Oxygen Evolution Reactions

Oxygen Evolution Reactions (OER) produce molecular oxygen via chemical reactions. These reactions are being studied because of their importance to the development of energy-storage systems.

During photosynthesis, molecular oxygen is produced from water as shown in the equation below:
2H₂O → 4e + 4H⁺ + O_2(g)so photosynthesis is an Oxygen Evolution Reaction.

The electrolysis of water, is another Oxygen Evolution Reaction. Oxygen gas is produced according to the equation shown below:
2H₂O_(l) → O_2(g) + 2H_2(g)
In either case, the chemical reaction to split water in order to produce oxygen gas is not spontaneous, it requires the addition of energy. In the case of the electrolysis of water in the laboratory, electrical energy is supplied from a battery or a power pack. In the case of photosynthesis, sunlight provides the energy for the reaction in the form of photons.
A catalyst can be used to speed up the reaction. In the case of photosynthesis, chlorophyll is the catalyst present in green plants. Scientists are continually working to find a good catalyst that will work just like chlorophyll. TiO₂, SrTiO₃, and BaTiO₃ have all been investigated as possible catalysts.

A team of MIT researchers have just found one of the most effective catalysts ever discovered for an Oxygen Evolution Reaction. The new catalyst is composed of cobalt, iron, oxygen and some other metals.

Reference
J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science, 2011; DOI: 10.1126/science.1212858

Further Reading
Carbon Cycle
Electrolysis
Mass-Mole Calculations
Mole Calculations
Gas Tests

Suggested Study Questions

1. Assuming 18g of water is to be split to provide oxygen gas. Calculate the maximum mass of oxygen gas that could be obtained.
2. Assuming 1L of water is to be electrolyzed at 25^oC and 1 atmosphere pressure. What is the maximum volume of oxygen gas that could be produced?
3. During the electrolysis of water, what is produced at the
   • cathode
   • anode
4. During the electrolysis of water, what is the charge on the
   • cathode
   • anode
5. During electrolysis name the electrode at which each of the processes below occurs:
   • oxidation
   • reduction
6. Write balanced equations to represent the reaction that occurs at the
   • cathode
   • anode
7. Calculate the minimum voltage that would be required in order for the electrolysis of water to produce both hydrogen gas and oxygen gas.
8. Describe a test that you could conduct in the laboratory that would allow you to say which of the gases evolved during the electrolysis of water is oxygen and which is hydrogen.
   
   
   
   
   

Defining Electronegativity

Electronegativity is defined as the power of an atom to attract electrons to itself, but it cannot be directly measured so it must be calculated using other properties of atoms. There are many different ways of calculating electronegativity.

Pauling Electronegativity
In 1932 Pauling proposed the concept of electronegativity to explain why the covalent bond formed between two different atoms, eg A-B bond, is stronger than you might expect if you were to take an average of the A-A and B-B bond strengths. Since Pauling's electronegativities are based on the differences in bond strengths between atoms, it is necessary to choose an arbitrary reference point so that these electronegativities can be compared and used to make predictions. Hydrogen, with an electronegativity of 2.20, has been used as the reference to build up a table of electronegativities. Pauling electronegativities have no units.

Mulliken Electronegativity
Mulliken electronegativities are based on the mean of the first ionization energy and the electron affinity and has the units kJ/mol or eV.

Allred-Rochow Electronegativity
Allred-Rochow electronegativity is related to the charge experienced by an electron on the 'surface' of an atom and is calculated using the ratio of effective nuclear charge experienced by the valence electrons and the square of the covalent radius.

Sanderson Electronegativity
Similar to the Allred-Rochow electronegativity, Sanderson's calculations use atomic volume instead of the square of the covalent radius.

Allen Electronegativity
Allen electronegativity is related to the average energy of the valence electrons in a free atom.

Solid State Energy Scale
In 2011, Oregon State University scientists created a new method to describe electronegativity. In this approach, electronegativity is characterised as the solid state energy of elements in a compound, and shows that electrons simply move from a higher energy to a lower energy.

Reference
Brian D. Pelatt, Ram Ravichandran, John F. Wager, Douglas A. Keszler. Atomic Solid State Energy Scale. Journal of the American Chemical Society, 2011; : 111003131629001 DOI: 10.1021/ja204670s

Further Reading
Electronegativity Trends
Electronegativity and Bond Polarity
Interactive Periodic Table of the Elements
Ionization Energy and Electron Configuration

Suggested Study Questions

1. Use the Interactive Periodic Table to find the Pauling Electronegativity for each of the following elements:
   • sodium
   • potassium
   • magnesium
     
   • calcium
   • oxygen
   • sulfur
   • chlorine
   • fluorine
2. Use the values for the electronegativities above to describe each of the following compounds as ionic or covalent:
   • sodium chloride
   • magnesium oxide
   • potassium fluoride
   • calcium oxide
   • sulfur dioxide
   • chlorine dioxide
   • ozone
3. For each of the covalent compounds above, describe the compound as non-polar or polar covalent.
4. What is meant by the term ionization energy?
5. What is meant by the term electron affinity?
6. How is electronegativity differen to electron affinity?
7. If an element has a very low first ionization energy, do you expect it to have a high of low Pauling electronegativity? Explain your answer.
8. If an element has a high Pauling electronegativity, do you expect it to have a high or low value for its electron affinity? Explain your answer.