To Determine the Activation Energy of Reaction between

Bromide ion and Bromate(V) ion in Acid Solution

Theory:

Activation energy is the minimum energy which reacting species must possess in order to be able to form an 'activated complex', or 'transition state', before proceeding to the products. [The activation energy (Ea) may be derived from the temperature dependence of the reaction rate using the Arrhenius equation.]

The series of steps for the conversion of reactants to products is called a reaction mechanism.

As the bond rotates there is an interaction on the various strain energy. The maximum energy as the molecule progresses from one minima to another is called the Activation Energy. The arrangement of the atoms at the maxima is called the Transition State.Energy required to break existing bonds before forming new bonds

Actually, the Arrhenius equation, k = A e(-Ea/RT), does not require that the activation energy (Ea) changes with temperature. The rate certainly is temperature-dependent, but not the activation energy. In fact, rates of a reaction at different temperatures are sometimes used, in concert with this equation, to determine the temperature-independent Ea. All you do is plot ln k against 1/T, giving a (hopefully) straight line with a slope of Ea/R.

Activation Energy (Ea): the difference in potential energy between reactants and the transition state:

- Determines the rate of reaction.

- If activation energy is large, only a few molecular collisions occur with sufficient energy to reach the transition state, and the reaction is slow.

- If activation energy is small, many collisions generate sufficient energy to reach the transition state, and reaction is fast.

Equation of the reaction between Bromide ion and Bromate(V) ion in Acid Solution:

5Br-(aq)+ BrO3-(aq) + 6H+(aq) 3Br2(aq) + 3H2O(l)

Equation of the reaction between phenol and bromine:

3Br2(aq) + C6H5OH (aq) C6H2Br3OH(s) + 3H+(aq) + 3Br-(aq)

Procedure:

  1. 10.0cm3 of 0.01M phenol solution, 10.0 cm3 solution A (0.083M with respect to KBr and 0.017 M with respect to KBrO3) and 10 drops of methyl red indicator were placed into a boiling tube.
  2. 5.0 cm3 of 0.5M sulphuric(VI) acid was placed into a second boiling tube.
  3. Both boiling tubes were placed into a beaker of water, which is maintained at about 30ºC. The contents of the tubes were allowed to reach the temperature of the water bath.
  4. The sulphuric(VI) acid was poured into the first boiling tube, and at the same time the stopwatch was started. The boiling tube was swirled gently.
  5. The first boiling tube was kept in the water bath throughout the experiment. The time (t) taken was recorded, to the nearest second, for the complete disappearance of the red colour.
  6. The temperature (T) of the reaction mixture at the end of the experiment was also recorded, to the nearest degree.
  7. Steps (1) to (6) was repeated, maintaining the reaction temperature at about 35ºC, 40ºC, 45ºC and 50ºC.

Q and A

Q.1 Is it advisable to carry out the experiment at high temperature such as 80ºC? Why?

Q.2 Why is the temperature of the reaction mixture (phenol solution, KBr, KBrO3, 10drops of methyl red + H2SO4) measured when indicator(methyl red) becomes colorless? Why not the initial temperature measured?

Q.3Suggest an explanation why the indicator decolourize at the end point.

Q.4 Is it advisable to carry out the experiment at high temperature such as 80ºC? Why?