In another expedition of transition metal chemistry I shall be synthesising a small amount of copper(II) chloride dihydrate crystals – almost purely for their gorgeous green aesthetic. The simple combination of copper, hydrochloric acid and some hydrogen peroxide to act as the oxidiser is all you need, but I still find a way to screw up my yields. Nevertheless, I do get a nice crop of the beauties and I hope you can enjoy looking at them as much as I do!
I really have been getting into some transition metal chemistry lately; the fascination has probably stemmed from enjoying this branch of chemistry so much currently at college. Copper salts, although toxic, hit every shade of blue and green, and, after having feasted my eyes on the blues of copper(II) sulfate and copper(II) hydroxide, I thought I would attempt the synthesis of copper(II) chloride in my transition metal endeavours, extending more towards the beautiful greens of the metal.
I’ll treat you to the emerald-green of copper(II) chloride soon enough, but quickly first, what is the chemistry behind this synthesis? In a perfect world, just hydrochloric acid would be needed to react with copper to form copper(II) chloride and hydrogen. This does not happen. A strong oxidising agent such as hydrogen peroxide is also required to help with oxidising the copper.
Cu (s) + 2HCl (aq) + H2O2 (l) → CuCl2 (aq) + 2H2O (l)
I will be adding an excess of hydrochloric acid and hydrogen peroxide as I know once the reaction has ended when all of the shiny brown-red copper powder is consumed to produce the emerald-green solution. Apart from carrying it out safely, this reaction should be relatively straightforward.
It causes burns. The vapour irritates the lungs.
At low concentrations it irritates the eyes and skin. At higher concentrations it can be harmful if swallowed and presents a risk of serious damage to the eyes.
It is harmful if swallowed. Solid may irritate the eyes or skin.
- Gloves and goggles should always be worn when handling chemicals to protect your sensitive eyes and skin.
- You are potentially dealing with concentrated strong acids here mixed with a strong oxidising agent. Needless to say you must be wearing a lab coat – I would also recommend water proof shoes and a breathing mask – so you can quickly remove them, protecting you and your clothes in the accident of a spill. When heating up the solution to carry out the recrystallisation, be aware of any spitting of the solution as it will still contain excess hydrochloric acid and hydrogen peroxide. I only heated my solution up to 80oC instead of 100oC as, although I sacrified on the final product yield, this reduced any potentially dangerous bubbling and spitting.
- I recommend performing the reaction outside or in a well ventilated area due to the toxic and corrosive hydrogen chloride gas that is given off by the hydrochloric acid, especially if what you are using is concentrated. Concentrated hydrochloric acid is not essential in this reaction though, so more dilute solutions should work if preferred if the volume of acid used is increased in proportion.
Due to dealing with hydrogen chloride vapours from the concentrated hydrochloric acid, I moved to a different room when completing the reaction as it is better ventilated so please excuse the poor lighting and background! Safety first.
I set up my hotplate stirrer as I will require heat to carry out a recrystallisation later on and strong stirring is always useful for this kind of reaction.
I decided I would use 5g of copper powder for the synthesis, equating to a possible yield of 13.43g of copper(II) chloride dihydrate which should be more than plenty for me. After weighing out the copper I just dumped it into the beaker. I will be controlling the reaction rate by the addition of the hydrogen peroxide rather than the addition of the copper.
Any solid metallic copper should work for this reaction as long as it is pure enough. This is great as you could just use some copper wire, but I used some copper powder instead as this would dramatically increase the rate of reaction instead of waiting for the wire to be slowly eaten through via its relatively small surface area. Also copper wire is more of a pain to weigh with small accurate scales.
Initially, I poured 20ml of 32% (10.2M) hydrochloric acid all in one go into the beaker with the copper, and then I more slowly added 20ml of 9% hydrogen peroxide a little bit at a time. Each time I added some hydrogen peroxide, as soon as it hit the solution surface it would cause a moderate amount of bubbles and a light, grass-green colour in that small area, both of which would quickly fade away; otherwise there was only minute bubbling in the solution and a darker green colour.
A large amount of copper was still at the bottom of the beaker after 20ml of the hydrogen peroxide was added as I expected.
Another 10ml of hydrogen peroxide was measured out in a measuring cylinder, but, by the time I had added 5ml of it, the solution had formed a thick layer of bubbles which were quite permanent. I hypothesised this was excess break down of the hydrogen peroxide – but there was still some copper left – so there must not be an excess of hydrochloric acid.
I measured out a further 5ml of hydrochloric acid and added it to the beaker. Quickly I got results, and the remainder of the copper reacted and dissolved, also removing the presence of the large permanent bubbles.
Once I was happy with the completeness of the reaction, I set up for gravity filtration by putting a bit of cotton in a glass funnel with a 250ml receiving conical flask. It was extremely slow even for a gravity filtration for some reason. I expect it was due to the cotton being quickly clogged by a brown precipitate that could be seen.
The solution became much clearer and transparent after the filtration which was great. Bubbling could still be seen in the solution though, suggesting there was a decent amount of excess hydrogen peroxide. I also tested the pH with universal indicator paper which turned red, suggesting a pH of 1-2 and that there also is some excess hydrochloric acid.
You might be able to get away with using less hydrochloric acid and hydrogen peroxide than me but I encountered no problems. Most of the hydrochloric acid and hydrogen peroxide left will evaporate and decompose respectively in the high temperatures of the next step anyways.
That next step of course is a crystallisation to yield some pure product. Stirring was turned on lightly and the solution was heated until around 80oC.
Once the first precipitate started forming, a few drops of distilled water was added to the beaker to re-dissolve the solid and then the beaker was removed from the hotplate to allow to cool. The beaker was placed in the freezer for 5 minutes – after it had cooled to room temperature – to squeeze out even more copper(II) chloride crystals.
I set up again for a gravity filtration except this time using a small sheet of filter paper instead of cotton to allow any liquid to drain more easily. I would have washed the copper(II) chloride with acetone or similar to help dry and remove water but it seems very soluble in all sorts of organic solvents such as acetone so I left it.
I then removed the filter paper from the funnel and placed it onto two sheets of printer paper with two pieces of filter paper underneath, acting to help absorb any water and dry the product. The product was then left to air dry for a few days.
After a couple of days I was satisfied with how well the product had dried and transferred it to a small piece of printer paper to weigh.
The scales reported back 2.862g of beautifully green copper(II) chloride dihydrate crystals. The theoretical yield had been calculated to be 13.43g based off 5g of copper, representing a miserable 21.31% yield, although I guess that is what happens when you extract your product via a crystallisation. I only require a small amount though so I am satisfied.
Concerning the yield, I think I can put most of this down to the large amount of copper(II) chloride that is still dissolved in the solution. I have kept the solution so in my own time I might boil it down to extract as much copper(II) chloride as I can and store it in a different jar. It will most likely not be as pure but I am not too interested in purity in this instance and by storing the two purities seperate then I can choose between quality or quantity if needed.
Overall though the synthesis was a success, with the gorgeous green of the final copper(II) chloride dihydrate product to stand testimony. I will leave you with a couple of pictures to drool over – thank you for reading!