A student prepared copper(II) chloride crystals from green copper(II) carbonate and dilute hydrochloric acid. Table 9.1 lists the steps of the method in the...
Question 1 Report
A student prepared copper(II) chloride crystals from green copper(II) carbonate and dilute hydrochloric acid. Table 9.1 lists the steps of the method in the correct order, but some steps are missing.
| Step | What the student does |
|---|---|
| 1 | measure dilute hydrochloric acid into a beaker |
| 2 | |
| 3 | |
| 4 |
(a) Complete Table 9.1 by describing the missing steps 2, 3 and 4. [4]
(b) Name the gas given off in step 2 and describe a test for this gas. [2]
(c) Plan how the student can tell when all of the acid has reacted. [2]
Answer Details
This tests preparing a soluble salt by reacting an acid with an insoluble carbonate, then recovering the salt by crystallisation. The reaction is:
\[ \text{CuCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CuCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \](a) Missing steps 2, 3 and 4 [4].
| Step | What the student does |
|---|---|
| 2 | Add copper(II) carbonate a little at a time until it is in excess (no more dissolves) [1] |
| 3 | Filter the mixture to remove the excess (unreacted) copper(II) carbonate [1] |
| 4 | Heat the filtrate to the point of crystallisation [1], then leave it to cool and dry the crystals [1] |
Adding the solid in excess guarantees that all the acid is used up, so no corrosive acid contaminates the salt; the excess solid is insoluble and is simply filtered off.
(b) Gas and its test [2]. A carbonate plus an acid always releases carbon dioxide [1]. Test: bubble the gas through limewater; it turns milky (cloudy) [1].
(c) Telling when all the acid has reacted [2]. Keep adding the carbonate until some solid stays undissolved at the bottom [1], or until the fizzing (effervescence) stops when more solid is added [1]. Either shows there is no more acid left to react.
Examination tip: the "add solid in excess then filter" method only works when the metal compound is insoluble in water, as copper(II) carbonate is; you cannot use it with a soluble base.
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