Unformatted text preview: Law Expressions
Rate Determination of Rate Law Expressions
Determination Initial Rates Method Measured at the beginning of the reaction. Series of
Measured
experiments are run in which the concentration of
reactants are changed to study their effect on the
speed or rate of the reaction.
speed Graph Method Collected data is graphed in different forms to
Collected
investigate which one will give a straight line. Each
reaction order will generate a straight line under
different conditions.
different Initial Rates Method
Initial For the reaction 2 NO(g) + 2 H2(g) → N2(g) + 2 H2O(g) at 1280°C the following data has been collected. Determine
the rate law for the reaction and the rate constant.
the
Ex peri ment
[ NO ]
[ H 2]
I niti al Rate (M/ s)
–3
–3
–5
1
5.0 x 10
2.0 x 10
1.25 10
x
–3
–3
–5
2
10.0 x 10
2.0 x 10
5.00 x 1
0
–3
–3
–5
3
10.0 x 10
4.0 x 10
10.0 x 10 Initial Rates Method
Initial Establish the ratio of one experiment to the
Establish
other.
One substance’s concentration must remain
One
constant in the first step.
constant
General Equation: Rate2
Rate
Rate1
Rate =
= [A]2
[A]
[A]1 x [B]2
[B]
[B]1 y If [ ] remains constant, cancel it out. Initial Rates Method
Initial For the reaction: S2O82(aq) + 3 I(aq) → 2 SO42(aq) + I3(aq)
For use the following data to determine the rate law and rate
use
constant.
constant.
Experi ment
[I]
I niti al Rate (M/ s)
[S2O82 ]
1
2
3 A) =k[S2O82] 0.080
0.080
0.16 b) = k[S2O82][I] 0.034
0.017
0.017 c) = k[S2O82]2 4 2.2 x 10
4
1.1 x 10
4
2.2 x 10 d) k[S2O82]2[I] Order of Reactions
Order Zero Order Does not depend on any reactant First Order Depends on one substance directly Second Order Depends on one substance to the second
Depends
power.
power.
Depends on two substances directly Integrated Rate Laws
Integrated First Order Reactions Rate = k[A]
Rate
∆[ A ]
Rate = ∆t
∆[ A ]
−
= k [ A]
∆t
∆[ A ]
−
= k∆t
[ A] t ∫
0 ∆[ A ]
= ∫ − k∆t
[ A] 0
t [A]
ln
[ A] t = kt 0 ln[ A]t  ln[ A ]0 = kt Integrated Rate Laws
Integrated First Order Reactions ln[ A] t  ln[ A ] 0 = kt Equation of a line
Equation
Y = mx + b Integrated Rate Laws
Integrated How long will it take for a first order reactant with original
How
concentration of 0.157 M to decrease to 0.0565 M? The
rate constant is 1.34 x103 1/s
rate A) 268 s
b) 762 s
c) 3.72x103 s d) 245 s What will be the conc. of A after 12 min? A) 0.0598M b)0.412M c) 0.154 M d) 0.159M Halflife
Halflife
Time that it will take for the
Time
[ A]
concentration of reactant to
ln
decrease to ½ of its original
2[ A]
value. (t )
value. 0 = kt1 / 2
0 1/2 0 t1/2 when [A]t = [A]0/2
1/2 when Substituting in the integrated
Substituting
equation for first order.
equation [ A]
ln
[ A] t 0 = kt 1
ln = kt1 / 2
2 − ln 2 = kt1 / 2
0.693 = kt1 / 2
0.693
= t1 / 2
k HalfLife
HalfLife For First Order Reactions: t1/2 = 0.693
0.693
k Important for radioactive substances that are known to
Important
decay by first order reactions.
decay When...
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 Fall '08
 STAFF
 Reaction, Kinetics, Rate equation, Rate Law Expressions

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