## Lesson

Water

Without water life on earth would be impossible. Water is therefore the most important substance on the planet. The human body consists of approximately 70% water. Unfortunately water does not contain a lot of energy so we cannot use it as a fuel. We even use water to extinguish fires.

The Space Shuttle and water

During the preparation for this lesson you saw that hydrogen (H2) and oxygen (O2) together are an excellent fuel. They contain so much energy that it is used to send the Space Shuttle into space. You also saw that the product of the reaction between H2 and O2 is water (H2O) and lots of energy. Apparently H2 and O2 contain massive amounts of energy, which is released as the low energy product water is produced. This is shown in figure 21.

The reverse route

In figure 20 you can see that in the Solar Tree the reverse route is necessary because the key product to be produced is H2. Instead of producing water from H2 an O2, water now has to be split into H2 an O2. Instead of freeing a lot of energy by producing water, the splitting of water needs a lot of energy.

The reaction is as follows: 2 H2O(l) → 2 H2(g)+ O2(g)

Electrolysis

During the previous lessons we paid attention to producing sustainable energy in general and electricity in particular. Furthermore you learned about decomposition reactions in previous years: thermolysis, photolysis, electrolysis (lysis (Gr.) = to separate). If you combine all this knowledge things become clear:

1. H2 is very rich in energy and can excellently be used as a fuel;
2. To produce high energy products (e.g. H2) you need a lot of energy;
3. Hydrogen (H2) can be obtained through the decomposition of water (H2O);
4. Electrolysis is a widely used method to decompose water;
5. Through electrolysis you put a lot of (electrical) energy into a reactant to produce high energy products;
6. The electrical energy will be converted into chemical energy, that can be stored and transported easily.

The Hofmann Voltameter

The German chemist Hofmann (1818-1892) invented the voltameter, an apparatus for electrolysing water (see figure 22). This apparatus is still used to demonstrate the splitting of water by electrolysis. In the next experiment you will investigate how you can best electrolyse water. To determine exactly what happens during the three experiments, you have to know which gasses are produced. During the preparation for this lesson you have recollected your knowledge about the determination of several gasses.

Half reactions

Probably you have concluded that experiment 2 and 3 produced different gasses. You probably also noticed that different gasses are produced at the two electrodes. Apparently another reaction occurs at the anode than at the cathode. That is, of course, not very strange because the cathode had a surplus of electrons and the anode has a shortage of electrons.

Preferably at the cathode (-) a reaction will take place that accepts electrons (there is a surplus). At the anode (+) however a reaction will take place that wants to give away or donate electrons (there is a shortage).

At the right you see a part of table 48 from BINAS. In this table you can see some of the most common half reactions. These reactions are written in such a way that if you read the reaction from left to right it accepts electrons. If you read the reaction from right to left it donates electrons. This is just an easy way to categorise the reactions and it does not say anything about the direction or the reaction. It are all equilibrium reactions (⇔) so you can just as well read them from left to right as from right to left.

You performed the reactions with the Hofmann voltameter and analysed which gasses were produced in which experiment. Using the table together with the example you should now be able to write down the complete reaction per experiment.

Hydrogen production in practice

Commercial production of hydrogen out of water by electrolysis can be done using several methods. You have seen two methods:

1. Water + inert electrolyte, experiment 2: hydrogen + oxygen
2. Water + electrolyte , experiment 3: hydrogen + chlorine gas

Both methods have pros and cons. Oxygen can be collected and sold or you can let it escape into the air. Oxygen is however very dangerous in regard to fire. Especially in a production facility where a lot of current is used and sparks are likely to occur. Chlorine gas has to be collected. You cannot let it escape into the air. Chlorine is very aggressive and very toxic. You have to take a lot of precautions when working with chlorine gas. On the other hand you can get a good price for chlorine ($300 per ton) compared to oxygen ($30 per ton).

Economic and social aspects

Choosing for a specific production process, you have to take the economics into account like the buying price of the commodities. Method one uses sulphuric acid or another inert electrolyte. Method two uses the quite inexpensive sodium chloride which is a commodity. The selling price of chlorine gas is much higher than that of oxygen. It is important to have a profitable process and company.

But social aspect also play an important role. It all depends on where the gasses will be produced. Environmental organisations will be critical of chlorine production facilities and together with civilians protest against transport of chlorine gas through their neighbourhood. They will demand extra safety measurements which will make the production facility more expensive. A cheap production process will produce cheap and clean car fuel which makes everybody happy. But what if this process produces potentially toxic waste or side products? What is more important?

Hazard symbols

If you use or transport chlorine gas you have to use the symbols: (oxidising, gasses under pressure, acute Toxicity, hazardous to the aquatic environment)

If you use or transport oxygen you have to use the symbols: (oxidising, gasses under pressure)