Chapter 12 Group 17 Flashcards
Group 17 info
- The group 17 elements are called halogens
- The halogens have uses in water purification and as bleaches agents (chlorine), as flame-retardants and fire extinguishers (bromine) and as antiseptic and disinfectant agents (iodine)
Group 17: Physical Trends: Colour
All halogens have distinct colours which get darker going down the group
F2= pale yellow gas
Cl2= green/yellow gas
Br2 = orange/brown liquid
I2=grey/black solid, purple vapour)
Group 17: Physical Trends: Volatility
- 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
Volatility def
refers to how easily a substance can evaporate
A volatile substance will have a low boiling point
Group 17: Physical Trends in bond strength
- In a covalent bond, the bonding pair of electrons is attracted to the nuclei on either side and it is this attraction that holds the molecule together
- Going down the group, the atomic size of the halogens increases
- The bonding pair of electrons get further away from the halogen nucleus and are therefore less strongly attracted towards in
- The bond strength of the halogen molecules therefore decreases going down the group
halogens bonding
-Halogens are diatomic molecules in which covalent bonds are formed by overlapping their orbitals
Bond enthalpy
is the heat needed to break one mole of a covalent bond
-The higher the bond enthalpy, the stronger the bond
what elements are a exception to bond enthalpy and why
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 attracting between the bonding pair of electrons and two nuclei
Group 17: Dipole Forces & Volatility
- The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces
- 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
- This is why as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase
- As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group
Group 17: Oxidising Agents
-Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge
Ca(s) + Cl2(g) → Ca2+(Cl–)2(s)
- Halogens are therefore oxidising agents and get reduced themselves
- —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)
-The oxidising power of the halogens decreases going down the group (the halogens get less reactive)
The electronegativity of an atom refers
to how strongly it attracts electrons towards itself in a covalent bond
- The decrease in electronegativity is linked to the size of the halogens
- 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
displacement reactions group 17
A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
Group 17: Reaction with Hydrogen
Halogens react with hydrogen gas to form hydrogen halides
Due to the decrease in reactivity of the halogens going down the group, the reactions between halogen and hydrogen gas become less vigorous
Reaction between halogen & hydrogen gas
- hydrogen + flourine (reacts explosively even in cool, dark conditions)
- hydrogen + chlorine (reacts explosively in sunlight)
- hydrogen + bromine (reacts slowly on heating)
- hydrogen + iodine (forms an equilibrium mixture on heating)
Thermal stability
refers to how well a substance can resist breaking down when heated
-A substance that is thermally stable will break down at high temperatures
Thermal Stability of the 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
- As the bonds get weaker, the hydrogen halogens become less stable to heat going down the group
Thermal Stability of the Hydrogen Halides: 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
Halide Ions: 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 decreases going down the group
- This trend can be explained by looking at the ionic radii of the halides’ ions
Going down the group (halide ions)
- 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
Concentrated sulfuric acid
-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)
(general equation)
Where X is the halide ion
Reaction of chloride ions with concentrated sulfuric Acid
- Concentrated sulfuric acid is dropwise added to sodium chloride crystals to produce hydrogen chloride gas
- The reaction that takes place is:
H2SO4(l) + NaCl(s) → HCl(g) + NaHSO4(s)
-The HCl gas produces is seen as white fumes
Reaction of bromide ions with concentrated sulfuric acid
-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:
2HBr(g) + H2SO4(l) → Br2(g) + SO2(g) + 2H2O(l)
-The bromine is seen as a reddish-brown gas
Reaction of iodide ions with concentrated sulfuric acid
-The reaction of sodium iodide and concentrated sulfuric acid is:
H2SO4(l) + NaI(s) → HI(g) + NaHSO4(s)
- Hydrogen iodide decomposes the easiest
- Sulfuric acid oxides the hydrogen iodide to several extents:
- The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur dioxide gas:
2HI(g) + H2SO4(l) → I2(g) + SO2(g) + 2H2O(l)
- Iodine is seen as a violet/purple vapour
- The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur:
6HI(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:
8HI(g) + H2SO4(l) → 4I2(g) + H2S(s) + 4H2O(l)
-Hydrogen sulfide has a strong smell of bad eggs
Reaction of halide ions with silver nitrate & ammonia solutions Colour (Cl-, Br- and I-)
- Cl- = white
- Br-=cream
- I-=pale yellow
Reaction of halide ions with silver nitrate & ammonia solutions effect of adding dilute ammonia and concentrated ammonia (Cl-, Br- and I-)
effect of adding dilute ammonia solution to ppt
- Cl- = dissolves
- Br-=remains insoluble
- I-=remains insoluble
effect of adding concentrated ammonia solution to ppt
- Cl- =dissolves
- Br-=dissolves
- I-= remains insoluble
Reaction of Chlorine
-The reaction of chlorine with dilute alkali is an example of a disproportionation reaction
-In these reactions, the chlorine gets oxidised and reduced at the same time
Different reactions take place at different temperatures of the dilute alkali
A disproportionation reaction
is a reaction in which the same species is both oxidised and reduced
Chlorine in cold alkali (15 degrees)
-Cl2(aq) +2NaOH(aq) -> NaCl(aq) + NaClO(aq) + H2O(l)
- The ionic equation shows that the chlorine gets both oxidised and reduced
- 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)
Chlorine in hot alkali (70 oC)
3Cl2 + 6NaOH(aq) -> 5NaCl(aq) + NaCl3(aq) + 3H2O(l)
- The ionic equation shows that the chlorine gets both oxidised and reduced
- Chlorine gets oxidised as there is an increase in ox. no. from 0 to +5 in ClO3–(aq)
- Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl–(aq)
Chlorine in Water Purification
- Chlorine can be used to clean water and make it drinkable
- The disproportionation reaction of chlorine with water in which chlorine gets reduced to HCl and oxidised to HClO
- 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
