group 7 - halogens Flashcards
what is the name of the group 7
the halogens
how do the colours of the halogens vary
get darker going down the group
what is the appearance of F2
yellow gas
very reactive
toxic
what is the appearance of Cl2
green gas
very reactive
toxic
what is the appearance of Br2
orange liquid
very reactive
toxic
often in solution as aqueous bromine water
what is the appearance of I2
grey crystals
reactive
toxic
easily turns into a purple vapour upon heating
how does the volatility change down group 7
Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
This means that fluorine is the most volatile and iodine the least volatile
how does the bond strength change down the group
it decreases
Bond enthalpy is the energy needed to break one mole of covalent bonds
The higher the bond enthalpy, the stronger the bond
An exception to this is fluorine which has a smaller bond enthalpy than chlorine and bromine
Fluorine is so small that when two atoms of fluorine get together their lone pairs get so close that they cause significant repulsion counteracting the attraction between the bonding pair of electrons and two nuclei
what intermolecular forces exist between the halogens
van der Waals
The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces
how does the strength of the VDW change down the group?
The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
Therefore, the larger the molecule the stronger the van der Waals’ forces between molecules
what is the trend in melt + boiling points
increase
-bigger molecules
-more VDW
-higher melt/boil
more energy required to break forces
what is the trend in electronegativity?
electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
decrease
Going down the group, the atomic radii of the elements increase which means that the outer shells get further away from the nucleus
An ‘incoming’ electron will therefore experience more shielding from the attraction of the positive nuclear charge
The halogens’ ability to accept an electron (their oxidising power) therefore decreases going down the group
what is the trend in 1 IE
decrease
atoms get bigger
more shielding
weaker attraction from nuclear to electron in the outer shell
so less energy is needed to remove one mole of electrons from the outer shell of gaseous atom/ion
what happens to the oxidising power of the group
decrease
-more shells of electrons
weaker attraction bwt outer shell + nucleus
-harder to attract/gain an extra electron to gain a full outer shell
how do the halogens act as oxidising agents
Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge
Eg. Ca (s) + Cl2 (g) → CaCl2 (s) consisting of Ca2+ and 2Cl- ions
Halogens are therefore oxidising agents:
Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)
reactions of halogen with halide
Cl2 with Br- = orange solution
Cl2 with I- = brown solution
Br2 with Cl- = no visible change
Br2 with I- = brown solution
I2 with Cl- and Br- = no visible change
how do the halogens act as reducing agents
Halide ions can also act as reducing agents and donate electrons to another atom
The halide ions themselves get oxidised and lose electrons
The reducing power of the halide ions increases going down the group
This trend can be explained by looking at the ionic radii of the halide ions
Going down the group, the halide ions become larger
The outermost electrons get further away from the nucleus
The outermost electrons also experience more shielding by inner electrons
As a result of this, the outermost electrons are held less tightly to the positively charged nucleus
Therefore, the halide ions lose electrons more easily going down the group and their reducing power increases
what is thermal stability
Thermal stability refers to how well a substance can resist breaking down when heated
A substance that is thermally stable will break down only at high temperatures
how does the thermal stability change going down the group- hydrogen halides
The hydrogen halides formed from the reaction of halogen and hydrogen gas decrease in thermal stability going down the group
The decrease in thermal stability can be explained by looking at the bond energies of the hydrogen-halogen bond
Going down the group, the atomic radius of the halogens increases
The overlap of its outer shell with a hydrogen atom therefore gives a longer bond length
The longer the bond, the weaker it is, and the less energy required to break it
As the bonds get weaker, the hydrogen halogens become less stable to heat going down the group
how can u test for the reducing power of the halide ion
react with H2SO4
Cl- does not reduce
Br- reduce from S (+6) to S(+4)
I- reduce from S(+6) to S (-2)
how to write half equations for the reducing power
add water and h+ to balance
what solution can be used to test for halide ions
Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise
why is nitric acid used before adding silver nitrate
Remove carbonate ions
The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions
what is the general equation for halide test
Ag+ (aq) + X- (aq) → AgX (s)
what are the results of halide test
Silver chloride (AgCl) is a white precipitate
Silver bromide (AgBr) is a cream precipitate
Silver iodide (AgI) is a yellow precipitate
why do you add dilute and conc ammonia after silver nitrate test
Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present
Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
If the precipitate dissolves in dilute ammonia the unknown halide is chloride
If the precipitate does not dissolve in dilute, but does dissolve in concentrated ammonia the unknown halide is bromide
If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodide
what are the reactions with H2SO4 + general equation
Chloride, bromide and iodide ions react with concentrated sulfuric acid to produce toxic gases
These reactions should therefore be carried out in a fume cupboard
The general reaction of the halide ions with concentrated sulfuric acid is:
H2SO4(l) + X-(aq) → HX(g) + HSO4-(aq)
reaction of bromine with sulphuric
The reaction of sodium bromide and concentrated sulfuric acid is:
H2SO4 (l) + NaBr (s) → HBr (g) + NaHSO4 (s)
The concentrated sulfuric acid oxidises HBr which decomposes into bromine and hydrogen gas and sulfuric acid itself is reduced to sulfur dioxide gas:
2H+ 2Br- (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l)
The bromine is seen as a reddish-brown gas
reaction of iodine with sulphuric
The reaction of sodium iodide and concentrated sulfuric acid is:
H2SO4 (l) + NaI (s) → HI (g) + NaHSO4 (s)
Hydrogen iodide decomposes readily
Sulfuric acid oxidises the hydrogen iodide to form several products:
The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur dioxide gas:
2H+ 2I- (g) + H2SO4 (l) → I2 (g) + SO2 (g) + 2H2O (l)
Iodine is seen as a violet/purple vapour also as a black solid
The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur:
6H+ 6I- (g) + H2SO4 (l) → 3I2 (g) + S (s) + 4H2O (l)
Sulfur is seen as a yellow solid
The concentrated sulfuric acid oxidises HI and is itself reduced to hydrogen sulfide:
8H+ 8I- (g) + H2SO4 (l) → 4I2 (g) + H2S (s) + 4H2O (l)
Hydrogen sulfide has a strong smell of bad eggs
what is a disproportionation reaction
is a reaction in which the same species is both oxidised and reduced
The reaction of chlorine with dilute alkali is an example of a disproportionation reaction
how is chlorine useful
Chlorine can be used to clean water and make it drinkable
Chlorine is used in water treatment to kill bacteria. It has been used to treat drinking water and the water in
swimming pools. The benefits to health of water treatment by chlorine outweigh its toxic effects.
The reaction of chlorine in water is a disproportionation reaction in which the chlorine gets both oxidised and reduced
Chloric(I) acid (HClO) sterilises water by killing bacteria
Chloric acid can further dissociate in water to form ClO-(aq):
HClO(aq) → H+(aq) + ClO-(aq)
ClO-(aq) also acts as a sterilising agent cleaning the water
what is the reaction of chlorine in water
Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO-(aq)
Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)
what is the reaction of chlorine with sunlight
2Cl2 (g) + 2H2O(l) = 4HCl (aq) + O2 (g)
Pale green to colourless
chlorine with water
Cl2(g) + H2O (l) ⇌ HClO (aq) + HCl (aq)
chlorine with water and sunlight
occurs.
2Cl2 + 2H2O = 4HCl + O2
Reaction of chlorine with cold dilute NaOH solution
Cl2 (and Br2, I2) in aqueous solutions will react with cold sodium hydroxide. The colour of the halogen
solution will fade to colourless.
Cl2 (aq) + 2 NaOH (aq) NaCl (aq) + NaClO (aq) + H2O (l)
reaction of Cl- with H2SO4
HCl and steamy fumes
equation of Br- and Cl2
Cl2 + 2 Br- = Br2 + 2Cl-
orange solution
equation of I- with Cl2
Cl2 + 2I- = 2Cl- + I2
brown solution
equation of I- and Br2
Br2 + 2I- = 2 Br- + I2
brown solution forms
overall equation for the reaction of aqueous bromine and sulfuric acid
2H2SO4 + 2NaBr → Na2SO4 + SO2 + Br2 + 2H2O
