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Unformatted text preview: Reaction Rates Determining Rate Laws rate = ![product ] " ! [reactant ] = !t !t
2 HBr(g) ! H2(g) + Br2(g) •! For example: For every 2 moles of HBr that react, one mole of each product forms. Rx rate = ![B r2] ! [H 2 ] " ! [H B r ] = = !t !t 2!t Reaction Rates
•! The rate law tells us how the speed of a reaction varies with the concentrations of the reactants. Determining Rate Laws
•! There are three (3) main categories of rate laws that we will use: !! Zero-Order ! First-Order! Second-Order •! For example, suppose we have a reaction: A + B ! products the differential form of the rate law for this reaction would be: •! There are TWO forms of the rate laws: Differential form : (relates differences in concentration and time) Integrated form : (relates concentrations, rather than changes in concentration, to time) Determining Rate Laws Determining Rate Laws
•! For example, suppose we have a ! reaction: A + B ! products
[A] 0.1 [B] 0.1 0.1 0.1 0.2 0.3 Ms-1 1 2 3 4 9 •! If we have the data shown here: •! The exponents m and n are called reaction orders. •! Analyzing this data shows that: •! The overall reaction order is the sum of the exponents. •! The only way for us to know for sure what these exponents are is to determine them experimentally. 0.2 0.3 0.1 0.1 1)! when [A] is doubled, the rate doubles, when [A] is tripled the rate triples. What exponent can account for this? 2)! when [B] is doubled, the rate increase by a factor of 4 (22), when [B] is tripled, the rate increase by a factor of 9 (33). What exponent can account for this? [A]1 [B]2 •! This means that the rate law for this reaction must be: Determining Rate Laws
•! For the data given the rate is: Determining Rate Laws
•! Another example, suppose we have the ! reaction: 2A + B + C ! 2D •! If we have the data shown here:
[A] [B] 0.1 0.1 0.2 0.2 [C] 0.1 0.1 0.1 0.2 Ms-1 x 4x 32x 64x 0.1 0.2 0.2 •! This reaction is first-order with respect to A and secondorder with respect to B. •! Overall this reaction is a third-order reaction.
(1 + 2 = overall reaction order) •! What will the rate law be? 0.2 rate = k [ A] m [ B] [C ]
3 n o •! What will the exponents of the rate law be? ! rate = k [ A] [ B] [C]
6 2 1 Doubling A increases rate by 4, so [A]2 Doubling B increases rate by 8, so [B]3 Doubling C doubles the rate, so [C]1 •! What will be the overall order of the reaction?
(2 + 3 + 1) ! Determining Rate Laws
Now it’s time to practice Reaction Order-1 ...
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