Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.