Group 7 (17), the Halogens (AQA A Level Chemistry): Flashcards
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What does volatility mean in the context of Group 7 elements?
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What does volatility mean in the context of Group 7 elements?
A standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions (298 K, 100 kPa), with all reactants and products in their standard states.
Going down Group 7, the colours of the halogens get ........... Fluorine is .......... and iodine is .......... .
Going down Group 7, the colours of the halogens get darker. Fluorine is pale yellow and iodine is grey-black.
True or False?
The boiling points of the halogens decrease going down Group 7 because the molecules become smaller.
False.
Boiling points increase going down Group 7. Molecules become larger, so instantaneous dipole-induced dipole forces between molecules are stronger, requiring more energy to separate them.
Why does bond strength in the halogens generally decrease going down the group?
As atomic size increases, the bonding pair of electrons gets further from both nuclei and experiences more shielding. The attraction between the bonding pair and the nuclei weakens, so the bond enthalpy decreases.
Halogens are .......... molecular structures held together by weak .......... forces caused by .......... dipole-induced dipole interactions.
Halogens are simple molecular structures held together by weak van der Waals' forces caused by instantaneous dipole-induced dipole interactions.
True or False?
Fluorine has a lower bond enthalpy than expected for its position in Group 7 because of lone-pair repulsion in the F2 molecule.
True.
Fluorine atoms are so small that the lone pairs on each atom get very close together, causing significant repulsion that reduces the overall bond strength.
What are instantaneous dipole-induced dipole forces?
Instantaneous dipole-induced dipole forces are a type of van der Waals' force arising when a temporary imbalance in electron distribution in one molecule creates an instantaneous dipole, which induces a dipole in a neighbouring molecule. These forces increase with molecular size.
What does it mean to describe a halogen as an oxidising agent?
A halogen acts as an oxidising agent by accepting an electron from another species. In doing so, the halogen itself is reduced (its oxidation number decreases from 0 to -1).
True or False?
Chlorine can displace iodide ions from solution because chlorine is a stronger oxidising agent than iodine.
True.
A more reactive (stronger oxidising) halogen displaces a less reactive one from solution. Chlorine oxidises I- to I2, while Cl2 is itself reduced to Cl-.
The oxidising power of the halogens .......... going down the group. This is because atomic radius ........., causing greater .......... of the nucleus by inner electrons, so incoming electrons are attracted less strongly.
The oxidising power of the halogens decreases going down the group. This is because atomic radius increases, causing greater shielding of the nucleus by inner electrons, so incoming electrons are attracted less strongly.
Write the ionic equation for chlorine displacing bromide ions from solution.
Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)
What is the trend in reducing power of halide ions down Group 7?
Reducing power increases going down the group. Larger ionic radius and greater electron shielding mean the outermost electrons are held less tightly, making them easier to lose (donate to another species).
Halide ions act as .......... agents by .......... electrons. Going down Group 7, the ionic radius ........., so the outer electrons are held .......... tightly and the reducing power .......... .
Halide ions act as reducing agents by donating electrons. Going down Group 7, the ionic radius increases, so the outer electrons are held less tightly and the reducing power increases.
True or False?
Iodide ions are stronger reducing agents than chloride ions.
True.
I- has a larger ionic radius and greater electron shielding than Cl-, so its outer electrons are held less tightly and it more readily donates electrons (acts as a reducing agent).
Electronegativity
The ability of an atom to attract electrons towards itself in a covalent bond. Electronegativity decreases down Group 7 as atomic radius increases and shielding reduces the attraction for incoming electrons.
What reagents are used to test for halide ions in solution?
The unknown solution is first acidified with dilute nitric acid, then silver nitrate solution is added dropwise. A coloured precipitate of silver halide confirms the presence of a halide ion.
When testing for halides, dilute .......... acid (not hydrochloric acid) is used to acidify the solution first. This prevents .......... ions from giving a false positive result.
When testing for halides, dilute nitric acid (not hydrochloric acid) is used to acidify the solution first. This prevents carbonate ions from giving a false positive result.
State the colours of the precipitates formed when silver nitrate is added to solutions of Cl-, Br-, and I-.
Cl- → white precipitate (AgCl)
Br- → cream precipitate (AgBr)
I- → pale yellow precipitate (AgI)
True or False?
Silver iodide precipitate dissolves in concentrated ammonia solution.
False.
Silver iodide does not dissolve in either dilute or concentrated ammonia. Silver chloride dissolves in dilute ammonia; silver bromide dissolves only in concentrated ammonia.
Ag+ (aq) + X- (aq) → ..........
Ag+ (aq) + X- (aq) → AgX (s)
where X- is the halide ion. The product is a solid precipitate.
Why is it important to say the precipitate dissolves in ammonia, rather than the ions or solution?
Mark schemes penalise students who write that the "solution" or "ions" dissolve. The correct observation is that the precipitate dissolves in ammonia solution.
True or False?
Adding concentrated sulfuric acid to sodium bromide produces only HBr gas as the observed product.
False.
The HBr initially formed is further oxidised by concentrated H2SO4. Orange-brown Br2 gas and SO2 are also produced: 2HBr (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l).
What is the trend in ease of oxidation of hydrogen halides by concentrated sulfuric acid?
It becomes easier to oxidise the hydrogen halide going down the group: HCl is not oxidised further; HBr is oxidised to Br2; HI is oxidised to I2 and further reduced products (S, H2S) also form. Halide ions become stronger reducing agents down the group.
Silver halide
An insoluble precipitate formed when silver nitrate solution is added to a halide ion solution: AgCl is white, AgBr is cream, and AgI is pale yellow. Their different solubilities in ammonia solution allow identification of the specific halide.
What is a disproportionation reaction?
A standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K, 100 kPa).
When chlorine reacts with cold dilute NaOH, the products are NaCl, .......... and ......... . This is an example of a .......... reaction.
When chlorine reacts with cold dilute NaOH, the products are NaCl, NaClO and H2O. This is an example of a disproportionation reaction.
True or False?
In the reaction of chlorine with cold alkali, the chlorine is both oxidised (to ClO-) and reduced (to Cl-).
True.
Chlorine starts at oxidation state 0. It is oxidised to +1 in ClO- and reduced to -1 in Cl-. This simultaneous oxidation and reduction defines disproportionation.
Write the equation for the disproportionation of chlorine in water.
Cl2 (aq) + H2O (l) → HCl (aq) + HClO (aq)
Chlorine is reduced to HCl (oxidation state -1) and oxidised to HClO (oxidation state +1).
How does chlorine make water safe to drink?
Chlorine reacts with water to form chloric(I) acid (HClO), which sterilises the water by killing bacteria. HClO also dissociates to ClO- (aq), which also acts as a sterilising agent.
In shallow swimming pools, chlorine is rapidly lost because sunlight causes the photodecomposition of HClO: 2HClO (aq) → .......... + .......... . This means chlorine must be .......... regularly.
In shallow swimming pools, chlorine is rapidly lost because sunlight causes the photodecomposition of HClO: 2HClO (aq) → 2HCl (aq) + O2 (g). This means chlorine must be topped up regularly.
True or False?
The risks of adding chlorine to drinking water outweigh its benefits because it can form toxic chlorinated hydrocarbons.
False.
The benefits of killing harmful bacteria outweigh the risks. Chlorine is used in small, controlled amounts, and the risk from chlorinated hydrocarbons is considered acceptable.
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