Reactivity Trends (OCR A-Level Chemistry A): Revision Notes

Qualitative Analysis

What is qualitative analysis?

Qualitative analysis is a method used to identify substances based on observations rather than measurements. Unlike quantitative analysis (which uses titration and gives numerical results), qualitative analysis can be carried out quickly using simple test-tube scale experiments.

The observations you make might include:

• Gas bubbles forming (effervescence)
• Precipitates appearing
• Colour changes
• Identifying gases produced

This makes qualitative analysis particularly useful for identifying unknown compounds in the laboratory.

Tests for anions

Carbonate test (testing for CO₃²⁻ ions)

Carbonates react with acids to produce carbon dioxide gas. This gas production forms the basis of the test for carbonate ions.

Procedure:

1. Add dilute nitric acid (HNO₃) to the solid or solution being tested
2. If bubbles appear, the unknown compound could be a carbonate
3. To confirm the gas is carbon dioxide, bubble it through lime water (a saturated aqueous solution of calcium hydroxide, Ca(OH)₂)
4. If carbon dioxide is present, the lime water turns cloudy or milky white

Chemical equations:

The reaction with dilute nitric acid:

$\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HNO}_3\text{(aq)} \rightarrow 2\text{NaNO}_3\text{(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$

The confirmation test with lime water:

$\text{CO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$

The cloudiness forms because calcium carbonate is a fine white precipitate that makes the lime water appear milky.

Sulfate test (testing for SO₄²⁻ ions)

Most sulfates dissolve in water, but barium sulfate (BaSO₄) is very insoluble. The formation of a white precipitate of barium sulfate forms the basis of this test.

Procedure:

1. Add aqueous barium ions (Ba²⁺) to the solution being tested
2. The barium ions are usually added as either barium chloride (BaCl₂) or barium nitrate (Ba(NO₃)₂)
3. If sulfate ions are present, a white precipitate of barium sulfate forms
4. The precipitate is very dense and sinks to the bottom of the test tube

Chemical equation:

The ionic equation for the test:

$\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}$

Halide tests (testing for Cl⁻, Br⁻, and I⁻ ions)

Most halides are soluble in water, but silver halides are insoluble. Aqueous silver ions react with halide ions to form precipitates of different colours.

Procedure:

1. Add aqueous silver nitrate (AgNO₃) to the solution being tested
2. If halide ions are present, a precipitate forms
3. The colour of the precipitate indicates which halide is present:
   • Silver chloride (AgCl) is white
   • Silver bromide (AgBr) is cream
   • Silver iodide (AgI) is yellow
4. Add aqueous ammonia to test the solubility of the precipitate (this helps distinguish between the three halides as their colours can be difficult to tell apart)

Chemical equations:

General equation (where X⁻ represents any halide):

$\text{Ag}^+\text{(aq)} + \text{X}^-\text{(aq)} \rightarrow \text{AgX(s)}$

Specific equations for each halide:

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

$\text{Ag}^+\text{(aq)} + \text{Br}^-\text{(aq)} \rightarrow \text{AgBr(s)}$

$\text{Ag}^+\text{(aq)} + \text{I}^-\text{(aq)} \rightarrow \text{AgI(s)}$

Solubility in ammonia:

The three silver halide precipitates have different solubilities in ammonia, which helps confirm which halide you have:

Halide	Ionic equation	Colour of precipitate	Solubility in NH₃(aq)
Chloride, Cl⁻	Ag⁺(aq) + Cl⁻(aq) → AgCl(s)	white	soluble in dilute NH₃(aq)
Bromide, Br⁻	Ag⁺(aq) + Br⁻(aq) → AgBr(s)	cream	soluble in conc. NH₃(aq)
Iodide, I⁻	Ag⁺(aq) + I⁻(aq) → AgI(s)	yellow	insoluble in conc. NH₃(aq)

Sequence of tests - why order matters

When analysing an unknown inorganic compound, you must carry out the anion tests in the correct order. Otherwise, you could get confusing results or make an incorrect identification.

Let's understand why this order is essential:

Why test for carbonate first

When you add dilute acid in the carbonate test, you look for bubbles from carbon dioxide gas. Neither sulfate ions nor halide ions produce bubbles with dilute acid. This means:

• The carbonate test can be carried out without the possibility of an incorrect result
• If the test shows no bubbles, there is no carbonate present and you can safely proceed to the next test
• You have not added any reagents that might interfere with later tests

Why test for sulfate second

In the sulfate test, you add Ba²⁺ ions and look for a white precipitate of barium sulfate (BaSO₄).

However, barium carbonate (BaCO₃) is also white and insoluble in water. If you carry out the sulfate test on a carbonate, you will get a white precipitate even though no sulfate is present. This is a false positive result.

Therefore, it is important to:

• Carry out the carbonate test first
• Only proceed to the sulfate test when you know that no carbonate is present

Why test for halides last

In the halide test, you add Ag⁺ ions (as AgNO₃) and look for a precipitate.

Both silver carbonate (Ag₂CO₃) and silver sulfate (Ag₂SO₄) are insoluble in water. They would form precipitates in this test even though no halide ions are present.

Therefore, it is important to:

• Carry out the carbonate and sulfate tests first
• Only proceed to the halide test after ruling out carbonate and sulfate ions

Testing mixtures of ions

If you need to analyse a mixture containing multiple ions, you carry out the tests in the same sequence, but on the same solution:

1. Carbonate test:

• Add dilute nitric acid (HNO₃) and continue adding until the bubbling stops
• All carbonate ions will have been removed and there will be none left to interfere with the next tests

2. Sulfate test:

• To the solution left from the carbonate test, add excess barium nitrate, Ba(NO₃)₂
• Any sulfate ions present will precipitate as barium sulfate
• Filter the solution to remove the barium sulfate precipitate

3. Halide test:

• To the solution left from the sulfate test, add silver nitrate (AgNO₃)
• Any carbonate or sulfate ions initially present have already been removed
• Therefore, any precipitate formed must be due to halide ions
• Add NH₃(aq) to confirm which halide you have

Tests for cations

Ammonium ion test (testing for NH₄⁺)

When heated together, ammonium ions (NH₄⁺) and hydroxide ions (OH⁻) react to produce ammonia gas (NH₃).

Procedure:

1. Add aqueous sodium hydroxide (NaOH) to a solution containing the ammonium ion
2. Warm the mixture by gently heating
3. Ammonia gas is produced (you are unlikely to see bubbles as ammonia is very soluble in water)
4. You may be able to smell the ammonia, which has a distinctive sharp odour
5. To confirm the gas is ammonia, test it with moist pH indicator paper
6. Ammonia is alkaline, so its presence will turn the paper blue

Chemical equation:

$\text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}$

Practical applications - barium meals

Qualitative analysis isn't just used in chemistry laboratories. One important medical application uses the precipitation reaction of barium sulfate.

Common exam mistakes

Key takeaways