Reactivity Trends - Quantitative Analysis Flashcards
(20 cards)
How do you test for carbonate ions?
Add any dilute acid and observe effervescence. Then bubble the gas through limewater; if CO₂ is present, it turns cloudy.
What is the reaction equation for carbonate testing? Use sodium carbonate.
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
Why does limewater turn cloudy when CO₂ is bubbled through it?
The CO₂ reacts with calcium hydroxide in limewater, forming insoluble CaCO₃, which causes cloudiness.
What colour precipitate forms when adding CO2 and Ca(OH)2 together?
CaCO3 (white precipitate) + H2O
How do you test for sulfate ions?
Add acidified BaCl₂ solution. If sulfate ions are present, a white precipitate (BaSO₄) forms.
What is the ionic equation for the sulfate test?
Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)
Why is the solution acidified before the sulfate test?
To remove carbonate impurities that could form BaCO₃ (a white ppt), giving a false positive.
Why can’t sulfuric acid be used to acidify the sulfate test?
Sulfuric acid contains sulfate ions, which would react with Ba²⁺ and form a precipitate, giving a false positive result.
How do you test for halide ions?
Add nitric acid (HNO₃) to remove carbonates, then add silver nitrate (AgNO₃) dropwise.
Ag+ + X- AgC
Why is nitric acid added before silver nitrate?
To react with carbonates, preventing the formation of Ag₂CO₃, which would mask the correct precipitate colors.
What precipitates are formed in the silver nitrate test?
Fluoride (F⁻): No precipitate.
Chloride (Cl⁻): White ppt (AgCl).
Bromide (Br⁻): Cream ppt (AgBr).
Iodide (I⁻): Pale yellow ppt (AgI).
What are the equations for halide precipitation reactions?
Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s) (white ppt).
Ag⁺ (aq) + Br⁻ (aq) → AgBr (s) (cream ppt).
Ag⁺ (aq) + I⁻ (aq) → AgI (s) (pale yellow ppt).
How can silver halide precipitates be further distinguished?
AgCl dissolves in dilute NH₃, AgBr dissolves in concentrated NH₃, AgI does not dissolve in NH₃.
What happens when silver halides react with ammonia?
AgCl + 2NH₃ → [Ag(NH₃)₂]⁺ + Cl⁻ (Colorless solution).
AgBr + 2NH₃ → [Ag(NH₃)₂]⁺ + Br⁻ (Colorless solution, only in concentrated NH₃).
AgI does not react (too insoluble).
Why can’t hydrochloric acid be used in the halide test?
HCl contains chloride ions, which would form a white AgCl precipitate, leading to false positive results.
What is the correct sequence of qualitative tests for anions?
Carbonate → Sulfate → Halide. This prevents false positives (since BaCO₃ and Ag₂SO₄ are both insoluble).
Carbonate 1st = Carbonates react with acids to form CO₂ gas, while sulfates and halides do not.
Sulfate 2nd = BaCO₃ is a white precipitate like BaSO₄, so carbonates must be removed first.
Halide 3rd = Ag₂CO₃ and Ag₂SO₄ also form precipitates, which would interfere with results.
How do you test for ammonium ions (NH₄⁺)?
Add warm NaOH and test for ammonia gas (NH₃) using red litmus paper (turns blue).
What are two ways to identify ammonia gas?
1) Pungent smell. 2) Turns red litmus paper blue.
What is the reaction equation of the ammonium test?
NH4+ + OH- –> NH3 + H2O
What is qualitative analysis?
A method used to identify ions or compounds in a substance based on observations rather than numerical measurements. Observations include:
✅ Gas bubbles (effervescence)
✅ Precipitates (solid formation)
✅ Colour changes
✅ Identification of gases
