Explanation for a basic decomposition of water experiment

Question

My son is trying to write a conclusion for a basic decomposition of water experiment using a 9 V battery (with a test tube placed over each terminal) placed inside a salt water solution. The results were: the tube over the negative terminal nearly completely filled with presumably hydrogen gas while the tube over the positive terminal only had a minute amount of presumably oxygen gas.

His textbook didn't explain these results, other than to hint at the fact that the results were due to there being twice as many hydrogen atoms in the product. He has several questions that I cannot answer.

1. If the hydrogen and oxygen molecules take up the same amount of space, then why did the tube over the negative terminal have much more than twice the gas as the other one?
2. The atomic mass of oxygen is nearly 16 times that of hydrogen, so then even if there was twice as much hydrogen in the product, wouldn't the oxygen still take up more space?

Answer 1

Electrolyzing a NaCl solution does produce hydrogen at the cathode, but no oxygen at the anode. Chlorine $\ce{Cl2}$ is produced at the anode, with maybe a small proportion of oxygen as an impurity. Unfortunately this $\ce{Cl2}$ gas is relatively soluble in water. That is why you have obtained relatively few gas at the anode. The mas of the atoms has no effect on the volume of the gases produced at each electrode.

Answer 2

1. If the hydrogen and oxygen molecules take up the same amount of space, then why did the tube over the negative terminal have much more than twice the gas as the other one?

Good questions but keep in mind that you did not mention the concentration of sodium chloride. If you have too much salt in water, chlorine gas is produced (eventually bleach) instead of oxygen. This is why you did not see the expected 1:2 vol. ratios.

If possible, use the lowest possible salt concentration for the experiment or even better, can you use a small amount of Epsom salt? Then there is no issue of chlorine formation and only oxygen gas will form. You will certainly see 1:2 vol. ratio.

2. The atomic mass of oxygen is nearly 16 times that of hydrogen, so then even if there was twice as much hydrogen in the product, wouldn't the oxygen still take up more space?

This issue was tackled by scientists long time ago. The key point is that if you have 1 billion molecules of $\ce{H2},$ and 1 billion molecules of $\ce{O2},$ they will occupy the same volume at a given temperature and pressure. There is a named law behind it. The volume of molecules way too small as compared to the volume they occupy.

The radius of oxygen molecule is 152 picometer (symbol: $\pu{pm}$) and that of hydrogen is 120 picometer $(\pu{1 pm} = \pu{10^{-12} m}),$ so masses and volumes do not correlate exactly.

Answer 3

Oxygen is soluble somewhat well in water, while hydrogen ist hardly soluble. Depending on how long the experiment lasted, this might explain the difference.

Regarding your second question: Gases you would reasonably encounter in school experiments can well be approximated by an ideal gas, that is, they all occupy the same volume per amount of substance.

These explanations are based in my recollection of my own school education, so take them with a grain of salt.

As for M. Farooq and Maurice’s remarks regarding the formation of chlorine gas: My teacher used a mild acid (citric acid or similar) to produce the required ions to conduct electric current in the water. This is something you might try as well.

Answer 4

From what you describe, I think the results depend a lot on (1) the salt added to the water to enhance conductivity, and (2) the composition of the wires used.

1. As other answers mention, using a table salt (NaCl) electrolyte can and usually will produce chlorine gas at the positive terminal (anode). This is because chlorine is easier to produce than oxygen for reasonably high salt concentrations. You will probably notice a chlorine smell if there is chlorine being produced. If you used baking soda as the electrolyte (which I recommend and think makes for an easier-to-explain experiment) only oxygen gas should be produced at the anode. Even with only oxygen, you might not get it all in a test tube, because...
2. The wires may be reacting with and using up the oxygen or chlorine produced at the anode. If your solution has turned green/blue (likely copper wires) or rusty orange/brown (iron) during the experiment, then this is likely the culprit for where your missing oxygen is. You are extracting pure oxygen or chlorine right at the metal surface, so it makes sense that the metal would react! Whatever doesn't react with the electrode would float up into your collecting tube.

As previous answers have already mentioned, for gases at reasonably low pressures (less than ~5 atmospheres), all the molecules regardless of weight take up about the same space, so you can get away with using the Ideal Gas Law. So double the number of molecules means double the volume.

I'm sure you've already looked at the Electrolysis of Water Wikipedia page, but I include the link because it's probably helpful in writing up your son's results. Plus, there are a lot of links in it to more references if that's not enough for you. Good luck with the report!