qualitative analysis WJEC A-LEVEL Flashcards
(16 cards)
What is a flame test, and what colours are produced for the following cations?
Lithium (Li⁺): Red flame
Sodium (Na⁺): Yellow flame
Potassium (K⁺): Lilac flame
Calcium (Ca²⁺): Brick red flame
Barium (Ba²⁺): Green flame
Copper (Cu²⁺): Blue-green flame
What is the test for cations using sodium hydroxide (NaOH), and what are the color changes?
Al³⁺: White precipitate (Al(OH)₃), soluble in excess NaOH forming colorless solution
Ca²⁺: White precipitate (Ca(OH)₂), not affected by excess NaOH (remains insoluble).
Cu²⁺: Blue precipitate (Cu(OH)₂)
Fe²⁺: Green precipitate (Fe(OH)₂)
Fe³⁺: Red-brown precipitate (Fe(OH)₃)
Zn²⁺: White precipitate (Zn(OH)₂), soluble in excess NaOH forming colorless solution
Why do we use sodium hydroxide (NaOH) before silver nitrate (AgNO₃) in the test for halides?
NaOH is used first to remove any other metal ions (like Ca2+ or Fe2+) that could interfere with the test by forming precipitates with the silver nitrate solution.
What color change occurs when silver nitrate (AgNO₃) is added to halide ions, and why is nitric acid (HNO₃) used beforehand?
Chloride (Cl⁻): White precipitate of AgCl
Bromide (Br⁻): Cream precipitate of AgBr
Iodide (I⁻): Yellow precipitate of AgI
HNO₃ is used to remove any impurities, such as carbonate ions, that could interfere with the silver Nitrate test, to dissolve any coloured hydroxide ppt formed, and so only halide ions react with Ag+.
How do silver halides react with ammonia, and what is the role of ammonia in these reactions?
Ammonia is used to dissolve silver halides in the following ways:
Silver chloride (AgCl): Forms a soluble complex in dilute ammonia (white precipitate dissolves in dilute ammonia to form [Ag(NH₃)₂]⁺). Insoluble in excess ammonia.
Silver bromide (AgBr): Forms a soluble complex in concentrated ammonia (cream precipitate dissolves in concentrated ammonia to form [Ag(NH₃)₂]⁺). Insoluble in excess ammonia.
Silver iodide (AgI): Does not dissolve in ammonia (yellow precipitate remains insoluble, as iodine is less reactive with ammonia and insoluble in both dilute and concentrated ammonia).
What is the test for sulfates (SO₄²⁻), and why is dilute HCl used with BaCl₂?
Add barium chloride (BaCl₂) to the solution.
If sulfates are present, a white precipitate of barium sulfate (BaSO₄) forms.
Dilute HCl is used to remove carbonate ions (CO₃²⁻) or sulfite ions (SO₃²⁻) that could interfere by forming other white precipitates.
What is the test for ammonium ions (NH₄⁺)?
Warm sodium hydroxide (NaOH) solution with the sample.
If ammonium ions are present, ammonia gas (NH₃) is released, which has a pungent smell.
Red litmus paper over the vapor will turn blue (indicating ammonia).
What happens when group 1 and 2 metals react with oxygen?
Group 1 metals (Li, Na, K) form metal oxides (e.g., Na₂O, K₂O).
Group 2 metals form metal oxides and peroxides (e.g., CaO, SrO).
How do Group 1 and 2 elements generally react with water?
They generally form metal hydroxides and release hydrogen gas, causing effervescence. Group 1 metals react more vigorously down the group, with hydrogen from potassium onwards able to ignite.
How do Group 1 and 2 elements generally react with water?
They generally form metal hydroxides and release hydrogen gas, causing effervescence. Group 1 metals react more vigorously down the group, with hydrogen from potassium onwards able to ignite.
What is the general equation for Group 1 metals reacting with water?
2M + 2H₂O → 2MOH + H₂
(M = Li, Na, K) Example: 2Li + 2H₂O → 2LiOH + H₂
How do Group 2 elements react with water, and how is magnesium different?
Beryllium doesn’t react. Magnesium reacts slowly with cold water but reacts with steam to form magnesium oxide instead of hydroxide. Other Group 2 metals (Ca, Sr, Ba) react with cold water to form hydroxides.
What is the general equation for Group 2 metals reacting with steam or water?
With steam (e.g., magnesium): Mg + H₂O → MgO + H₂
With water (e.g., Ca, Sr, Ba): M + 2H₂O → M(OH)₂ + H₂
How do Group 1 metals react with oxygen, and why are they stored under oil?
They form metal oxides rapidly, visible as a black coating. They’re stored under oil to prevent reaction with air.
General equation: 4M + O₂ → 2M₂O (M = Li, Na, K) Example: 4Li + O₂ → 2Li₂O
What is the general reaction of Group 2 metals with oxygen?
Group 2 metals react with oxygen to form oxides.
General equation: 2M + O₂ → 2MO (M = Mg, Ca, Sr, Ba) Example: 2Sr + O₂ → 2SrO
Are oxides of Group 1 and 2 metals acidic or basic?
They are basic oxides. They react with acids to form a salt and water.
Example: CaO + 2HCl → CaCl₂ + H₂O
