9701 · 11.2
The chemical properties of the halogen elements and the hydrogen halides flashcards
Revision flashcards for Cambridge 9701 The chemical properties of the halogen elements and the hydrogen halides (syllabus 11.2). Flip, recall, then mark a real past-paper question.
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Define an oxidising agent in terms of electron transfer.
A species that accepts electrons from another species, causing the other species to be oxidised. The oxidising agent itself is reduced.
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What is the trend in oxidising power of the halogens down Group 17?
It decreases. Fluorine is the strongest oxidising agent, and iodine is the weakest.
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Why does the oxidising power of halogens decrease down the group?
Atomic radius increases and electron shielding increases. This makes it harder for the nucleus of a larger halogen atom to attract an incoming electron to its outer shell.
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What is a disproportionation reaction?
A redox reaction where an element in a single species is simultaneously oxidised and reduced.
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Write the ionic equation for the reaction of chlorine with cold, dilute NaOH(aq).
$Cl_2 + 2OH^- \rightarrow Cl^- + ClO^- + H_2O$
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What are the oxidation states of chlorine in $Cl_2$, $Cl^-$, and $ClO^-$?
0 in $Cl_2$, -1 in $Cl^-$, and +1 in $ClO^-$.
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What is the trend in thermal stability of the hydrogen halides (HX)?
It decreases down the group. HF is the most stable, and HI is the least stable.
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Why does the thermal stability of HX decrease down the group?
The H-X bond enthalpy decreases. As the halogen atom gets larger, the overlap between its p-orbital and the hydrogen's 1s orbital becomes less effective, resulting in a weaker, longer bond that requires less energy to break.
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What is the trend in acidity of the hydrogen halides in aqueous solution?
Acidity increases down the group: HF < HCl < HBr < HI. HF is a weak acid, while the others are strong acids.
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Why is HF a weak acid, while HCl, HBr, and HI are strong acids?
The H-F bond is very strong and requires a lot of energy to break (high bond enthalpy). This is the dominant factor and means HF dissociates only partially in water, despite the bond's high polarity.
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A student adds aqueous chlorine to aqueous potassium iodide. What is observed?
The colourless solution turns brown/black. Chlorine displaces iodine: $Cl_2(aq) + 2I^-(aq) \rightarrow 2Cl^-(aq) + I_2(aq)$.
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What colour is iodine in an aqueous solution versus in an organic solvent like cyclohexane?
Brown in aqueous solution (due to formation of $I_3^-(aq)$ ions). Purple/violet in a non-polar organic solvent.