The chemical properties of the halogen elements and the hydrogen halides — practice questions

Step 1: Identify the relative oxidising strengths. The order of oxidising power is $Cl_2 > Br_2 > I_2$. This means bromine is a stronger oxidising agent than iodine, but a weaker oxidising agent than chlorine. Step 2: Determine which displacement reactions are possible. Bromine can displace iodide ions ($I^-$) because it is more reactive. Bromine cannot displace chloride ions ($Cl^-$) because it is less reactive than chlorine. Step 3: State the observation. The initial solution containing the salts is colourless. When orange-brown aqueous bromine is added, the solution will turn a dark brown or black as iodine ($I_2$) is formed. The orange colour of the excess bromine may also be present. Step 4: Write the ionic equation. $Br_2(aq) + 2I^-(aq) \rightarrow 2Br^-(aq) + I_2(aq)$.

Part 1: Physical State. Fluorine is the most electronegative element, so the H-F bond is highly polar. This leads to the formation of strong hydrogen bonds between HF molecules. Hydrogen bonds are the strongest type of intermolecular force and require significant energy to overcome, resulting in HF having an unusually high boiling point (293 K) and being a liquid at room temperature. HCl has permanent dipole-dipole forces, which are weaker than hydrogen bonds. These weaker forces are easily overcome, so HCl is a gas at room temperature (boiling point 188 K). Part 2: Acidity. The strength of an acid in aqueous solution depends on the extent of its dissociation. The key factor for hydrogen halides is the H-X bond enthalpy. The H-F bond is very strong (568 kJ mol⁻¹) due to effective orbital overlap. This strong bond is difficult to break in water, so HF only partially dissociates and is a weak acid. The H-Cl bond is weaker (432 kJ mol⁻¹). This bond breaks much more readily in water, so HCl fully dissociates and is a strong acid. The strength of the bond enthalpy outweighs the effect of bond polarity in determining acidity.