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)
