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Rate Laws

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Rate = k [A] x [B] y [A] and [B] represent the concentrations of reactants x and y often (but not always) represent the coefficients in front of [A] and [B] k is a constant specific to the reaction.

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Rate Laws Rate = k [A] x [B] y k A constant specific to the reaction. A constant specific to the reaction. Rate constants may have a variety of strange units using L, mol, and sec. This is so the rate will always work out in mol/L sec. Rate constants may have a variety of strange units using L, mol, and sec. This is so the rate will always work out in mol/L sec.

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Rate Laws For example: 2NO + O 2 2 NO 2 The rate expression is: Rate = k [NO] 2 [O 2 ]

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Rate Laws For example: 2 N 2 O 5 4 NO 2 + O 2 The rate expression is Rate = k [N 2 O 5 ] 2

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Rate Laws Order in reactions: refers to the exponents on the compounds in the rate law Rate = k [NO] 2 [O 2 ] This reaction is second order for NO and first order for O 2

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Rate Law This rate expression Rate = k [NO] 2 [O 2 ] Rate = k [NO] 2 [O 2 ] is third order overall. is third order overall.

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Rate Laws If a reactant is first order, doubling the concentration will double the rate. If a reactant is second order, doubling the concentration will quadruple the rate. If changing the concentration has no effect the chemical does not appear in the rate law.

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Rate Laws To determine the rate constant, one must collect data using different concentrations of reactants.

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Rate Laws For example: H 2 O 2 + 2HI 2H 2 O + I 2 Trial[H 2 O 2 ][HI]Rate 10.1 M0.1 M.0076 20.1 M0.2 M.0152 30.2 M0.1 M.0152

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Rate Laws If [HI] doubles, the rate doubles, so the rate expression is first order for HI. If [H 2 O 2 ] doubles, the rate doubles, so the rate expression is first order for H 2 O 2.

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Rate Laws The rate expression must be Rate = k [H 2 O 2 ] [HI]

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Rate Laws Plug in data from any trial to solve for k:.0076 = k (0.1M) (0.1M).0076 =.01k K =.76

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Rate Laws If a reaction has two or more steps, the rate-determining step is the slowest one.

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Rate Laws Rate-determining Step 2 NO (g) + 2H 2 (g) N 2 (g) + 2 H 2 O (g) Elementary Steps 2 NO N 2 O 2 (fast) N 2 O 2 + H 2 N 2 O + H 2 O + H 2 (slow) N 2 O + H 2 N 2 + H 2 O (fast)

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Rate Laws Rate-determining Step Which is the rate-determining step? 2 NO N 2 O 2 (fast) N 2 O 2 + H 2 N 2 O + H 2 O + H 2 (slow) N 2 O + H 2 N 2 + H 2 O (fast) ____________________________________________________________________________________ 2 NO (g) + 2H 2 (g) N 2 (g) + 2 H 2 O (g)

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Rate Laws Rate-determining Step Which is the rate-determining step? 2 NO N 2 O 2 (fast) N 2 O 2 + H 2 N 2 O + H 2 O + H 2 (slow) N 2 O + H 2 N 2 + H 2 O (fast) ____________________________________________________________________________________ 2 NO (g) + 2H 2 (g) N 2 (g) + 2 H 2 O (g)

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