what are a few properties of Halogens?
- non-metals
- exist as diatomic molecules
- flourine = pale yellow gas
- chlorine = pale green gas
- bromine = brown liquid
- iodide = black solid
explain the trend in boiling point as we go down group 7?
- the boiling and melting point increases as we go down group 7
- as molecules become larger down the group
- as they have a greater surface area and greater/ stronger Van der waals forces
explain the trend in electronegativity as we go down group 7?
- electronegativity decreases
- this is due to a greater atomic radii
- so there is greater shielding
- so therefore less able to withdraw electron density from a covalent bond
explain the trend in oxidising ability as we go down group 7?
- oxidising ability decreases as we go down group 7 elements
- due to increased atomic radii
- so there is greater shielding
- so the outer electron is attracted less strongly to the nucleus
Explain what happens in displacement reactions with halide ions?
- chlorine water displaces both potassium bromide to make an orange solution of bromine (Br2) and potassium iodide to make a brown solution of iodine (I2)
- bromine water has no reaction with sodium chloride but displaces potassium iodide to make a brown solution of iodine (I2)
- iodine has no reaction with the rest
describe the test for identifying halide ions?
Acidified silver nitrate
(nitric acid and silver nitrate)
describe the reaction when silver nitrate reacts with Cl-, Br- , and I-
Cl- = white precipitate
Br- = cream precipitate
I- = yellow precipitate
write an equation to show each equation
cl = Ag+ (aq) + Cl- —-> AgCl (s)
br = Ag+ (aq) + Br- ——> AgBr (s)
I = Ag+ (aq) + I- —-> AgI (s)
why is nitric acid added before the silver nitrate?
to remove any carbonate ions which may produce a false positive result
why is nitric acid instead of hydrochloric acid?
hydrochloric acid contains Cl- ions so may produce a false positive result
if we later added dilute ammonia to the test tubes what would be observed
sodium chloride - dissolves to form colourless solution
potassium bromide - sparingly soluble - add concentrated ammonia - would dissolve to form a colourless solution
potassium iodide - insoluble remains insoluble when conc. ammonia is added
why can acidified silver nitrate not be used to identify Fluoride ions?
because silver fluoride which is formed is soluble in water, so solutions containing fluoride ions remain colourless
reactions of halide ions
why does the reducing power of halide ions increase as we go down the group?
- reducing power of the halides increases as we go down the group
- due to increasing atomic radii
- there is greater shielding
- so more able to donate an electron
what do all halides react with?
concentrated sulphuric acid
but it depends on..?
the halide ion used and reflect the reducing power of the halide
what type of reaction do all halides undergo?
Acid-base reactions
write down an equation to show this acid base reaction
NaX (s) + H2SO4 (aq) —> NaHSO4 (s) + HX (g)
X = halide
what is formed?
NaX = white solid
forms
NaHSO4 = white solid and HX = steamy white fumes
some halides are strong enough to reducing agents to reduce sulphuric acid to…?
sulphur dioxide and water
show this reaction in an equation
2NaX (s) + 2H2SO4 (l) ——-> X2 (g) + SO2 (g) + 2H2O (l) + Na2SO4 (s)
what type of reaction is this?
a redox reaction
what has been reduced and what has been oxidised?
sulphur has been reduced, halide has been oxidised so the halide is the reducing agent
why can chloride not reduce sulphuric acid?
chloride is a weak reducing agent
