1) Compound A reacts in a first order process. The rate constant for the reactio
ID: 575791 • Letter: 1
Question
1) Compound A reacts in a first order process. The rate constant for the reaction is determined by experiment to be 0.029s-1. How many seconds will it take for the amount of A to decrease to 1/4 of its original amount?
2) NOBr decomposes in a second-order reaction.
3) Which of these statements is/are true?
a)The overall order for the rate law, rate = k[A]2[B] is 3.
b)The half-life of a second order reaction is constant.
c)The rate law of a reaction can be determined from the balanced equation.
d)For a first order reaction, when the concentration is halved, the rate is also halved.
Thanks, any help is appreciated!
Explanation / Answer
) Compound A reacts in a first order process. The rate constant for the reaction is determined by experiment to be 0.029s-1. How many seconds will it take for the amount of A to decrease to 1/4 of its original amount?
for any order decomposition of A, -dCA/dt= K[A]n, where n is order and -dA/dt is rate and K is rate constant.
when n=1, -dA/dt= K[A], when integrated noting that at t=0, A= Ao and t= t, A= A
lnA= lnAo-Kt, or lnA= lnAo-0.029t,
A/Ao= exp(-Kt) (1)
given A=0.25Ao
0.25= exp(-0.029t), t= 47.8 sec
2. The reaction is 2NOBr ----->2NO+Br2, the rate constant suggests the reactiion to be second order and for second order -dNOBr/dt= K[NOBr]2, when integrated noting at t=0, [NoBr]=[NoBr]0 and at t=t, [NoBr]= [NoBr]
1/[NOBr] =1/[NoBr]0+Kt
from the data given 1/NOBr= 1/0.05+0.8*17
[NOBr] =0.029 at t= 17seconds
3.
The overall order for the rate law, rate = k[A]2[B] is 3.
Order is defined as the sum of powers of concentration terms in the rate expression. so the sum of powers = 2+1=3.
b)The half-life of a second order reaction is constant.
for second order reaction 1/A= 1/Ao+Kt,
half life is the time required for the concentration to drop to 50% of the initial value.
hence A= Ao/2
ot 2/Ao =1/Ao+K* half life, 1/KAo= half life. So half life depends on initial concentration.
c)The rate law of a reaction can be determined from the balanced equation.
This is possible for only elementary reaction where there is correspondence between the rate and stoichiometry of the reaction.
d)For a first order reaction, when the concentration is halved, the rate is also halved.
for 1st order- dA/dt= KA, rate is proportional to concentration
when concentration is doubled, the rate is also doubled.
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