From last time heating curves see phase information and phase change information extract numerical information chemical reactions thermochemical calculations are a lot like stoichiometric calculations—use the ratios given to you. ChemActivity 21: Equilibrium CA21 and CA22 we look at reactions as events (rather than as equations) We will look at ways to characterize reactions and what parameters and behaviors these reactions have. today is about looking at the transformation event itself. All reactions happen via collisions. This is the incident that starts the reaction event. The energy imparted by this collision is important if one wants to extract numbers about the reaction. For us (for now) it is sufficient to say that the initial collision must have enough energy to start the reaction. A weak collision accomplishes nothing. Model 1: The conversion. . . Complete this model in 10 minutes. Notice the formulas of the reactants and products of our reaction. They are called isomers of each other. We, of course, do not have model kits. Either tear off your own body parts to construct the molecules, or use your imagination. Your call. What is the answer to CTQ 3b? At a high enough temperature, you will start to see the cis- convert to the trans-. But how much will convert? Tune in to Model 2! Model 2: The number of . . . Work on Model 2 up to the information on page 90. Complete this section in 30 minutes. What stays constant from one second to the next? What does equilibrium then mean? Complete the rest of Model 2. Work on the exercises when you are finished. Some constants The kA, kB, and Keq are very important numbers. The units of these numbers are less important. They get rather complicated when you consider reactions that produce more than one species. These constants can, in fact, be measured in many different units. Relative rates If we have a reaction that (at equilibrium) has many more product molecules than reactant molecules, what can we say about kA and kB? What can we say about Keq? This kind of reaction “favors the products.” Let’s try the same rates but with different amounts of starting materials Equilibrium constants another way The rate constants kA and kB are hard to measure. There is another way to get Keq that is much easier. And you only need the balanced equation to get it. Equilibrium expressions I’m not justifying where the expression comes from. This form is MUCH easier to measure But for a reaction: 2 N2O3(g) + O2(g) 4 NO2(g) the equilibrium constant can be found by: products over reactants all concentrations are measured after reaction is “over”, at equilibrium. Every concentration is raised to a power equal to its stoichiometric coefficient in the balanced reaction. Let’s try a few: Equilibrium constant Much Greater than one favors products Much Less than one favors reactants Roughly equal to one favors a mixture