Unit 5: Qualitative Analysis (Acid-Base Reactions)
Using characteristic acid-base reactions to identify unknown ions and gases.
5.17 Identifying H⁺ with CO₃²⁻ & SO₃²⁻
A fundamental property of acids ($H^+$ ions) is their reaction with carbonates and sulphites to produce a gas. This provides a simple test for the presence of an acid.
- Reaction with Carbonates ($CO_3^{2-}$): Acids react with any solid or aqueous carbonate to produce carbon dioxide gas, which is observed as effervescence (fizzing).
Equation: $2H^+(aq) + CO_3^{2-}(s/aq) \rightarrow H_2O(l) + CO_2(g)$ - Reaction with Sulphites ($SO_3^{2-}$): Acids react with sulphites to produce sulphur dioxide gas, which also causes effervescence.
Equation: $2H^+(aq) + SO_3^{2-}(s/aq) \rightarrow H_2O(l) + SO_2(g)$
Therefore, if you add a sample of a carbonate (like calcium carbonate) to an unknown solution and observe fizzing, it indicates the solution is acidic. Conversely, adding an acid to an unknown solid and observing fizzing suggests the solid is a carbonate or sulphite.
Solved Examples:
- How would you test if an unknown clear solution is acidic using a solid?
Solution: Add a small amount of a solid carbonate, such as sodium carbonate ($Na_2CO_3$) or calcium carbonate ($CaCO_3$). If fizzing occurs, the solution is acidic. - A student adds hydrochloric acid to an unknown white powder and sees bubbles. What two anions might be present?
Solution: The carbonate ion ($CO_3^{2-}$) or the sulphite ion ($SO_3^{2-}$). - Write the ionic equation for the reaction between hydrogen ions and a solid carbonate.
Solution: $2H^+(aq) + CO_3^{2-}(s) \rightarrow H_2O(l) + CO_2(g)$. - What is the term for the fizzing or bubbling produced in these reactions?
Solution: Effervescence. - A solution of nitric acid is added to solid potassium sulphite. What gas is produced?
Solution: Sulphur dioxide ($SO_2$). - Why is reacting with a carbonate a good test for an acid?
Solution: Because the production of a gas (fizzing) is an easily observable result. - Does the reaction of an acid with a carbonate also produce a salt?
Solution: Yes. For example, $2HCl + CaCO_3 \rightarrow CaCl_2 + H_2O + CO_2$. The salt is calcium chloride. - What would you observe if you added vinegar (a weak acid) to baking soda ($NaHCO_3$)?
Solution: You would observe fizzing as carbon dioxide gas is produced. - Write the full chemical equation for the reaction of sulphuric acid with sodium carbonate.
Solution: $H_2SO_4(aq) + Na_2CO_3(s) \rightarrow Na_2SO_4(aq) + H_2O(l) + CO_2(g)$. - Can this test distinguish between a strong and a weak acid?
Solution: Qualitatively, yes. A strong acid will typically react much more vigorously (fizz faster) than a weak acid of the same concentration.
5.18 Identifying CO₂ & SO₂ with Limewater
Once a gas is produced from the reaction of an acid with an unknown substance, it must be identified. Both carbon dioxide ($CO_2$) and sulphur dioxide ($SO_2$) give a characteristic positive test with limewater, which is a saturated aqueous solution of calcium hydroxide, $Ca(OH)_2$.
When either gas is bubbled through limewater, a white precipitate is formed, making the limewater turn milky or cloudy.
- For Carbon Dioxide: The precipitate is insoluble calcium carbonate.
$CO_2(g) + Ca(OH)_2(aq) \rightarrow CaCO_3(s) + H_2O(l)$ - For Sulphur Dioxide: The precipitate is insoluble calcium sulphite.
$SO_2(g) + Ca(OH)_2(aq) \rightarrow CaSO_3(s) + H_2O(l)$
If you continue to bubble the gas through the milky solution, the precipitate will redissolve and the solution will turn clear again. This is due to the formation of soluble hydrogencarbonate or hydrogensulphite salts.
Distinguishing between $CO_2$ and $SO_2$: While both turn limewater milky, they can be distinguished by their smell. $CO_2$ is odourless, whereas $SO_2$ has a sharp, choking smell (like a burning match).
Solved Examples:
- What is the chemical name for limewater?
Solution: Calcium hydroxide solution. - What is the positive result for the limewater test?
Solution: The clear limewater turns milky or cloudy. - What is the chemical formula of the white precipitate formed when $CO_2$ reacts with limewater?
Solution: $CaCO_3$. - A gas produced in a reaction is bubbled through limewater, which turns milky. The gas has no smell. What is the gas?
Solution: Carbon dioxide ($CO_2$). - A gas with a choking smell turns limewater cloudy. What is the gas?
Solution: Sulphur dioxide ($SO_2$). - What happens if you bubble $CO_2$ through limewater for a very long time?
Solution: The milky precipitate disappears and the solution becomes clear again. - Write the equation for the reaction when excess $CO_2$ is bubbled through limewater.
Solution: $CaCO_3(s) + CO_2(g) + H_2O(l) \rightarrow Ca(HCO_3)_2(aq)$. - How would you test for the gas produced when hydrochloric acid reacts with marble chips ($CaCO_3$)?
Solution: Bubble the gas produced through limewater. It will turn milky, confirming the gas is $CO_2$. - Why are these gases (acidic oxides) able to react with calcium hydroxide (a base)?
Solution: It is a neutralisation reaction between an acidic gas and a basic solution. - Could you use this test to identify ammonia ($NH_3$) gas?
Solution: No. Ammonia is a basic gas and will not react with limewater to form a precipitate.
5.19 Identifying NH₄⁺ with OH⁻
The ammonium ion ($NH_4^+$) can be identified by its reaction with a strong base (a source of $OH^-$ ions), such as sodium hydroxide.
When an ammonium salt is warmed with a strong alkali, it liberates ammonia gas ($NH_3$). Ammonia has a characteristic sharp, pungent smell.
Equation: $NH_4^+(aq) + OH^-(aq) \xrightarrow{Warm} NH_3(g) + H_2O(l)$
The Test Procedure:
- Add sodium hydroxide solution to the sample solution in a test tube.
- Gently warm the mixture in a water bath or with a low Bunsen flame.
- Carefully smell the gas being produced. A pungent smell indicates ammonia.
- For confirmation, hold a piece of damp red litmus paper in the mouth of the test tube. If ammonia is present, the litmus paper will turn blue.
This test can be used to identify either ammonium ions in an unknown salt or hydroxide ions in an unknown solution (by adding a known ammonium salt).
Solved Examples:
- What is the test for the ammonium ion?
Solution: Add sodium hydroxide solution and warm gently. A pungent gas (ammonia) is produced which turns damp red litmus paper blue. - Write the full chemical equation for the reaction between ammonium chloride and sodium hydroxide.
Solution: $NH_4Cl(aq) + NaOH(aq) \rightarrow NH_3(g) + H_2O(l) + NaCl(aq)$. - Why is the mixture warmed?
Solution: To increase the rate of reaction and to help the ammonia, which is very soluble in water, escape as a gas. - Why must damp litmus paper be used?
Solution: Ammonia gas must dissolve in the water on the paper to form ammonium and hydroxide ions, which then cause the indicator to change colour. Dry litmus paper will not change colour. - A student adds $NaOH$ to an unknown solution, warms it, and a gas is produced that turns damp red litmus paper blue. What ion was present in the unknown solution?
Solution: The ammonium ion ($NH_4^+$). - What property of ammonia gas causes the litmus paper to change colour?
Solution: Ammonia is an alkaline (basic) gas. - How could you use ammonium chloride to test for an unknown solution being a strong alkali?
Solution: Add the ammonium chloride solution to the unknown solution and warm. If ammonia gas is produced, the unknown solution is a strong alkali. - What is the characteristic smell of ammonia?
Solution: Sharp and pungent. - What are the products of the reaction between ammonium sulphate and potassium hydroxide?
Solution: Ammonia gas, water, and potassium sulphate. - Would this test work if you used a weak base like magnesium hydroxide?
Solution: It would be much less effective or might not work at all, as a high concentration of $OH^-$ ions is needed to drive the equilibrium to produce ammonia gas.
5.20 Identifying NH₃ & HCl Gases
The reaction between ammonia gas ($NH_3$, an alkaline gas) and hydrogen chloride gas ($HCl$, an acidic gas) is a classic example of a gas-phase neutralisation reaction. They react to form a solid salt, ammonium chloride ($NH_4Cl$).
This reaction produces a thick white smoke or cloud, which is actually a fine suspension of solid ammonium chloride particles.
Equation: $NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$
This provides a distinctive confirmatory test for either gas:
- To test for $NH_3$ gas: Bring a glass rod dipped in concentrated hydrochloric acid near the source of the gas. The appearance of a white smoke confirms ammonia.
- To test for $HCl$ gas: Bring a glass rod dipped in concentrated ammonia solution near the source of the gas. The appearance of a white smoke confirms hydrogen chloride.
Solved Examples:
- What is observed when ammonia gas and hydrogen chloride gas mix?
Solution: A dense white smoke is formed. - What is the chemical name and formula of the white smoke?
Solution: Ammonium chloride, $NH_4Cl$. - What is the physical state of the white smoke?
Solution: Solid. It is a fine particulate solid suspended in the air. - A gas is produced in an experiment. A student brings a glass rod dipped in concentrated $HCl$ to the gas, and a white cloud forms. What is the gas?
Solution: Ammonia ($NH_3$). - How would you confirm the presence of hydrogen chloride gas escaping from a test tube?
Solution: Bring a glass rod dipped in concentrated ammonia solution to the mouth of the test tube. A white smoke confirms $HCl$. - Is the reaction between $NH_3$ and $HCl$ a redox reaction or an acid-base reaction?
Solution: It is an acid-base (neutralisation) reaction. - Why must concentrated solutions of the acid and base be used for this test?
Solution: To ensure there is a high enough concentration of the gases above the solutions to react and produce a visible smoke. - What role does ammonia play in this reaction according to the Brønsted-Lowry theory?
Solution: It acts as a base, accepting a proton from $HCl$. - What role does hydrogen chloride play?
Solution: It acts as an acid, donating a proton to $NH_3$. - Could this test be used for sulphuric acid gas ($H_2SO_4$)?
Solution: No, because sulphuric acid is not volatile and does not exist as a gas under normal lab conditions.
5.21 Identifying Cl⁻ ions with Conc. H₂SO₄
The test for halide ions using silver nitrate is more common, but the chloride ion ($Cl^-$) can also be identified using concentrated sulphuric acid. This is an example of an acid-salt reaction where a less volatile acid displaces a more volatile one.
When concentrated sulphuric acid is added to a solid chloride salt, steamy fumes of hydrogen chloride gas are produced.
Equation: $H_2SO_4(l) + NaCl(s) \rightarrow NaHSO_4(s) + HCl(g)$
The hydrogen chloride gas can be confirmed by holding a glass rod dipped in concentrated ammonia solution near the mouth of the test tube. A white smoke of ammonium chloride will form, confirming the gas is $HCl$ and therefore the solid contained chloride ions.
Safety Note: This test should be performed in a fume hood as it produces a corrosive gas ($HCl$). Concentrated sulphuric acid is also extremely corrosive.
Solved Examples:
- What is observed when concentrated sulphuric acid is added to solid sodium chloride?
Solution: Steamy fumes of hydrogen chloride gas are seen. - What is the role of the sulphuric acid in this reaction?
Solution: It acts as a strong, non-volatile acid that donates a proton to the chloride ion. - How can you confirm that the steamy fumes are indeed hydrogen chloride?
Solution: Test the gas with a glass rod dipped in concentrated ammonia. A white smoke ($NH_4Cl$) will form. - Why is this test performed on a solid sample of the chloride salt?
Solution: The reaction requires the concentrated acid to displace the $HCl$ as a gas. In a solution, the water would simply dissolve the $HCl$. - Write the ionic equation for the reaction between $H_2SO_4$ and the $Cl^-$ ion.
Solution: $H_2SO_4(l) + Cl^-(s) \rightarrow HSO_4^-(s) + HCl(g)$. - Would this test work with concentrated nitric acid?
Solution: No. Nitric acid is also volatile, so it cannot be used to displace another volatile acid like $HCl$. - What safety precaution is essential for this test?
Solution: It must be performed in a well-ventilated area or a fume hood. - A student adds concentrated $H_2SO_4$ to an unknown white solid. Steamy fumes are produced that form a white smoke with concentrated ammonia. What is the anion in the solid?
Solution: The chloride ion ($Cl^-$). - What is the other product of the reaction besides $HCl$ gas?
Solution: Sodium hydrogensulphate ($NaHSO_4$). - Does this reaction involve oxidation and reduction?
Solution: No, it is an acid-base displacement reaction. The oxidation states do not change. (Note: With bromides and iodides, redox reactions do occur).