Simplified Rate Law

University:
Eastern Michigan University
Course:
HIST 375 | Modern India
Academic year:

2024
Views:

0

Pages:

2
Author:

Navine Saleh

Simplified Rate Law Calculations (Converting a rate law from standard format to a more usable one) (Evaluating a proposed rate law) In general for a chemical equation A+BC+ D the initial rate law can be written: rate = k[A]"[B]m where k is the rate constant (independent of concentration), [A] and [B] are the molar concentrations of reactant A and B, respectively, and n and m are exponents -usually but not necessarily integers. The constants k, n and m can be determined from experimental data obtained from measuring the rate for various starting concentrations of A and B. Usually [A] is varied while [B] is held constant, and [B] is varied while [A] is held constant. The temperature is held constant for all trials. From the data one obtains a set of rate equations, one for each trial: rate₁ = k[A]"[B]1" rate2= k[A]2"[B]2 up to trial p rate, = k[A]p"[B]p etc., which can be solved for k, m, and n. If the various concentrations of A and B are obtained by dilution of stock solutions with the concentrations [A] and [B]o, then by using the dilution formula, MconcVconc = Mdil Vdil the concentration of A in trial number p, [A]p, for example, is given by [A]VAp-[A]pVTp where Vap is the volume of stock solution A used in rate trial p and Vip is the total final volume of all solutions mixed in rate trial p. So, [A], = [ALV V and [B], [B]VB for [A] and [B] in any trial p. These expressions can be substituted into the general rate equations for each trial giving rate =k [A]VAD [B]V Br VT If we always hold [A] and [B], constant this equation becomes rate=k P V V A where k' is a new constant, k' = k[A][B].. Setting up and testing a hypothesis is an integral part of the scientific method. For the reaction studied in experiments 6, 7, and 8 you will test the hypothesis that the reaction is first order in both components, i.e. n = m = 1. This results in a simpler formula: rate, k Tr In Experiment 7 this equation is referred to as the law of mass action. A stands for HIO, and B stands for H2SO3. The rate can be given by 1/t where t is the time required for the ratek V V blue 12-starch complex to appear after mixing the two acids. In this experiment, the mixture containing 5.00 mL A and 5.00 mL B will be taken as the "anchor" point. The time for this mixture, labeled mixture "d", will be used to determine the value of k' in the law of mass action. Using the law of mass action and the information from mixture "d" gives: 1 ta and k' = k (10)(1500)=(0.250) 4 La If the measured value for ta is 16.0 s, then, k=0.250 s¹. With k' evaluated from one trial, the rate for any mixture of A and B can be predicted using the law of mass action by simply substituting for k' and the volume ratios. For example, using k' from above the mixture composed of 2.00 mL of A, 5.00 mL of B and 3.00 mL of deionized water would be predicted to have the rate: rate = (-0.250(12000)(1500) = 0.02505-1 k V From this the expected time is then 1 rate 1 0.0250 = 40.0.5 When the calculated rates are compared with the experimentally determined rates - in experiment 7 you will be doing this graphically it can be decided whether the law of mass action is the correct rate law for this system. If it is not, the correct rate law can be derived from the experimental data.

Simplified Rate Law

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