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# Lecture 18.5- rate laws

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Section 18.5 Lecture for Honors Chem

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### Lecture 18.5- rate laws

1. 1. BELLWORK - Spontaneous? ΔG = ΔH - TΔS Would the following reactions be spontaneous? a)Exothermic with an increase in entropy b)Endothermic with decrease in entropy c)Endothermic with a large increase in entropy at insanely high temperatures.
2. 2. The rate law is an expression for the rate of a reaction in terms of the concentration of reactants. The rate constant (k) is specific for each reaction and temperature. Molar concentration of reactants A and B
3. 3. The rate law and the rate constant must be determined experimentally.
4. 4. 18.5 k is large if the products form quickly; k is small if the products form slowly.
5. 5. first-order reaction = reaction rate is directly proportional to the concentration of only one reactant. Rate = k[A] As concentration of that reactant decreases, reaction rate also decreases.
6. 6. More about reaction order Rate = k [A]2 The reaction is 2nd order in A and 2nd order overall. Rate = k [A] [B] The reaction is 1st order in A, 1st order in B, and 2nd order overall Rate = k [A][B]2 The reaction is 1st order in A, 2nd order in B, and 3rd order overall
7. 7. More about reaction order Rate = k [A]2 The rate law can Rate = k [A] [B] not be determined from the chemical equation for the reaction. Rate = k [A][B]2
8. 8. An elementary reaction is a reaction in which reactants are converted to products in a single step. reaction mechanisim - the series of elementary reactions or steps that take place during the course of a complex reaction An intermediate is a product of one step in a reaction mechanism and a reactant in the subsequent step.
9. 9. A reaction progress curve shows an activation- energy peak for each elementary reaction. Each valley shows the energy of an intermediate.
10. 10. The rate law for an elementary reaction can be determined from the equation for that elementary step. The rate law for the entire reaction (all elementary reactions occurring in a row) will depend on which elementary reaction is the slowest. Unless you know the entire reaction mechanism up to the rate limiting step, rate laws must be determined by experimental data from the LAB.
11. 11. 18.5 Section Quiz. 1. Choose the correct words for the spaces. If the specific rate constant, k, is _________, then products form __________. a. large, slowly b. large, quickly c. small, quickly d. small, a first order reaction
12. 12. 18.5 Section Quiz. 2. Choose the correct words for the spaces. In a graph that shows the progress of a reaction, the peaks in the curve correspond to the energies of the _______ and the valleys correspond to the energies of the ________. a. activated complexes, reactants b. intermediates, activated complexes c. reactants, products d. activated complexes, intermediates
13. 13. 18.5 Section Quiz. 3. Which of the following statements is true? a. Most reactions are elementary reactions. b. In most reactions, reactants are converted to products in a single step. c. Most reactions take place in a series of steps called intermediates. d. Most reactions take place in a series of steps called a reaction mechanism.
14. 14. 18.5 Section Quiz. 4. The reaction mechanism can best be determined from a. the specific rate constant. b. the overall chemical reaction. c. the rate law. d. experimentation.