QUALITATIVE ANALYSIS PRE-LAB 23.04.2017

Learning Goals Students will be able to:   Describe how the reaction coordinate can be used to predict whether a reaction will proceed including how the potential energy of the system changes. Describe what affects the potential energy of the particles and how that relates to the energy graph. Describe how the reaction coordinate can be used to predict whether a reaction will proceed slowly, quickly or not at all. Use the potential energy diagram to determine: The approximate activation energy for the forward and reverse reactions. The sign difference in energy between reactants and products. Draw a potential energy diagram from the energies of reactants and products and activation energy. The concept questions only address some of these things; the textbook problems are used for reinforcement of many of these.

Up to now... We have been able to determine reaction rates from our study of Chemical Kinetics We know how to find the equilibrium constant for a reaction at a fixed temperature. REMEMBER: (1) Equilibrium means that the reaction has reached a point where the rate of converting reactants to products is equal to that of the rate of converting products to reactants; and, (2) that both reactants and products are maintaining constant concentrations 23.04.2017

What Don't we know at this point? Up to now... We can tell what direction the reaction will proceed by comparing the ratio of concentrations between reactants and products (Qc) to the equilibrium constant (Kc) What Don't we know at this point? 23.04.2017

How do you speed up or slow down a reaction? FIRST, we must look at the reaction process? The activation Energy can be a thought of as a barrier to any reaction. We need something to happen to get over this hump... to do this requires: - a collision, or... - a sudden increase in temperature, or... - a higher concentration of reactants, etc., etc., etc... WE NEED SOMETHING! 23.04.2017

Factors affecting reaction rates 1. Chemical Nature 2. Concentration 3. Pressure 4. Temperature 5. Surface Area 6. Catalysts 23.04.2017

FACTOR 1 23.04.2017

FACTOR 2 23.04.2017

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FACTOR 3 A change in pressure (or volume) will result in an attempt to restore equilibrium by creating more or less moles of gas. For example, if the pressure in a system increases (or the volume decreases), the equilibrium will shift to favor the side of the reaction that involves fewer moles of gas. Similarly, if the volume of a system increases (or the pressure decreases), the production of additional moles of gas will be favored 23.04.2017

FACTOR 4 23.04.2017

FACTOR 4 23.04.2017

FACTOR 5 23.04.2017

FACTOR 6 23.04.2017

FACTOR 6 23.04.2017

Which reaction would probably appear to be quickest? A B C D B C because the reaction is exothermic and the first law of thermodynamics will predict more products. In addition, there is not enough activation energy for many reactants to reform

Using the heater would Increase the number of & Have no effect B because more collisions would have enough activation energy to form products and be stable with the higher energy required to maintain stability Increase the number of & Have no effect

What would best describe what is in the container after several minutes have passed ? Answer choices on next slide

Container will have mostly & Container will have mostly & Answer choices C because the reaction is thermally neutral, the first law of thermodynamics will predict equal products and reactants since there is enough activation energy for reactants to form Container will have mostly & Container will have mostly & Container will have a mixture of all four with nearly equal amounts No reaction will occur since the products and reactants have the same energy

rate ὰ (number of collisions)/second Post-Lab Chemical reactions occur as a result of collisions between reacting molecules. Collision Theory of chemical kinetics... rate ὰ (number of collisions)/second The rate of a reaction is directly proportional to the number of molecular collisions per second 23.04.2017

ΔH is the difference between the final potential energy of the products and the initial potential energy of the reactants BUT... An activation energy is required to start a reaction. When the potential energy of the products is greater than the reactants, ΔH is positive. Positive means heat or energy is absorbed during the reaction. The stability of reactants and products are determined by how easily they can crest the hump of the "activated complex." 23.04.2017

interpreting energy diagrams ΔH is the difference between the final potential energy of the products and the initial potential energy of the reactants interpreting energy diagrams An activation energy is required to start a reaction. When the potential energy of the reactants is greater than the products, ΔH is negative. Negative means heat or energy is released during the reaction. The stability of reactants and products are determined by how easily they can crest the hump of the "activated complex." 23.04.2017

Stability An activation energy is required to start a reaction. The endothermic reaction on top has very stable reactants because it takes a lot of activation energy to start the reaction. It takes less energy to have the reaction go from right to left The reactants are more stable! 23.04.2017

Stability An activation energy is required to start a reaction. The opposite is true for exothermic reactions. The distance between the starting potential energy and the crest of the "activated complex" is much less than that of the potential energy of the products. It will take more activation energy to get the reaction to go to the left than it would to get the reaction to go to the right. The products are more stable! 23.04.2017