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Rate Laws

Rate Laws. Chemical reactions are reversible. 2NO 2 (g) → 2NO(g) + O 2 (g). O 2 (g) + 2NO(g) → 2NO 2 (g). The reverse reaction becomes important when enough products accumulate. Now Δ [NO 2 ] depends on the difference in the rates of the forward and reverse reactions.

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Rate Laws

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  1. Rate Laws • Chemical reactions are reversible. 2NO2(g) → 2NO(g) + O2(g) O2(g) + 2NO(g) → 2NO2(g) • The reverse reaction becomes important when enough products accumulate. • Now Δ[NO2] depends on the difference in the rates of the forward and reverse reactions. • To avoid this complication reactions are studied at a point soon after the reactants are mixed, before products build up to significant levels. • Therefore, the reaction rate will depend only on the concentrations of the reactants.

  2. 2NO2(g) → 2NO(g) + O2(g) • Neglect reverse reaction and the rate can be expressed as: Rate Law k = rate constant n = order of the reactant Both must be determined by experiment. n can be an integer (including zero) or a fraction Concentrations of products do not appear because the rate is being studied under conditions where reverse reaction does not contribute to overall rate. Value of n must be determined by experiment; it cannot be written from balanced equation.

  3. Types of Rate Laws • Differential rate law or rate law: rate depends on concentration (from previous slide). • Integrated rate law: concentrations depend on time. • Both must be determined by experiment. • If one rate law is determined, we also know the other one. • Which one is determined by experiment depends on what types of data are easiest to collect.

  4. Method of Initial Rates • Common method for determining the form of the rate law. NH4+(aq) + NO2-(aq) → N2(g) + 2H2O(l) Values of n and m can be determined by observing how the initial rate depends on the initial concentration of NH4+ and NO2-. error on chart

  5. Rate order for NO2-: use #1 and 2, [NH4+] stays the same and [NO2-] changes Ratio of rates: Value of m = 1 Reaction is first order for NO2-

  6. Rate order for NH4+: use #2 and 3, [NO2-] stays the same and [NH4+] changes Ratio of rates: The value of n is also 1.

  7. The values of m and n are both 1 and the rate law is written: • Rate = k[NH4+][NO2-] • The rate law is first order in both NO2- and NH4+. • The overall reaction order is the sum of n and m. • For this reaction, n + m = 2. • The reaction is second order overall.

  8. The value of the rate constant, k, can be calculated using the results of any of the experiments. • Using data for #1, • Rate = k[NH4+][NO2-] • 1.35 x 10-7 mol/L∙s = k(0.100 mol/L)(0.0050 mol/L) • Then Note: the units for kare dependent upon the rate orders for the reactants.

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