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Chapter 7 Reaction Rates and Chemical Equilibrium.

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Presentation on theme: "Chapter 7 Reaction Rates and Chemical Equilibrium."— Presentation transcript:

1 Chapter 7 Reaction Rates and Chemical Equilibrium

2 2 Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (M/s). A B rate = -  [A] tt rate =  [B] tt  [A] = change in concentration of A over time period  t  [B] = change in concentration of B over time period  t Because [A] decreases with time,  [A] is negative.

3 3 A B rate = -  [A] tt rate = [B][B] tt Every reaction has its own rate

4 4 Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) time 393 nm light Detector  [Br 2 ]   Absorption red-brown t 1 < t 2 < t 3

5 Collision Theory Collision theory: In order for a reaction to take place, the following conditions must be met: 1. Two molecules must collide 2. The molecules must have the proper orientation during collision 3. The molecules must collide with enough energy Effective Collision: a collision that results in chemical reaction

6 6 Importance of Molecular Orientation effective collision ineffective collision

7 7 Exothermic ReactionEndothermic Reaction The activation energy (E a ) is the minimum amount of energy required to initiate a chemical reaction. A + B AB C + D + +

8 Transition State ( AB ) One or more original bonds are partially broken and one or more new bonds may be in the process of formation + +

9 Rate of Reaction The rate of reaction depends on several conditions: 1. Nature of Reactants 2. Concentrations 3. Temperature 3. Catalysts-speeds up a reaction by providing an alternate pathway that has a lower activation energy

10 Chemical Reaction A reversible reaction is one which moves in both the forward and backward direction. Both forward and backward reactions have different initial rates. Eventually the forward reaction and backward reactions have the same rate. When this happens, there is no more change in concentration. This is called equilibrium.

11 11 Equilibrium is a state in which there are no observable changes as time goes by. Chemical equilibrium is achieved when: the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant Physical equilibrium H 2 O (l) Chemical equilibrium N 2 O 4 (g) H 2 O (g) 2NO 2 (g) NO 2

12 12 N 2 O 4 (g) 2NO 2 (g) Start with NO 2 equilibrium

13 13 N 2 O 4 (g) 2NO 2 (g) = 4.63 x 10 -3 K = [NO 2 ] 2 [N 2 O 4 ] aA + bB cC + dD K = [C] c [D] d [A] a [B] b K = Equilibrium Constant Equilibrium Expressions Problems 7.3, 7.4, 7.5 pages 224 and 225

14 14 K >> 1 K << 1 Lie to the rightFavor products Lie to the leftFavor reactants Equilibrium Will K = [C] c [D] d [A] a [B] b aA + bB cC + dD

15 15 Le Châtelier’s Principle Changes in Concentration Change in Pressure Change in Temperature Use of a catalyst LeChatelier’s Principle: When a stress is placed on a system in equilibrium, the system responds to minimize the stress. What’s a stress?

16 16 Le Châtelier’s Principle Changes in Concentration continued ChangeShifts the Equilibrium Increase concentration of product(s)Left (Reactants) Decrease concentration of product(s)Right (Products) Decrease concentration of reactant(s) Increase concentration of reactant(s)Right (Products) Left (Reactants) aA + bB cC + dD Add Remove N 2 (g) + 3H 2 (g) 2NH 3 (g)

17 17 Le Châtelier’s Principle Changes in Volume and Pressure A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas

18 18 Le Châtelier’s Principle Changes in Temperature ChangeExothermic Rx Increase temperatureK decreases Decrease temperatureK increases Endothermic Rx K increases K decreases colder hotter N 2 O 4 (g) 2NO 2 (g)

19 19 Catalyst lowers E a for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium. Adding a Catalyst does not change K does not shift the position of an equilibrium system system will reach equilibrium sooner Le Châtelier’s Principle

20 20 Le Châtelier’s Principle - Summary ChangeShift Equilibrium Change Equilibrium Constant Concentrationyes no Pressureyes* no Volumeyes* no Temperature yes Catalyst no *Dependent on relative moles of gaseous reactants and products

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