • Increase reactant surface area – which allows the reactants to react faster • Increase the concentration of the reactants – more stuff, more chance for a reaction to happen • Adding a catalyst • That’s 3 yes votes in a row, but #4 is a big no here! Increasing temp will speed up the particles, making them move faster will increase the power of collisions, and the speed will make them make more collisions (good). When there’s more surface area, there are more places for the collisions to happen, so that too will work to speed up the rate of the reaction. (rate ≠ time!) With more concentration, that will also increase the collisions, the stuff is everywhere! (not like being on a deserted island at all) What about catalysts??? Not today. They do not increase collisions, but still work!

12. Demo Diagram #1 of 4 Temperature increase will increase the collisions, speeding up the rate of the chemical reaction. We’ll drop 2 alka-seltzer tabs into two beakers of water, one hot, and one room temperature. We’ll time how long it takes to fizz away, and then decide upon the RATE of REACTION. Alka seltzer tab Hot Cold water water

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14. Demo Diagram #1 Temperature increases the rate of reaction. It took __________ seconds for the alka seltzer tab to fizz away in the cold water But it only took ________ seconds for an identical tab to fizz away in the hot water. It’s clear, hotter temperature made the reaction rate higher. Or, the hotter temperature made the time it took to completely react shorter. Did I mention yet that rate of reaction ≠ time of reaction?

15. Last part of Demo Diagram #1 Temperature affects the rate of reaction. Hotter temperatures give particles more kinetic energy. They move faster. This gives them more chance to collide, and makes the collisions stronger. Both of those reasons would indicate a higher rate of reaction.

16. How to draw a potential energy diagram (we’ll do lots of these, don’t worry now) Title goes here… this one will be called: Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

17. The reactants, in this case methane and oxygen, have some potential to explode. This “energy” is known, but for now, we’ll just use a line without exact kJ/mole. Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

18. You can carry around your methane and oxygen all day relatively safely. As long as you don’t add some heat, the explosion won’t happen. To react, the particles need to collide with sufficient energy and proper orientation. They don’t have that yet. Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

19. If we begin to warm the reactants up (with some heat) this is what happens… Dangerous, but not a kaboom yet… Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

20. Here we are literally at the “point of no return”. One more spot of energy and we won’t be able to hold the reaction back. It’s like being at the very top of the roller coaster as it gets to the top of the first big hill! Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

21. At this point the kaboom has started, and the reaction has “paid back” the start up energy. We’re back to where we started from an energy point of view. But, we’re not able to stop here. Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

22. The kaboom keeps happening, the reaction releases all the heat it has to. The products form and they are dull, dull, dull (from an energy point of view). Potential energy diagram for combustion of methane Potential Energy kJ/mole 0 kJ Time zero The time of the reaction End of reaction

23. The products, water and CO2 are mostly unreactive. They have a low potential energy. The products have a much greater potential energy. The difference between the potential of the reactants and products is the ΔH. A –ΔH is needed here, and that indicates EXOTHERMIC. Potential energy diagram for combustion of methane Potential Energy kJ/mole reactants -ΔH products 0 kJ Time zero The time of the reaction End of reaction