A reaction between A(aq) and B(aq) is represented stoichiometrically by: Observa
ID: 972357 • Letter: A
Question
A reaction between A(aq) and B(aq) is represented stoichiometrically by: Observations on the rate of this reaction are obtained in three (3) separate experiments and shown below. Note: All data are recorded at the same temperature. The total volume is the same for all trials.Also, [ ]_0 denotes initial concentration (in M). Determine the order with respect to each reactant and the overall order. Also, write down the experimental (differential) rate law, according to the above data. Determine the experimental rate constant (k_exp) of the above reaction, in units of M and s. Consider a solution initially 0.160 M in A(aq) and 0.600 M in B(aq). Approximately how long will it take for 25.0 % of B(aq) to react at the temperature maintained above? Express your answer in minutes. Determine the concentration (in M) of all species at this time. [Note: One of the integrated rate laws derived in class, cannot be used in this instance (why?). You must work with the rate law itself to seek an approximate solution.]Explanation / Answer
Data.
Experiment
[A] (M)
[B] (M)
Initial Rate (m/s)
1
4,000
2,000
1,00E-03
2
4,000
3,000
2,25E-03
3
0,250
2,000
2,50E-04
Chemical Reaction:
2 A (aq) + 3 B (aq) ==> A2B3 (aq)
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Theory.
Suppose Rate’s law have the following shape:
Rate = k[A]x[B]y
Where x and y are experimentally determined numbers. Look that x and y are not the same as the stoichiometry coefficients a y b. When the values of x, y and k are known is possible to use this equation. This coefficients specify the relationship between concentrations of A and B reagents, when sum them up you got the globally order of reaction.
A method to study the reagent concentration effect against rate of reaction is determining the dependency of the initial rate with initial concentrations. It is better to measure the initial rates, since, as the reaction proceeds, lower concentrations of the reactants and then it is difficult to accurately measure changes.
The data shows three velocity measurements for the formation of A2B3, if values 1 and 3 are observed is noted that with decreasing concentration [A] up to 6.25% of it, while keeping constant [B] the reaction rate decreases more than 4 times. In the same way the data 1 and 2 show that increasing [B], keeping constant [A], the rate increases or double up.
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Answer.
1. The rate law will be given as follows:
Rate law = k[A]2[B]1
This reaction is given by a second order regard to A, and first order regard to B and with an overall order of;
x + y = 2+1
overall order = 3.
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2. The rate contant is calculated as follows;
K = rate/[A]2[B]
From data of experiment 2 we have;
K = 2.25x10-3/(4)2(3)
K = 4.68x10-5 M2.s
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3. Now we have a relationship between concentration of reactants and time.
Let’s use data from experiment 3 and find time with the following formula for a first order reaction regard to [B];
2 A + 3 B ==> A2B3
0.160 0.600 -
75% 25% 100%
Ln[B]/[B]o = -kt
Ln (0.15)/(0.600) = -(4.68x10-5) x t
t = 29621.67 sec = 493.69 min.
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It is not easy to find the order of reactions but with more experience, you can conclude faster the order of a specific reaction. Hope you'll find this helpful, tough!
Experiment
[A] (M)
[B] (M)
Initial Rate (m/s)
1
4,000
2,000
1,00E-03
2
4,000
3,000
2,25E-03
3
0,250
2,000
2,50E-04
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