lecture5 equil - Chemical Equilibrium Lesson Objectives...

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Chemical Equilibrium
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Lesson Objectives Lesson Objectives Describe the three possibilities that exist when reactants come together. Identify the three possibilities by looking at a chemical equation. Describe what is occurring in a system at equilibrium.
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Reaction rate Example:Consider the hypothetical reaction A(g) + B(g) C(g) + D(g) the reaction rate expression would be R = k[A][B]. Calculate the reaction rate for this reaction given the initial concentrations [A] = 0.10 M and [B] = 0.20 M and given the reaction constant, k = 0.015 L/mol·s. R = k[A][B] = (0.015 )(0.10 )(0.20 ) = 0.00030 mol/L·s
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Reverse reaction If the reaction between A and B (g) is carried out in a closed reaction vessel, no products can escape. reaction runs, quantities of C and D are formed and the concentrations of C and D increase to provide an opportunity for a reverse reaction to occur. now it is possible for particles of C and D to collide and form A and B.
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Reverse reaction so, not only is this possible, A(g) + B(g) C(g) + D(g)(Equation 1) but this is also possible, A(g) + B(g) C(g) + D(g)(Equation 2) .The reaction constant, k, is only correct for this re- action at an indicated temperature. If the temperature of a reaction is changed, k will also change.
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Reverse reaction This indicates that for this reaction,
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Reverse reaction Eventually these two reaction rates must become
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Chemical equilibrium Chemical equilibrium is often referred to as dynamic chemical equilibrium to indicate that the molecular processes continue even though we cannot see any change in the contents of the reaction vessel. When a chemical reaction has reached equilibrium, we represent the reaction with arrows going both directions. A(g) + B(g) C(g) + D(g) (Equation 3)
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Reactions That Appear to Stop Before Reaching Completion Many chemical reactions fall under a third possibility. These are the reactions where a reaction does occur but it does not proceed all the way. In these reactions, some products and some reactants remains in the flask. When this happens the reaction is said to have reached a state of equilibrium. Chemical equilibrium occurs when the rate of the forward reaction is equal to the rate of the reverse reaction.
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CHEMICAL EQUILIBRIUM CHEMICAL EQUILIBRIU M Reactions
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CHEMICAL EQUILIBRIUM Step 3 A + B → C + D Forward reaction slows down further as the number of A and B molecules decreases. A + B ← C + D Reverse reaction speeds up as the number of C and D molecules increases.
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CHEMICAL EQUILIBRIUM Step 4 forward A + B ←→ C + D reverse RATE OF FORWARD REACTION = RATE OF REVERSE REACTION
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CHEMICAL EQUILIBRIUM Characteristics of a Chemical Equilibrium System: 1. A mixture of Reactants and Products is present 2. The composition of the reaction mixture no longer changes: Concentration of reactants is constant Concentration of products is constant NOTE : Concentration of reactants = Concentration of products
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CHEMICAL EQUILIBRIUM 4. The Dynamic Equilibrium may be
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