What happens when copper is treated with dilute HCl?

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Questions of this kind typically require you to identify the type of reaction that could potentially occur, then use published reference material on that type of reaction to determine whether the materials react or not.

When discussing chemical reactions it is almost always necessary to know the chemical reaction equation, or as much of it as possible. Thus we begin by writing down what we know. The symbol for copper is Cu, and it is a solid at room temperature, so we write Cu(s). The HCl is described as ”dilute,” implying a solution in water, so we label it aqueous, HCl(aq). We can write

`Cu(s) + HCl(aq) -> ?`

What reaction could potentially occur? Copper is a metal, and when metals are present in compounds, they are normally in the form of positive ions. Furthermore, they are paired with negative ions. There is only one negative ion in the equation that could potentially combine with a copper cation: chloride ion, `Cl^-` . You should know that when hydrochloric acid is dissolved in water, it is ionized, that is, exists as positive hydrogen ions and negative chloride ions.

Here you have an element, copper, and a compound, hydrochloric acid. The copper could combine with the chloride from the acid, forming an ionic compound, and leaving hydrogen by itself. Elemental hydrogen has the formula `H_2` and is a gas. To summarize, an element and a compound would react to give a different element and compound. A potential (unbalanced) reaction would look like this:

`Cu(s) + HCl(aq)-> Cu_?Cl(aq) + H_2(g)`

I hope by now you have recognized the many features that identify this reaction as a Single Replacement: there are an element and a compound on each side, and the compound is dissolved in aqueous solution. To determine if a single replacement reaction will occur or not, we look at the activity series.

The activity series (link attached) is arranged so that elements higher in the series will replace elements lower in the series. If the compound contains the lower element, the higher element will react with it. Here we are looking for copper to be higher than hydrogen. If copper is higher, it will replace hydrogen from HCl, and a reaction will occur.

The activity series shows that copper is below hydrogen. Copper will not replace hydrogen in a compound. That means no reaction will occur between copper and dilute hydrochloric acid. Nothing will happen when these materials are put together! And we are spared having to figure out if copper would form copper(I) or copper(II) ions.

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