The reaction of bromide and bromate ions in acid solution

[sheanhung]

The first test tube with 10cm3 of 0.01M phenol, 10cm3 of bromide/bromate solution(0.0833M KBr & 0.0167M KBrO3) and 4 drops of Methyl Red are prepared. The second test tube fill with 5cm3 of 0.3M H2SO4. The two test tube immersed in water bath (75*C) and mix the two content of the test tubes. Repeat the experiment for 65*C, 55*C.........15*C.

 

Some questions:

1. What is the function of phenol?

 

2. How much of the reaction between bromide and bromate ions is completed after time t? (what is the meaning and how to calculate??)

 

3. If the activation energy is doubled, would the rate of reaction be expected to increase or decease or remain unaffected?

 

Thx ~

[woelen]

The amount of bromide and bromate are chosen, such that they cancel each other precisely:

 

5Br(-) + BrO3(-) + 6H(+) ---> 3Br2 + 3H2O

 

This reaction is not instantaneously, it takes some time to react all of the reactants. The amount of acid added is in excess. Not all acid will be reacted. After the reaction, 66.7% of the acid remains.

 

If the activation energy rises, then the reaction can be expected to run at a lower rate, but the total amount of energy produced (or consumed) in the reaction will not be affected.

 

The role of the phenol is not clear to me. It might be that it is used as an indicator, which tells something about the concentration of the bromine. Phenol forms many highly colored species when it is oxidized, but someone with more knowledge about organical chemistry may give a more complete answer on this.

[sheanhung]

woelon, thx for ur help.

 

well, what i understand about the role of phenol in this experiment is it react with bromine to form tribromophemol(C6H2Br3OH) and HBr.

 

when all the phenol has reacted, bromine contimuously produced in the first reaction bleaces the methyl red.

 

The aim of this experiment is to determine the activation energy of the reaction between bromide and bromate ions in acid solution. I dont understand why phenol is needed.

 

How much of the reaction between bromide and bromate ions is completed after time t? (what is the meaning and how to calculate??)

[sheanhung]

I have already understand the function of phenol. It provides an intermidiate state before Bromine molecules able to bleach the methyl red. Or in other words, it is use to observe the time taken for Bromine molecules to react completely with phenol and free bromine molecules appear in the solution.

 

Since all the volume and concentration of reactants remain the same, the experiment can be done on different temperature to compare the rate of reaction.

[jdurg]

From everything I can tell, the introduction of bromide and bromate in an acidic environment does indeed produce elemental bromine. The phenol would appear to be there in order to react with the bromine forming ortho-bromophenol and hydrogen bromide. The HBr would escape from the phenol portion of the testtube and dissolve in the water with the methyl red (a pH indicator). The change in pH from the gaseous HBr escaping and dissolving in the aqueous layer should indicate if a reaction is taking place or not.