# Kinetics of bromide-bromate reaction

1. Dec 10, 2008

### MaherBj

1. The problem statement, all variables and given/known data
The aim of my exercise is to explore the kinetics of the reaction between bromide and bromate (V) ions in acid solution as shown below:
5Br– + BrO3– + 6 H+ → 3 Br2 + 3 H2O (Reaction 1)
To do this,To do this, a fixed amount of phenol (C6H5OH) and the dye methyl red (indicator) are added to the reaction mixture, the bromine reacts instantaneously with the phenol to form tribromophenol according to this reaction:

3 Br2 + C6H5OH → C6H2Br3OH + 3 H+ + 3 Br- (Reaction 2)

Once all the phenol added has reacted there will be excess Br2 released by the reaction. This will oxidize the methyl red causing the disappearance of the red color in the reaction mixture: (reaction 3)
Br2 + methyl orange → bleached methyl orange + 2Br–
(Red) (Colorless)
Thus, if the time (t) taken for the red color to disappear is measured from the start of the reaction then this will closely correspond to the time taken for the reaction to produce enough bromine to react with the known quantity of phenol.
I prepare the following mixture and add KBr at t = 0:
KBr: C0 = 0.02 M and V0=10 mL.
KBrO3: C1 = 0.005 M and VI = 10 mL
H2SO4: C2 = 1 M and V2 = 15 mL.
three drops of methyl orange indicator.
Phenol solution: C3 = 0.0001 M and V3 = 10mL
• C0, C1, C2 and C3 are the molar concentrations before mixing.
• The temperature is kept constant at 0 C during all the time of the reaction.

it takes 35 sec for the reaction-mixture to decolorize.

2. Relevant equations
Question 1:
Explain why the red color of the reaction-mixture disappears only at 35 sec.

Question 2:
how does the quantity (mole) of bromide ions that disappeared by reaction 1 vary with time?

Question 3:
What is the final quantity of bromine that could be produced at the end of the reaction??

Question 4:

At instant t1 = 35 sec,i re-add 10 mL of the phenol solution,it takes 50 sec (from t1) for the mixture to disappear. why this decolorization takes more time than before?(50> 34).

Question 5:

3. The attempt at a solution

for question 1:
when all phenol present reacts with the formed Br2, the red color disappears because reaction 2 is no more proceeding. Is this true???

for question 2:
I predict that it remains constant during the kinetic study because for every 5 moles of bromide ions that disappear in reaction 1 ,there are 5 moles of bromide ions appear by reaction 2 and 3. IS THIS TRUE??

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 11, 2008

### Staff: Mentor

Does reaction 3 proceed before all phenol is used?

3. Dec 11, 2008

### MaherBj

reaction (3) proceed when all phenol are consumed

4. Dec 11, 2008

### Staff: Mentor

Can it produce any Br- when the phenol is still present?

5. Dec 11, 2008

### MaherBj

As long as phenol is present,three moles of bromide ions are produced (according to rxn 2) and once the phenol is consumed, 2moles of bromide ions are produced (according to reaction 3).

6. Dec 11, 2008

### epenguin

Q1 I guess it is true but is it an answer? hard to know what exactly they want, but probably something a bit more focused like why is it no longer proceeding and why only after a time?

Q2 Your answer does not seem true to me, count them again.

Q3 Surely you do not need our help to reason out and calculate that one?

Q4 (I suppose you had to add another drop of methyl red, I don't think it unbleaches does it?). Before attempting to answer this question I recommend to write out (here!) either to number of moles or the molarity and also the atomic molarity IYKWIM of all the things there (only the ones that change) at start and after 35sec and reason abokut that. I think they are testing your chemical reasoning rather than knowledge.

7. Dec 11, 2008

### Staff: Mentor

So not 5, but either 3 or 2, depending on the moment.

8. Dec 11, 2008

### MaherBj

Thx epenguin..
But the answer of question 3 depends on part 2:
If the the quantity of bromide ions varies with time, then bromide ions determine the end of the reaction because it is the limiting reactant (according to the initial quantities introduced at t =0 and the coefficients of the reaction 1).
Then n (Br2)final = (3/5) × n0 (Br-)

But,If the quantity of bromide ions remain constant, then the excess reactant (bromate ions) determine the end of the reaction.
And n (bromine) at end of reaction = 3 n (bromate ions) at t = 0

9. Dec 11, 2008

### MaherBj

So Mr Borek, which reaction in your opinion proceeds faster 2 or 3?

10. Dec 11, 2008

### epenguin

....................

Last edited: Dec 11, 2008
11. Dec 11, 2008

### Staff: Mentor

I am not referring to the reaction speed, but to the amount of Br- produced. Not 5 moles, but either 3 or 2, depending on the reaction.