3.2.3 - Group 7, the Halogens Flashcards
Why does electronegativity and reactivity decrease down group 7? (3)
- Atoms get larger
- More distance between nucleus and electrons (ionic radius increase)
- Weaker attraction to pair of electrons in covalent bond
Halogen displacement principle
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt
Test for halides
Add dilute nitric acid and then silver nitrate solution.
White ppt in test for halides
Chloride ions
Cream ppt in test for halides
Bromide ions
Yellow ppt in test for halides
Iodide ions
Purpose of nitric acid while testing for halides
Reacts with any anions (e.g. carbonates) which would give false positive
What is the solubility in ammonia of Cl,Br,I ?
Cl- dissolves in dilute NH3
Br- dissolves in conc NH3
I- is insoluble in NH3
Test for ammonium compounds
Add NaOH, gently heat
Damp red litmus paper will turn blue in presence of ammonia gas
Test for hydroxides
Red litmus paper turns blue
Test for carbonates
Add dilute HCl
Bubble gas through limewater, will turn cloudy
Test for sulfates
Add HCl + barium chloride
White precipitate will form
Chloride + conc sulfuric acid product
White gas
Bromide + conc sulfuric acid product
Orange gas
Iodide + conc sulfuric acid produces a…
Purple gas
Why does BaCl2 need to be acidified?
Both sulfites and sulfates produce a white precipitate with BaCl, acid prevents that.
What order should you test group 7 compounds?
Carbonate test - sulfate test - halide test
Bleach disproportionation reaction equation
2NaOH + Cl2 —-> NaCl + NaOCl + H2O
Why is the bleach reaction a disproportionation reaction?
Chlorine is both reduced and oxidised simultaneously
What do iodide ions reduce concentrated sulphuric acid into? (3)
Sulphur dioxide - SO2 (+4)
Sulfur - S (0)
Hydrogen sulfide (H2S) (-2)
How do you formulate the half equation for the reduction of sulphuric acid producing … ?
SO4(2-) —– ____ + H20
Balance hydrogens, then charge using e-
Water + Cl2 equation
CL2 + H20 —-> HCl + HOCl
Overall ionic reaction for reduction with sulphuric acid
H2SO4 + (hydrogens) + (Halide ions) = (reduction product) + (Br2/Cl2/I2) + H20
Sodium chloride + conc sulfuric acid equation + role of chloride
NaCl + H2SO4 —-> NaHSO4 + HCL
Act as base (proton accepter)
What should you do before testing compounds with silver nitrate?
Add acid, add water, then add silver nitrate.
