Chemistry: Group 7 Halogens Quiz

Group 7: The Halogens

• Group 7 are the halogens, highly reactive non-metals
• Their atomic radius increases down the group due to the addition of electron shells
• Each element needs to gain an electron to form a 1- ion to achieve a full outer shell
• Reactivity decreases down the group as the atomic radius increases, meaning it becomes harder to attract an electron
• The first ionisation energy decreases down the group due to a greater atomic radius and increased shielding.
• Halogens are simple covalent molecules held together by van der waals forces
• The strength of these van der waals forces increases down the group, which requires more energy to overcome and results in a higher boiling point
• Fluorine is a gas at room temperature, whereas iodine is a solid
• Halogens are good oxidising agents as they accept electrons from the species being oxidised and are reduced.
• Their oxidising power decrease down the group as their ability to attract electrons decreases due to shielding and a greater atomic radius
• A halogen will displace any halide beneath it in the periodic table
• Halide ions are the negative ions formed from halogens
• They are good reducing agents which donate electrons to the species being reduced
• Their reducing power increases down the group as electrons are easier to lose from larger ions due to shielding and a larger atomic radius
• Halide ions react with sulfuric acid to form different products depending on how strong a reducing agent they are, with stronger reducing agents reacting further
• Acidfied silver nitrate is used to test for halide ions
• Silver nitrate reacts with halide ions to form coloured precipitates
• The colour of precipitate can be hard to distinguish, they can be further tested using ammonia
• Cl- with silver forms a white precipitate (AgCl) that dissolves in dilute and concentrated ammonia
• Br- with silver forms a cream precipitate (AgBr) that dissolves in concentrated ammonia only
• I- with silver forms a yellow precipitate (AgI) that doesn't dissolve in concentrated ammonia
• Chlorine reacts with cold water to produce chlorate(I) ions (ClO-) and chloride ions
• This is a disproportionation reaction as the chlorine is oxidised and reduced, with its oxidation state going from 0 to +1 and -1
• In the presence of UV light chlorine decomposes water to produce oxygen and hydrochloric acid
• Chlorine is reduced in this reaction
• Chlorine is used in water treatment to kill bacteria but it can be toxic
• Chlorine can be mixed with cold aqueous sodium hydroxide to produce sodium hypochlorite, a key ingredient in bleach
• Anions are tested using acidified silver nitrate and ammonia
• Anions (SO42-) are tested using BaCl2, which reacts to form a white precipitate
• Anions (OH-) can be identified with red litmus, which turns blue
• Anions (CO32-) can be identified by reacting with an acid which will cause fizzing and release CO2 gas
• The gas can be collected and bubbled through limewater, which will turn cloudy, confirming it as CO2
• Cations (Ca2+) can be identified with a flame test and will produce a brick red flame
• Cations (Sr2+) can be identified with a flame test and will produce a red flame
• Cations (Ba2+) can be identified with a flame test and will produce a pale green flame
• Cations (NH4+) can be identified with a red litmus test because they release ammonia gas which is a base, the litmus will turn from red to blue
• Cations (NH4+) can also be identified by adding NaOH, which will produce ammonia gas quickly

Group 7 Elements - The Halogens

• Group 7 elements are highly reactive non-metals that need to gain an electron to form a 1- ion and achieve a full outer shell of electrons.
• As you go down group 7, the atomic radius increases due to the addition of electron shells.
• Reactivity decreases down the group because the attraction of the nucleus weakens due to shielding making it harder for atoms to attract an electron.
• The first ionization energy of group 7 elements decrease down the group due to a greater atomic radius and increased amounts of shielding.
• Group 7 elements are simple covalent molecules held together by Van der Waals forces, which increase in strength as you go down the group. This means more energy is required to overcome the intermolecular forces resulting in a higher boiling point.
• Fluorine is a gas at room temperature, whereas iodine is a solid.
• Halogens act as strong oxidizing agents because they accept electrons from the species being oxidized and are reduced.
• Oxidizing power decreases down the group as their ability to attract electrons decreases due to shielding and a greater atomic radius.
• A halogen will displace any halide beneath it in the periodic table, meaning a higher halogen will displace a lower halogen.

Halide Ions

• Halide ions are the negative ions of halogens and act as good reducing agents.
• This is because they donate electrons to species being reduced and are themselves oxidized.
• Reducing power increases down the group as electrons are easier to lose from larger ions (due to shielding and larger atomic radius).

Silver Nitrate

• Acidified silver nitrate is used to test for halide ions as it reacts to form different colored precipitates depending on the ion present.
• The precipitates formed may not be clear to distinguish, so they can be tested further using ammonia.

Chlorine and Chlorate(I) ions

• Chlorine reacts with cold water to produce chlorate(I) ions (ClO-) and chloride ions.
• This is a disproportionation reaction because the chlorine is both oxidized and reduced. The oxidation state goes from zero to +1 and -1.
• In the presence of UV light, chlorine decomposes water to produce oxygen and hydrochloric acid. The chlorine is reduced in this reaction.
• Chlorine is used in water treatment processes, but it can be toxic.
• Chlorine can be mixed with cold, aqueous sodium hydroxide to produce sodium hypochlorite, which is a key ingredient in bleach.

Tests for Ions

• Halides can be tested using acidified silver nitrate and ammonia.
• Sulfates can be tested using BaCl2, which reacts to form a white precipitate.
• Hydroxides can be tested using red litmus paper (which turns blue) or a universal indicator (which turns blue-purple).
• Carbonates can be tested by adding acid (such as HCl), which will cause the substance to fizz (effervescence) and produce CO2 gas. This gas can be collected and bubbled through limewater, which will turn cloudy, confirming it as carbon dioxide.

Cations

• Calcium (Ca2+), Strontium (Sr2+), and Barium (Ba2+) ions can be identified using flame tests.
• Ammonium (NH4+) ions can be identified by holding red litmus paper over the substance being tested. If ammonium ions are present, the litmus paper will turn blue. Alternatively, NaOH can be added to produce ammonia gas faster.

Test your knowledge on Group 7, the halogens, and their properties. This quiz covers their reactivity, atomic radii, and ionization energies, along with their behavior as oxidizing agents. Understand the trends within this group of highly reactive non-metals.