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EMAIL ARCHIVE MESSAGE 006

I have performed the experiment described below. 

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The objective of this experiment is to obtain the activation energy for the reaction between Br- and BrO₃- in acid solution.  The equation is as follows:

5Br- (aq) + BrO₃- (aq) + 6H+ --> 3Br₂ (aq) + 3H₂O (l)   Eqn 1

In the reaction between Br- and BrO₃- as represented in Eqn 1, the time required for the reaction to proceed to a given extent at different temperatures is found by adding a fixed amount of phenol and a small amount of methyl red indicator to the reaction mixture.

The bromine produced during the reaction reacts very rapidly with phenol (forming tribromophenol).  Once all of the phenol is consumed, any further bromine bleaches the indicator immediately.

Procedure

Using a measuring cylinder, put 10 mL of 0.01 M phenol and 10 mL of the bromide/bromate (V) solution into a boiling tube, and then add 4 drops of methyl red indicator.  In a second boiling tube, put 5 mL of 0.5 M H₂SO₄.   Fill a 1 liter beaker with water, warm to about 75 ⁰C and adjust the heating to keep the temperature of the water steady.  Immerse the boiling tubes in the water and when their contents reach the water temperature, mix the contents of the tubes and time the reaction until the color of methyl red disappears.  Repeat the experiments at about 65 ⁰C, 55 ⁰C, 45 ⁰C, 35 ⁰C, 25 ⁰C, and 15 ⁰C.

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I have the following questions regarding this experiment:
1.  What thermodynamic sign does activation energy (EA) have?  What does this signify?
2.  The reaction did not start until the contents of the boiling tubes are mixed, is it because sulfuric acid acts as a catalyst in this experiment as it lowers the activation energy between the reactants and the products?
3.  What is the function of the methyl red in this experiment?

The main reaction for this experiment is:
5Br- (aq) + BrO₃- (aq) + 6H+ --> 3Br₂ (aq) + 3H₂O (l)
One of the tubes contains the Br- and the BrO₃- while the other tube contains sulfuric acid, will supply the H+.  Thus the sulfuric acid is one of the chief reactants, not a catalyst as you had asked.  The definition of a catalyst is a substance that increases the reaction rate of a reactoin without participating in the reaction.  This reaction requires no catalyst as it takes place very fast.

Once the tubes are mixed, the reaction proceeds to the right.  The Br₂ produced reacts instantly with the phenol.  As soon as the Br2 is produced from the first reaction, it immediately becomes a reactant in the following reaction:

3Br₂ + phenol --> tribromophenol
Since it takes 3 moles of bromine to react with one mole of phenol to produce one mole of tribromophenol, you can measure the reaction rate accurately.  When you measure the time you are finding the time to produce 3 moles of bromine.  The indicator tell you when 3 moles of the bromine are produced because it changes color when the second reaction is complete.  How does it know it is complete?  Because there is more bromine produced than there is phenol to react with it.  Therefore as soon as the second reaction is completed, the bromine left over reacts with the indicator and changes its color.  So, the indicator just tells you, "O.K. 3 moles of bromine have been made"

The activation energy is the minimum amount of energy that the reacting molecules must pocess before the reaction will proceed.  It is expressed as an amount of heat per amount of reactant (Ex/ kJ/mol).  As far as the thermodynamic sign, the convention really isn't agreed upon.  Heat is heat and although you can't have negative heat, the sign of the energy variable is used to indicate the direction of the heat's movement.   For example, most people use a positive sign to indicate that an entity 'takes in heat'.  This is always the case for 'activation energy'.  You see, reactants aren't going to have to give off heat before a reaction will proceed, they will need heat.   A different type of energy is the heat of reaction.  This is different than the activation energy.  If a reaction requires additional energy after it has started in order to proceed, it is termed 'endothermic' and the heat of reaction has a positive sign, again because heat must be put into the reaction..  However, many reactions are are 'exothermic' meaning that they give off heat as they proceed, in this instance the heat of reaction has a minus (-) sign because the reaction is giving off heat to the environment.  Another thing worth mentioning is that in an industrial seeting, most exothermic reactions must have the heat that they produce carried away from the reactor because (as you saw in your experiment), additional heat will make the reaction proceed faster (in most cases).  If this heat isn't taken away and builds up, the reaction can reach a point called "runaway" where the temperature become dangerously high and the reactor may explode.  The heat carried away from the reactor is used to produce steam from water (which can be used to make electricity for the plant) or it can be used to heat another stream that needs heat.