3.2.3.1 Trends in properties (group 7) Flashcards
What is the formula and name for all halide ions?
1-, -ide
What is the state and appearance of fluorine at room temperature and its characteristics?
Very pale yellow gas
Highly reactive
What is the state and appearance of chlorine at room temperature and its characteristics?
Greenish and reactive gas
Poisonous at high concentrations
What is the colour of fluorine in a solution?
No colour, doesn’t really form solutions
What is the colour of chlorine in solutions?
Often colourless, other times very pale green
What is the state and appearance of bromine at room temperature and its characteristics?
Red liquid
Gives off dense brown poisonous fumes
What is the colour of bromine in solutions?
Yellow solutions
What is the state and appearance of iodine at room temperature and its characteristics?
Shiny grey/black solid, sublimes to purple gas
Solid sublimes to purple gas
What is the colour of iodine in solutions?
Brown solution (sometimes black solid present)
Describe the trend of electronegativity down group 7
Decreases, number of shells increases down the group, shielding effect increases, attraction between nucleus and shared electron pair decreaes
Describe the trend of boiling points down group 7
Increases from F to I, molecules have more electrons, Van der Waal’s forces become stronger, so more energy needed to overcome forces
What are the chemical properties of halogens?
Gain electrons in chemical reactions
Reactivity of halogens decreases down group 7
They are good oxidising agents
What is the trend in oxidising abilities of halogens?
F2>CL2>Br2>I2
Describe the trends in reducing ability of the halide ions
Going down the group, reducing ability increases
because halide ion becomes larger as they have more electron shells. Halide ions have greater shielding effect, so it is easier to lose outer electrons as they are less strongly held by nucleus.
Describe the reaction of NaF with conc. H2SO4
Acid-base step: NaF(s) +H2SO4 (l) -> NaHSO4(s) +HF(g)
F- ions are not strong enough reducing agents to reduce the S in H2SO4, so no redox reaction occurs
Observation: White steamy fumes
Describe the reaction of NaCl with conc. H2SO4
Acid-base step: NaCl(s) + H2SO4(l) -> NaHSO4(s) +HCl(g)
Cl- ions are not strong enough reducing agents to reduce the S in H2SO4, so no redox reaction occurs.
Observation: White steamy fumes
Describe the reaction of NaBr with conc. H2SO4
Acid-base step: NaBr(s) + H2SO4(s) -> NaHSO4 (s) + HBr(g)
Br- ions are stronger reducing agents, so after acid-base reaction they reduce the sulfur in H2SO4 from +6 to +4 in SO2
Redox step: 2HBr + H2SO4 -> SO2 (g) + Br2 (g) + 2H2O(l)
role of H2SO4 - oxidising agent
Observation:
Orange fumes of bromine evolved and a colourless acidic gas (SO2)
Describe the reaction of NaI with conc. H2SO4
Acid-base step: NaI (s) +H2SO4(l) -> NaHSO4(s) + HI(g)
I- ions are the strongest halide reducing agents and can reduce sulfur from +6, to +4, to 0, to -2
Redox steps:
2HI + H2SO4 -> SO2(g) + I2(s) + 2H2O(l)
6HI + H2SO4 -> S(s) + 3I2(s) + 4H2O(l)
8HI + H2SO4 -> H2S(g) + 4I2(s) + 4H2O(l)
role of H2SO4: oxidising agent
Observations: White steamy fumes, black solid and purple fumes of iodine evolved, colourless acidic gas (SO2), H2S has with foul egg smell
What are the redox equations for the reaction of NaI with conc. H2SO4?
Oxidation: 2I- -> 2e- + I2
Reduction:
H2SO4 + 2H+ + 2e- -> SO2 + 2H2O
H2SO4 + 6H+ + 6e- -> S + 4H2O
H2SO4 + 8H+ + 8e- -> H2S + 4H2O
What is the reaction when chlorine reacts with water?
Redox reaction:
Cl2(g) + H2O(l) <-> HClO(aq) +HCl(aq)
Oxidation half-equation:
H2O + Cl2 -> 2HClO + 2H+ +2e-
Reduction half-equation:
Cl2 + e- -> 2Cl-
What is used to test for halide ions?
Dilute nitric acid then silver nitrate solution
What are the observations when halide ions react with acidified silver nitrate?
F- = colourless solution
Cl- = white ppt
Br- = cream ppt/ pale yellow
I- = yellow ppt
What is the ionic equation for halide ions reacting with acidified silver nitrate?
Ag+ (aq) + X-(aq) -> AgX(s)
- if the halide is given as a solid, make sure it is dissolved in water before doing the test.
Why is nitric acid added?
To remove carbonate ions that will interfere with the test
Why is HCl not used?
It interferes with the test (forms a white precipitate)
How can you distinguish between ions in a precipitate?
Add dilute ammonia solution or concentrated ammonia solution
What is the effect of adding dilute ammonia solution to precipitate?
Cl- = ppt dissolves
Br- = ppt remains insoluble
I- = ppt remains insoluble
What is the effect of adding concentrated ammonia solution to precipitate?
Cl- = ppt dissolves
Br- = ppt dissolves
I- = ppt remains insoluble
What is the equation when halide precipitate dissolves in ammonia solution?
AgCl(s) + 2NH3(q) -> Ag(NH3)2+(aq) + Cl-(aq)
