Group 7: The Halogens

Transcription

1 Group 7: The Halogens Fluorine (F), Chlorine (), Bromine (Br), Iodine (I-) and Astatine (At). The halogens are very reactive elements and so, just like the Group 2 metals, they are not found in nature in elemental form. Instead they occur as halide ions (fluoride (F ), chloride ( ), bromide (Br ), iodide (I )). Astatine is radioactive and only exists briefly in nature (as At ) when uranium, in uranium ores, decays radioactively. In fact, astatine is the rarest naturally occurring element in the Earth s crust and estimates suggest that no more than 44 mg of astatine exists in the first kilometre of the Earth s crust! Abundance Fluoride and chloride are particularly common in rocks. Chloride and bromide are particularly abundant in salt water, from which they are also extracted. Chloride is also abundant as Na and K salt deposits. Iodine is less abundant, but can be mined as iodates, e.g. the mineral lauterite is calcium iodate (Ca(IO 3 ) 2 ) and iodine can also be extracted from brine wells and seaweeds. Physical Properties In elemental form, the halogens exist as simple molecular compounds, containing diatomic molecules (i.e. molecules of two atoms): F 2, 2, Br 2, I 2. Fluorine is a paleyellow gas of F 2 molecules, chlorine is a greenish-yellow gas of 2 molecules. Bromine is a reddish-orange liquid of Br 2 molecules with a foul choking odour (Greek bromos means stench ) and iodine is a purple-black solid with a metallic lustre. Q.1. What forces hold the halogen molecules, X 2 (where X is a halogen), together? Q.2. Draw these forces on the diagram below, for chlorine:

2 Q.3. The diagram below illustrates the molecular structure of iodine, which is a simple molecular solid of I 2 molecules arranged in layers in which the orientations of the iodine molecules alternate. What forces hold these molecules together in this rigid lattice? Why is bromine a liquid and iodine a solid? (In other words: why does the volatility decrease down the group?). [Iodine derives its name from the Greek for violet). Top layer Middle layer Bottom layer

3 Reactivity Fluorine is the most reactive of all elements and it often reacts explosively with other elements. (Only He, Ar and Ne do not react with fluorine at all). Organic compounds (such as what our bodies are made of!) often inflame and burn in the presence of fluorine. Metals, including iron, may also ignite and burn violently in the presence of fluorine. Fluorine is highly toxic, causing serious burns to the lungs if inhaled! Fluorine may be handled in special metal vessels, which become coated with a protective layer of metal flouride (in a similar way that Mg and Al form protective oxide layers in air). Fluorine may only be handled if HF has impurity is first removed, since this rapidly attacks glass. 1. Reaction with water Fluorine reacts easily with water to produce hydroflouric acid. The other halogens dissolve slightly in water, with chlorine reacting slowly to form a mixture of hydrochloric acid and hypochlorous acid: H: hydrochloric acid HO: hypochlorous acid (chloric(i) acid) 2 (g) + H 2 O(l) HO(aq) + H(aq) The hypochlorous acid decomposes slowly (rapidly in sunlight) to produce oxygen: 2HO(aq) 2H(aq) + O 2 (g) Q.4. What is the oxidation state of chlorine in H? Q.5. What is the oxidation state of chlorine in HO? Q.6. Has the chlorine been oxidised or reduced? Q.7. What do we call this type of reaction? The hypochlorous acid produced is a powerful oxidising agent, in which the hypochlorous acid is reduced to. Q.8. Write an ionic half-equation for the reduction of hypochlorous acid to chloride. Q.9. Why is this reaction useful?

4 2. Reaction with alkali Bleach consists of chlorine reacted with excess cold and dilute NaOH to form a mixture of chloride ( ) and hypochlorite (O ). Q.10. Write the equation for the reaction between chlorine and alkali here: The strength of bleach is due largely to the oxidising power of the hypochlorite ions. When the hypochlorite oxidises another chemical, the hypochlorite itself is reduced to chloride: O (aq) + 2e + 2H + (aq) (aq) + H 2 O(l) Q.11. Why is this oxidising power useful in bleach?

5 3. Displacement Reactions Bromine was first prepared (by A. J. Balard in 1826) by reacting bromide ions with a solution of 2 dissolved in water: Q.12. What type of reaction is this? 2Br (aq) + 2 (aq) Br 2 (aq) + 2 (aq) Q.13. Is Br oxidised or reduced? Q.14. Is 2 oxidised or reduced? Q.15. What would you see in this reaction? The chlorine has displaced the bromide because chlorine is a stronger oxidising agent than bromine (equivalently bromide is a stronger reducing agent than chloride). The oxidising power of the halogens decreases in the order: F 2 > 2 > Br 2 > I 2 (oxidising strength) Q.16. When a halogen oxidises another chemical, the halogen itself is reduced to halide. Thus, the trend in oxidising power can be explained by the fact that the ease with which a halogen (X) forms the halide ion (H-) decreases down the group. Explain this trend. Q.17. Will fluorine displace iodide ions from solution? If so, then write the equation for this reaction. What would you see? Q.18. Will chlorine displace fluoride from solution? If so, then write the equation for this reaction. What would you see?

6 4. The silver nitrate test for halides A mixture of silver nitrate (AgNO 3 (aq)) in dilute nitric acid (HNO3(aq)) is added to a test solution containing an unknown halide (X ). In all cases, except with F, a precipitate of silver halide is seen. Complete the table below to summarise what you would see: Reagent Halide Product AgNO 3 (aq) + HNO 3 (aq) F - (aq) no reaction AgNO 3 (aq) + HNO 3 (aq) - (aq) AgNO 3 (aq) + HNO 3 (aq) Br - (aq) AgNO 3 (aq) + HNO 3 (aq) I - (aq) In practise it is difficult to distinguish the Ag(s), AgBr(s) and AgI(s) from each other by colour alone. Additional tests are required. One such test involves the addition of dilute and concentrated ammonia solution (NH 3 (aq)). Complete the table below to summarise what you would see: Silver halide Dilute ammonia solution Concentrated ammonia solution Ag(s) soluble AgBr(s) AgI(s) insoluble Another distinction is the effect of sunlight on the silver halide precipitate. Complete the table below to summarise the effect of sunlight on each silver halide: Silver halide Ag(s) AgBr(s) AgI(s) Effect of sunlight turns black