Reaction Rate. Products form rapidly. Products form over a long period of time. Precipitation reaction or explosion

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1 Reaction Rate Products form rapidly Precipitation reaction or explosion Products form over a long period of time Corrosion or decay of organic material

2 Chemical Kinetics Study of the rate at which a reaction proceeds Change in concentration of a reactant or product per unit time [reactant] or [product] t t The coefficients of the reaction must be taken into consideration in calculating rate 2SO 3 2SO 2 + O 2 So [SO 3 ]= [SO 2 ] = 2( [O 2 ]) t t t

3 If you were to plot this, the graph would not be a straight line. Indicating that the rate is not constant throughout the reaction The reaction rate is always linked to a specific time interval

4 Collision Theory Rate of reaction depends on how often and how energetically the reacting particles collide with one another Effective collisions depend on Energy Orientation of molecules

5 Factors affecting Collision Theory Concentration Greater the concentration of reactants, higher the rate of reaction because more likely the collisions in a given volume Pressure Only for gaseous reactants; pressure in gases directly related to concentration Temperature Higher temp, higher KE, more energetic collisions

6 Factors, cont. Bond types Ionic > covalent State of Subdivision The more finely divided the solid, faster the reaction rate Catalysts speed up a reaction without undergoing a permanent change Homogeneous reactants and catalyst are in the same phase Heterogeneous reactant and catalyst are in different phase solid catalyst provides surface area for formation of activated complex

7 Factors, last Inhibitors substances present that block effective collisions Inhibition of a catalyst The inhibitor is so similar to the reactant that it binds the catalyst and prevents it from speeding up the reaction This only hinders the catalysts function Catalyst poisoning The inhibitor destroys the catalyst by undergoing an irreversible chemical reaction

8 Reaction Progress

9 Rate Law Calculations The rate of the reaction is dependent on the concentration For an elementary reaction one with no intermediate steps in its reaction mechanism as shown in the previous graphs, A generalized reaction could be calculated as Rate = k [A] a [B] b [C] c where k is the reaction constant and the exponents represent the order of the reaction

10 Reaction Mechanisms Complex reaction a reaction that has multiple activated complexes All the elementary reactions of a complex reaction make up the reaction mechanism of the reaction The reaction progress is a series of hills and valleys The hills represent the energies of the activated complexes The valleys correspond to the energies of the intermediate products

11 Rate Order and the Rate Law First order reactions The rate is proportional to the first power of the concentration (exponent is 1) Doubling the concentration, doubles the rate Second order reactions Rate is proportional to the second power of the concentration (exponent is 2) Doubling the concentration, increases the rate by a factor of 4

12 Zero order reactions Reactions where rate is independent of concentration Usually dependent on the presence of a catalyst Rate law is experimentally determined and cannot in general be inferred from the chemical equation for the reaction The overall order of a reaction is the sum of the orders of each reactant and product

13 In the decomposition reaction of HI, in an interval of 100.0s, the concentration of HI decreases from 4.00 mmol/l to 3.50mmol/L. What is the average reaction rate? Equation 2HI (g) H 2(g) + I 2(g) Calculation R = -ΔR Δt R = -( ) mmol/l s R = 5.0 x 10-3 mmol/l/s

14 2NO(g) + O 2 (g) 2 NO 2 (g) When the NO concentration is doubled, the rate of the reaction increases by a factor of 4. When both the O 2 and the NO concentrations are doubled, the rate increases by a factor of 8. What are (a) the reactant orders and (b) the overall order of the reaction? A) NO is second order, O 2 is first order B) overall is third order

15 Complex Reaction Example 2N 2 O (g) 2 N 2(g) + O 2(g) Intermediates do not appear in the overall chemical equation the above reaction occurs in two steps N 2 O (g) N 2(g) + O (g) N 2 O (g) + O (g) N 2(g) + O 2(g) The oxygen atoms are intermediates they disappear in the final equation The overall equation for a complex reaction gives no information about the mechanism

16 Chemical Equilibrium The system for a chemical reaction is at equilibrium when the rate of formation of products and the rate of formation of reactants is equal

17 Reversible Reactions Some reactions do not go to completion as we have assumed They may be reversible a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously Forward: 2SO 2(g) + O 2(g) 2SO 3(g) Reverse: 2SO 2(g) + O 2(g) 2SO 3(g)

18 The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction: 2SO 2(g) + O 2(g) 2SO 3(g) A chemical equilibrium occurs, and no net change occurs in the actual amounts of the components of the system The concentration of any reactant or product depends on the difference in the rates of the forward and reverse reactions

19 Even though the rates of the forward and reverse are equal, the concentrations of components on both sides may not be equal It depends on which side is favored; almost all reactions are reversible to some extent

20 Le Chatelier s Principle The French chemist Henri Le Chatelier ( ) studied how the equilibrium position shifts as a result of changing conditions Le Chatelier s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress

21 What items did he consider to be stress on the equilibrium? 1)Concentration 2)Temperature 3)Pressure Concentration adding more reactant produces more product, and removing the product as it forms will produce more product

22 Temperature increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat If heat is one of the products (just like a chemical), it is part of the equilibrium so cooling an exothermic reaction will produce more product, and heating it would shift the reaction to the reactant side of the equilibrium: C + O 2 (g) CO 2(g) kj

23 Pressure changes in pressure will only effect gaseous equilibria Increasing the pressure will usually favor the direction that has fewer molecules N 2(g) + 3H 2(g) 2NH 3(g) For every two molecules of ammonia made, four molecules of reactant are used up this equilibrium shifts to the right with an increase in pressure

24 Equilibrium Constants Consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient): aa + bb cc + dd The equilibrium constant (K eq ) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power (which is the balancing coefficient).

25 Consider this reaction: aa + bb cc + dd Thus, the equilibrium constant expression has this general form: [C] c x [D] d [A] a x [B] b (brackets: [ ] = molarity concentration)

26 The equilibrium constants provide valuable information, such as whether products or reactants are favored: if K eq > 1, products favored at equilibrium if K eq < 1, reactants favored at equilibrium

27 Expressing Keq A colorless gas dinitrogen tetroxide (N 2 O 4 ) and the dark brown gas nitrogen dioxide (NO 2 ) exist in equilibrium with each other. N 2 O 4 2 NO Liter of a gas mixture at equilibrium at 10 o C contains mol of N 2 O 4 and mol of NO 2. Write the expression for the equilibrium constant and calculate the equilibrium constant (K eq ) for the reaction.

28 K eq = [NO 2 ] 2 [N 2 O 4 ] K eq = (0.030) 2 = 0.20 (0.0045) Does this reaction favor the reactants or the products?

29

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