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Section
III
UNIT 5
Questions 23 - 26
This series of questions
concerns various aspects of chemical kinetics. In general the rate of a
reaction is measured by looking at the rate at which products are being formed,
or the rate at which reactants are being used up. However it is often found
that the rate is not necessarily proportional to the concentration of the
reactants in the reaction equation.
This gives rise to the general form
of the rate equation, given as rate = k [A]a[B]b ,
where k is the rate constant, [A] and [B] the concentration
of the reactants and a,b
the orders with respect to each reactant. Consider
then the experimental data given for the following
reaction. 5Br -(aq) + BrO3 -(aq) +
6H+(aq)  3Br2(aq) + 3H2O(l)
| Experiment |
Volume Br -cm3 |
Volume BrO3-cm3 |
Volume H=cm3 |
Rate of Br2 Formation |
| 1 |
125 |
25 |
150 |
3 x 10-3 |
| 2 |
125 |
25 |
300 |
1.2 x 10-2 |
| 3 |
125 |
50 |
300 |
2.4 x 10-2 |
| 4 |
62.5 |
25 |
300 |
6 x 10-3 |
In all four experiments the total volume
was made up to 500cm3 using distilled water.
Question 23
What units would be used to measure the rate of formation of Br2
A mol2d-6m-1s
B
mold-3m-1s
C mold-6m-1s
D mo-6ldm3s-1
Question 24
The rate equation for this reaction is given
by
A rate = k [H+][Br -][BrO3-]
B rate = k
[Br -][BrO3-]
C rate = k [H+]2[Br -][BrO3-]
D rate = k
[Br -]2[BrO3-]
The following possible mechanism has been suggested for
this reaction
Step 1 H+ + Br - HBr
Step2 H+ + BrO3- HBrO3
Step 3 HBr + HBrO3 HBrO + HBrO2
Step 4 HBrO2 + HBr 2HBrO
Step 5
HBrO + HBr H2O + Br2
Question 25
For this mechanism to be consistent with the experimental data, which
step must be the slowest
A step 1
B step 2
C step 3
D step
4
These ideas also apply to many biochemical reactions we encounter.
Often such biological reactions are also dependent on the presence of acid or
base to allow the reaction to occur at speed. As such the pH of the solutions
these reactions take place in is often a key factor in determining their rate
of reaction.
Consider the following reaction
Compound A +
H2O + H+ Compound B + Compound C
The rate
equation for this reaction is found to be rate = k [Compound A][H+]
Question 26
If the pH of the solution this reaction is
taking place in is increased by 2 pH units, what will happen to the rate of the
reaction
A increases 100 fold
B decreases 100 fold
C increases 4 fold
D decreases 4 fold
SECTION
ANSWERS
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