Oxidation Reduction Reactions to Prepare Inorganic Compounds Questions

Synthesis of Copper(I) Chloride

Chm 122

Oakton Community College

Oxidation reduction reactions are often used to prepare inorganic compounds. A fairly uncommon compound, copper(I) chloride, can be prepared by a series of oxidation reduction reactions.

Copper metal reacts with nitric acid, in what is commonly known as the Ira Remsen reaction. Ira Remsen (1846-1927) was an influential chemist in America. He founded the chemistry department at Johns Hopkins University and initiated the first center for chemical research in this country. In this experiment, he describes his experience with nitric acid.

Remsen, who was also the co-discoverer of saccharin, recounts the reaction (Equation 1) in his account.

While reading a textbook on chemistry, I came upon the statement “nitric acid acts upon copper.” I was getting tired of reading such absurd stuff and I determined to see what this meant. Copper was more or less familiar to me, for copper cents were then in use. I had seen a bottle marked “nitric acid” on a table in the doctor’s office where I was then “doing time!” I did not know its peculiarities but I was getting on and likely to learn. The spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words “act upon” meant. Then, the statement, “nitric acid acts upon copper” would be something more than mere words.

All was still. In the interest of knowledge I was even willing to sacrifice one of the few copper cents then in my possession. I put one of them on the table, opened the bottle marked “nitric acid”, poured some of the liquid on the copper, and prepared to make an observation.

But what was this wonderful thing which I beheld? The cent was already changed, and it was no small change either. A greenish blue liquid foamed and fumed over the cent and over the table. The air in the neighborhood of the performance became dark red. A great colored cloud arose. This was disagreeable and suffocating – how should I stop this? I tried to get rid of the objectionable mess by picking it up and throwing out of the window, which I had meanwhile opened. I learned another fact – nitric acid not only acts upon copper but it acts upon fingers. The pain led to another unpremeditated experiment. I drew my fingers across my trousers and another fact was discovered. Nitric acid also acts upon trousers.

Taking everything into consideration, that was the most impressive experiment, and, relatively, probably the most costly experiment I have ever performed. I tell of it even now with interest. It was a revelation to me. It resulted in a desire on my part to learn more about that remarkable kind of action. Plainly, the only way to learn about it was to see its results, to experiment, to work in the laboratory.

Cu(s) + 2H+ (aq) + NO3(aq) → Cu2+(aq) + NO2 (g) + H2O(l) (1)

See https://www.youtube.com/watch?v=qQAFRlVmIBI (accessed 10/22/20) to see the reaction.

This reaction of the copper metal is the first reaction of this process of synthesizing copper(I) chloride. The solution is then treated with excess sodium carbonate to neutralize the excess acid (Equation 2) and to precipitate the copper(II) ion (Equation 3).

2H+ (aq) + CO32-(aq) ⇋ CO2 (g) + H2O(l) (2)

Cu2+ (aq) + CO32-(aq) ⇋ CuCO3 (s) (3)

Solid copper(II) carbonate is then separated by “vacuum” filtration through Buchner funnel (please watch a short demonstration of using Buchner funnel at https://www.youtube.com/watch?v=Kj5YF2zPBbE). The filter cake is then rinsed and transferred to a beaker where it is treated with hydrochloric acid. Solid copper carbonate is first dissolved (remember that decreasing the pH will shift this heterogeneous equilibrium of a saturated solution of slightly soluble compounds) so that “free” copper ions are transferred from ionic crystal lattice to the solution (Equation 4). These copper(II) ions are then complexed with excess of hydrochloric acid (Equation 5).

CuCO3 (s) + 2H+(aq) → Cu2+(aq) + CO2 (g) + H2O(l) (4)

Cu2+(aq) + 4Cl (aq) → CuCl42-(aq) (5)

Solid copper is then added, which reduces the copper(II) ion to copper(I) ion while the solid copper is oxidized itself also to copper(I) ion (Equation 6).

CuCl42-(aq) + Cu (s) + 4 Cl(aq) → 2 CuCl43- (aq) (6)

Adding the complex to water will then destroy the complex (Equation 7).

CuCl43- (aq) CuCl(s) + 3Cl(aq) (7)

Prelab Questions:

  1. Considering the reaction shown in Equation 1, complete the following sentence.

During this reaction ____________ was reduced and oxidizing agent was ___________ . _____________ lost electron(s) and _____________ served as reducing agent.

  1. In the second step you would precipitate Cu2+ from the solution by adding large excess of sodium carbonate. Is carbonate ion the reactant in one or more processes? Briefly explain your answer in the space below.
  2. Assume that there is 0.0100 mol of nitric acid remaining in excess in the solution after the entire amount of copper was oxidized and you then start adding (very gradually in small portions!) sodium carbonate. Bubbles of gas evolve at the beginning. Write chemical formula of that gas (Line 1 below).

When you add the total amount of sodium carbonate, a light blue-green precipitate forms. Write the chemical equation of the precipitation reaction that takes place in this step (Line 2 below). Hint: Don’t forget that in each saturated solution there is a heterogeneous dynamic equilibrium so are you going to put arrow in one or in both directions?

If concentration of carbonate ions in the saturated solution obtained is 0.520M, what is concentration of Cu2+ that remains in the (saturated) solution (that was not precipitated)? Show your work and reasoning step by step below Line 2; Mandatory: Include corresponding chemical equation describing this heterogeneous equilibrium (Hint; look up Ksp value for copper(II) carbonate in the appendix at the end of your textbook).

Line 1: Chemical formula of the gas is ___________________________

Line 2: Net ionic chemical equation showing formation of light blue-green solid is:

_____________________________________________________________________

  1. Consider Equation 6 above.

In this reaction ______________ is the limiting reactant and ____________ is the excess reactant.

In this reaction ________________ is oxidizing agent and _________________ is reducing agent.

After completing the lab work, you obtain the following data.

Data And Observations:

Mass of Copper (initial) 1.0372g

Mass of final solid product (CuCl) 2.5945g

Post-Lab Questions

  1. The overall reaction can be written as

Cu(s) + 2 Cl(aq) + Cu(s) → 2CuCl(s)

Show all the species, in proper sequence, to which the original copper is transformed through the individual reactions in Equations 1 – 7 to synthesize the CuCl(s). How does your sequence support the overall reaction?

  1. Follow the string of chemical equations given above to answer the following question: The initial mass of solid copper reacted in the very first step of this synthesis was 1.0372 g.
    1. What mass of CuCl(s) would you expect to be produced (theoretical yield)? You will need to do a series of stoichiometry reactions tracing the copper through the reactions. Show each step.
    2. If you obtained 2.5945 g, what is the percent yield of this synthesis?
  2. If you performed this reaction in the lab, the first step would be to oxidize a known mass of elemental copper to copper(II) in the reaction with excess of concentrated nitric acid.

Compare the standard reduction potential of Cu(s)/Cu2+(aq) and 2H+(aq)/H2(g) and make a conclusion whether H+ ion is an oxidizing agent powerful enough to oxidize Cu to Cu2+. Write your answer and give brief reasoning, (mandatory), in the space below.

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