The Reaction in Question

2KMnO4 + 16H+ + 5K2C204 à 2Mn2+ + 8H2O + 10CO2

This reaction is an auto catalytic redox reaction, it as catalysed by the Mn2+ ion. The Mn2+ ion acts as an electron bridge between the Ethanedioate ion and the Manganate VII ion. In this reaction the Ethanedioate is oxidised and the Manganate VII is reduced. This process is show below.

Image showing mn2+ connecting Ethanedioate
with Manganate VII ions

Mn7+ + 5e à Mn2+ 

ask roger about half equation for Ethanedioate ion

The arrangement catalyses the reaction, but another complex can form, whereby 3 Ethanedioate ions surround the Mn2+ ion, preventing its catalytic action.

  image showing mn2+ surrounded by Ethanedioate
preventing Manganate VII from connecting

The formation of this complex inhibits the progress of the reaction, so the reaction is slow until there is so much Mn2+ that there isn’t enough Ethanedioate ions to surround it, and the Mn2+ can catalyse the reaction freely. Also since as the reaction progresses Ethanedioate is used to, so there is also less inhibitor as the reaction continues.

Aims

My aim is to investigate the reaction between Manganate VII ions and Ethanedioate ions.

2KMnO4 + 16H+ + 5K2C204 à 2Mn2+ + 8H2O + 10CO2

1. Investigate the effect of varying the concentration of Ethanedioate ions.
2. Look at the effect of temperature on the reaction.
3. Add catalyst at the start of the reaction and analyse outcome.
4. See how temperature effects the catalytic action.

Risk Assessment

Potassium Manganate VII (KMnO4)

Causes skin irritation and possible burns, wash thoroughly after handling. Do not allow to come into contact with eyes and do not ingest. Eye goggles and protective clothing must be worn.

Store away from sources of ignition, in a cool. dry and well ventilated. Stable under standard temperature and pressure.

Contact with combustible materials may cause fire, will accelerate burning if involved in a fire. Although not likely in this investigation since the Manganate VII will be in solution.

Disposed in sink with plenty of water.

Potassium Ethanedioate (K2C204)

Avoid direct physical contact, eye goggles and protective clothing must be worn.

Store in a cool, dry place away from light sources. Stable under standard temperature and pressure.

Non-flammable.

Disposed in sink with plenty of water.

Sulphuric Acid (H2S04)

Avoid direct physical; contact, eye goggles and protective clothing must be worn.

Although only 2M will be used, the substance is acidic and corrosive and must therefore not come into contact with eyes, skin or be ingested.

Disposed in sink with plenty of water.

Manganese Sulphate (MnSO4)

Store in tightly closed container, in a dry place away from heat and direct sunlight.

Wear appropriate protective safety goggles and protective clothing to minimise contact with skin. Wash hands and face after handling and before eating, drinking smoking or using toilet facilities.
Stable under standard temperature and pressure.
Material is non-flammable.
Avoid ingestion, inhalation and skin contact.

Disposed in sink with plenty of water.

Theory and Background Information

Collision Theory

The collision theory states that a reaction occurs when particles collide in the correct geometry and with a minimum amount of energy, the activation energy.

Activation Energy

Activation energy is the energy required for a reaction to occur (we write activation energy Ea). This is shown below.

activation energy graph

It is clear to see that the Products are in a lower energy state than the reactants. On the way up the curve bonds are being broken, this process requires energy. On the way down the curve bonds are formed. The activation energy is the energy required to break the bonds for the reaction to proceed. The above reaction has only two steps, this is shown by the two peaks in the activation energy curve.

Rates Of Reaction

The rate at which reactants are converted into products is called the rate of reaction. This rate can be manipulated when the reaction is put under different conditions. Industrial chemists require reaction to be faster in order to save money, money is wasted whilst waiting around for a generally slow reaction.

In order to react particles must have sufficient energy, ie the activation energy. So when the particle is provided with more energy its finds it easier to react. In order to provide the particles with energy, the temperature at which the reaction is carried out is raised. By increasing the number of particles reaction per unit time, increasing the temperature has increases the rate of reaction.

Concentration and pressure affect the rate of reaction in similar ways, both cause more collisions to be made. If you increase the concentration of a reactant, there will be more of them per unit volume, so more collisions will occur. With more collisions per unit time, there is more chance of particles colliding with the correct geometry and with sufficient energy to react. Increasing the pressure will cause the particles to be jammed closer together, so by increasing pressure you increase number of collisions, which will increase the number of reactions per unit time, and increase the rate of reaction.

If a reaction can lower it’s activation energy, it will react more freely. This can be done by adding a catalyst to the reaction. The catalyst offers a different route for the reaction to take.

Maxwell Boltzmann Distributions

Image showing Maxwell Boltzmann distribution
2 curves, red and blue
Red at higher temp than blue
Black vertical line. Ea
Green vertical line to left of Ea, Ea Cat
Areas Shaded to right of Black Ea Line to show proportion of particles reacting

Within a mixture of reactants, some particles will have sufficient energy to react and others will not. This distribution of energy between particles is show in the graph above. As you can see, most of the area under the curves lie to the left of the Ea line, these particles do not have sufficient energy to react.

The reactions representing both curves are the same. You will also notice that the two curves have different shapes, this is because the reaction represented by the red line is of a higher temperature than that of the blue line. As already discussed, particles with a higher temperature have an easier time reacting than others. This is shown by the shift to the right by the red curve, it is shorter and fatter to allow a higher proportion of particles into the range to the right of the Ea line.

When a catalyst is added to the reaction, particles find it much easier to react, on a Maxwell Boltzmann curve, this is represented by moving the Ea to the left and it can now be labelled Ea Cat. Since now a higher proportion of the particles are to the right of the vertical Ea Cat line, more of them can react.

Complexes

A complex is made up of a central metal ion and ligands datively bonded to it. A dative bond occurs when a lone pair of elections is attracted to the central metal ion. Molecules can be monodentate, bidentate and polydentate depending on the number of dative bonds the molecule can form to a central metal ion.

Colour

We perceive colour because our eyes can detect different levels of different wavelengths of light. In transition metals, colour arises from the splitting of the 3d sub shell by ligands. Different ligands and different oxidation states of different metals with an unfilled 3d sub shell give different colours. I will be able to use this in my investigation, since the Mn changes oxidation state, it will change colour, which can be picked up by a colourimeter.

Redox

A redox reaction has two half’s, reduction and oxidation. In reduction an atom or molecule gains electrons, and in oxidation the atom or molecule looses them. This results in a change in oxidation state. Oxidation cannot occur without reduction and visa versa, if something is loosing electrons, then something has to be taking them.

When an atom has the number of electrons as shown in the periodic table, it has an oxidation number of 0, this number decreases when the atom is reduced, since its gaining electrons which have a negative charge. The oxidation number increases when the atom is oxidised because it is loosing its negative charge by loosing electrons.

Catalysts and Auto Catalysts

activation energy with catalyst graph

Catalysts lower the activation energy of a reaction, but they have the same structure at both the beginning and end of the reaction, though they may change structure/oxidation state during the reaction. Catalysts lower the activation energy of a reaction by providing a different path for the reaction to take, which may involve more or fewer steps than the original reaction.

When a product of a reaction catalyses its own production, the reaction is described as auto catalytic.

Image for A plus B arrow cat C makes C and D

As show in the reaction above, A and B react to produce C and D. C catalyses the reaction. This mean that initially the reaction is slow, since not much catalyst has been produced, as the reaction progresses more catalyst is produced, so the rate of the reaction increases dramatically. An example of a graph from a reaction of this type is shown below.

auto catalyst example graph

Methods

Colourimetry

A colourimeter measures the light absorbance of a solution. Usually a filter is used, the filter ensure that a specific range of wavelengths of light reach the sample. The filter ensures that only wavelengths which give high levels of absorbance are used, this helps reliable results to be obtained. To check which filter gives the highest absorbance and in effect the most reliable results, I have tested several filters with a solution which mimics that which will be used in the experiment.

The 520 filter gave the highest absorbance, so I will use this filter throughout the investigation. The information in this table is shown in the graph below.

Graph showing filter against absorbance

Calibration Curve

In this investigation I will be able to used a colourimeter to show the concentration of Manganate seven ions at any given point in the reaction. To do this I will use a calibration curve, to set up a calibration curve I will need to prepare some known concentrations of Manganate ions, then take readings with a colourimeter. All the readings will need to come from the same colourimeter, since there is significant variation in readings between colorimeter’s. Once the reading had been taken, then were plotted onto the graph shown below.

I used colourimeter number 6.

Making Up Solutions

All the solutions in this investigation will be made by dissolving the solid in water. Using the solid means I can calculate specific concentrations easily. This is opposed to making my own solutions from a stock solution, which would require more calculations and be less accurate. The solutions are made by weighing the correct amount of sold in a beaker on a 2 point balance. This solid is then flushed into a 250ml conical flask using a funnel. The 250ml conical flask is made up to the neck and inverted 10 times to ensure all the solid has dissolved. The solution can then be filled till the meniscus is level with the 250ml marl. For my reaction I will make up the following solutions.

250ml of 0.0067 moldm-3 of Potassium Manganate VII 
250ml of 0.015 moldm-3 of Potassium Ethanedioate
250ml of 0.01 moldm-3 of Manganese Sulphate

The calculations for these solutions are shown below.

Concentration = Moles
		  Volume

Concentration x Volume = Moles

and

Moles = Mass
	    Mr
The volume measured in litres is the required volume, I will be making stock solutions of 250ml. Which is 0.25 of a litre.

Fill in this bit with hand writing cos its too difficult on computer

Precautions and Standardisations

Precautions and standardisations help reliable results to be obtained. If all factors effecting the outcome of an experiment are controlled and kept constant when they aren’t meant to vary, the results will be more reliable. An example of this would be temperature, I will make sure all experiments are carried out on the same day, so the room temperature will be the same within experiments.

I will also use my reactants from a stock solution, this ensures that the reactants in all the experiments have had the same concentration throughout. I will also need to use the same logit box and colourimeter throughout the experiments, because different colourimeter can give slightly different readings.

All glass wear must be cleaned before use, this ensures that any residue on the glass from the last time it was used will not effect the experiment. The glass wear will be washed twice with tap water, twice with deionised water, and then once with the solution about to be put into that piece of glass wear.

The repetition of anomalous results can ensure reliable results are obtained. If I suspect a result is anomalous because it does not follow a trend, I will repeat that experiment. This ensures random error does not effect the outcome of the experiment.

Tests

Experiment 1: Vary Concentration of Ethanedioate Ions

This reaction takes place in 3.5ml cuvettes, and will be monitored by loggit using a colourimeter. Whenever the amounts of reactants given do not sum to 3.5ml, the rest was made up with deionised water.

To make up the cuvettes with Ethanedioate and sulphuric acid I used graduated pipettes, once logit had started running I added the Manganate ions with a syringe, which didn’t have enough power upon squirting to mix the two sufficiently, so the cuvette is quickly shaken and returned to the colourimeter. It can be assumed that the reaction starts after shaking.

The following pages show the graphs the logit box plotted. The Y axis is absorbance and the X axis is time.

Experiment 2: Varying Temperature

Experiment 2 will look at the effect of temperature on the speed of the reaction. I will measure the length of time the reaction takes to complete at different temperatures.

Since Run 6 from experiment 1 was the slowest, I will make up this reaction x3 to fit a test tube. This means 4.5ml of Ethanedioate Ions, 3 ml of Manganate VII Ions and 3 ml of Sulphuric acid. Using the slowest reaction will allow me sufficient time to record results accurately.

Electronic water baths will be used to bring the reactants to accurate temperatures, the temperature will be double checked with a thermometer. I will use the temperatures 30, 40, 50 and 60 degrees centigrade. The reaction will be timed from the moment the Manganate VII ions are added to the Ethanedioate and Sulphuric acid.. Once added, the test tube will be shaken to ensure thorough mixing. The stop watch will be stopped when the test tube goes completely colourless.

The following page is a graph of the data above.

Analysis of Initial Results

Rate over Time with Catalyst

This set of runs is similar to Experiment 1, this time the reaction will already have a stock of catalyst, Mn2+ ions. The amount of catalyst is varied. The reactions will take place in 3ml cuvettes, whenever the values shown in the table do not sum to give 3.5 ml, the rest will have been made up with deionised water.

Experiment 3: Vary Temperature with Catalyst

This experiment is similar to Experiment 2, this time I will be looking at the effect of temperature on the rate of reaction with catalyst added.

For this experiment the concentrations and amounts of reactants will remain constant. The actual values will be Run 3 from Experiment 3 made up to fit a test tube.. This means 2ml of Manganese Sulphate, 1.5ml of Ethanedioate ions, 3ml of Sulphuric Acid and 3ml of Manganate VII ions.

The reaction at the higher temperatures will be very fast, so to give a good spread of results I will be using room temperature also. The reaction is timed from when the Manganate VII ions are added to the mixture of Sulphuric Acid, Ethanedioate ions and the Manganese Sulphate. The test tube will be shaken to ensure thorough mixing. The stop watch will be stopped when the test tube goes completely colourless.

Error

Calculations

Conclusions and Evolution

References

Chemical Ideas

Page on Colourimetry

Page on Catalysts

Maxwell Boltzmann