3.1.4 Qualitative Analysis Flashcards

A set of practice questions and answers derived from lecture notes on the qualitative analysis of anions and cations, focusing on test sequences, observations, and chemical equations.

What is the correct sequence for carrying out tests for unknown anions?

Carbonate test, followed by sulfate test, and finally halide tests.

Write the full equation, including state symbols, for the reaction between sodium carbonate and nitric acid.

$$Na_2CO_3(aq) + 2HNO_3(aq) \rightarrow 2NaNO_3(aq) + CO_2(g) + H_2O(l)$$

How can you prove that the gas being formed in a carbonate test is $CO_2$?

Bubble the gas through limewater ($Ca(OH)_2$); the water will turn milky due to the formation of a $CaCO_3$ precipitate.

What is the ionic equation for the sulfate test when a white precipitate forms?

$$Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$$

Why must the carbonate test be performed before the sulfate test?

Barium carbonate ($BaCO_3$) is also an insoluble white precipitate in water, so carbonates must be eliminated or identified first to avoid a false positive for sulfate.

In a halide test, what is the observation when silver nitrate is added to a solution containing chloride ($Cl^-$) ions?

A white precipitate ($AgCl$) forms.

What observation is recorded when silver nitrate is added to a solution containing bromide ($Br^-$) ions?

A cream precipitate ($AgBr$) forms.

Which halide precipitate will not dissolve in either dilute or concentrated aqueous ammonia?

Iodide ($I^-$) precipitate ($AgI$).

What is the ionic equation for the reaction between silver ions and chloride ions?

$$Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$$

Why is the halide test performed last in the sequence of anion tests?

Both silver carbonate and silver sulfate are insoluble white precipitates that could give false positive results for chloride ions.

What reagents and conditions are required to test for ammonium ($NH_4^+$) ions?

Aqueous sodium hydroxide ($NaOH$) is added and the mixture is heated.

What is the ionic equation for the reaction used to identify ammonium ions?

$$NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l)$$

How is the presence of ammonia gas confirmed during a cation test?

The gas is tested with moist pH indicator paper, which turns blue because ammonia is alkaline.

Why does the test for ammonium ions not produce visible gas bubbles?

Ammonia ($NH_3$) is much more soluble in water than carbon dioxide ($CO_2$).

To identify sulfate and halide ions in a single test tube sequence, why is barium nitrate used instead of barium chloride?

Barium chloride would introduce chloride ($Cl^-$) ions, which would subsequently react with silver nitrate to form a white precipitate, leading to a false positive for halides.