Reaction Kinetics

Title

Experiment 17
Reaction Kinetics- Determination of the Activation Energy of the Reaction Between Oxalic Acid and Potassium Permanganate.

Objective

To determine the activation energy of the reaction between oxalic acid and potassium permanganate.

Theory and Background

Activation energy is the minimum amount of energy that is required to activate atoms or molecules to a condition in which they can undergo chemical transformation or physical transport. In terms of the transition-state theory, the activation energy is the difference in energy content between atoms or molecules in a transition-state configuration and the corresponding atoms and molecules in their initial configuration. The activation energy is usually represented by the symbol Ea in mathematical expressions for such quantities as the reaction-rate constant, k = Aexp(-Ea/RT).The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction proceeds. This equation suggests that the activation energy is dependent on temperature, but this effect is cancelled by the temperature dependence of the reaction rate coefficient. A substance that modifies the transition state to lower the activation energy is termed a catalyst. A biological catalyst is an enzyme. It is important to note that a catalyst increases the rate of reaction, but isn’t consumed by it nor does it change the energies of the neither original reactants nor products. Instead, the reactant energy and the product energy remain the same while the activation energy is lowered. Arrhenius concept is about the acids and bases split into ions when they dissolved in water. The Arrhenius acid-base concept classifies a substance as an acid if it produces hydrogen ions H+ or hydronium ions in water. A substance is classified as a base if it produces hydroxide ions OH- in water. Arrhenius concept focuses on what the chemical contains or what is there in solution. Reaction between oppositely charged ions is often very fast but the reaction between similarly charged ions. Example permanganate and oxalate may proceed at a rate which is measurable: 2KMnO4 + 5H2C2O4 + 3H2SO4 → K2SO4+ 2MnSO4+ 8H20+ 10CO2

2MnO-4 + 16H+ + 5C2O2-4 → 2Mn2+ +8H20 + 10CO2

Apparatus

Test tubes, water bath, stopwatch, measuring cylinder and thermometer

Materials

0.02M Potassium permanganate, 1M Sulphuric acid and 0.5M Oxalic acid

Procedure

1. 2cm3 of 0.02M potassium permanganate and 4cm3 of 1 M sulphuric acid was measured into a test tube. 2. 2cm3 of oxalic acid was added to another test tube. 3. The test tubes were placed in a water bath at 350C. 4. The oxalic acid was poured into the acidified permanganate solution and the time was taken for the permanganate to decolorise. 5. The time taken was recorded. 6. The experiment was repeated at higher temperature of 40,45,50,55 and 600C.

Results

Temperature, T(K) | 308 | 313 | 318 | 323 | 328 | 333 | 1/T (K-1) | 3.247x10-3 | 3.195x10-3 | 3.145x10-3 | 3.096x10-3 | 3.049x10-3 | 3.003x10-3 | Reaction time, t(sec) | 96.56 | 55.22 | 26.47 | 23.28 | 14.47 | 11.53 | ln 1/t | -4.570 | -4.011 | -3.276 | -3.148 | -2.672 | -2.445 |

Calculations k= Ae –Ea/RT

ln k= ln A + (-Ea/RT) ln e ln k= ln A + (-Ea/R)(1/T) ln k= -Ea/R (1/T) + ln A----------1

from the equation

y= mx+ c-----------2

-Ea/R =m
-Ea= -mR

= -[ -4.0-(-3.1)0.003195-0.003096 ] x 8.31
=9090.9 J mol-1

Discussion

Permanganate ions are strongly oxidising in acidic (H+) solution MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Oxalic acid is only a weak acid and does not make the solution acidic enough on its own for the reaction to proceed at a reasonable rate. Sulphuric acid is a strong acid and produces a high concentration of H+ in solution so the reactions proceed rapidly. The purple colour potassium permanganate decolourise as the temperature is increases. As the temperature is increased the average speed of the particles increases because they have more energy. This increase in speed means they collide more frequently resulting in an increase in the reaction rate. Temperature, however, has a large influence on reaction rates a 10oC rise can result in the reaction rate being doubled. As the kinetic energy of a particle is proportional to its absolute temperature we can predict the relative increase in the number of collisions when the temperature is raised by 10K. From the graph the gradient was measured and activation energy was calculated. The activation energy is 9090.9 J mol-1. There several precautions should be taken into consideration throughout this experiment to ensure personal safety and also to obtain accurate results. Wear goggles and gloves at all times during experiment.

Conclusion

The activation energy for the oxalic acid and potassium permanganate is 9090.0 J mol-1. The higher the temperature the faster the time taken for the purple colour to disappear.

References 1. Jim Clark, 2010, Activation Energy. Available from: <http://www.chemguide.co.uk> (12th February 2013)

2. wiseGEEK n.d., What is Activation Energy. Available from: <http://www.wisegeek.com> (12th February 2013)

References: 1. Jim Clark, 2010, Activation Energy. Available from: &lt;http://www.chemguide.co.uk&gt; (12th February 2013) 2. wiseGEEK n.d., What is Activation Energy. Available from: &lt;http://www.wisegeek.com&gt; (12th February 2013)