![]() The equation for this reaction is given below:Ī solution in contact with one of the silver halide precipitates contains a very small concentration of dissolved silver ions. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right. The ammonia combines with silver ions to produce a complex ion called the diamminesilver(I) ion, +. The compounds are all quite insoluble, but become even less so down the group. The table below lists solubility products from silver chloride to silver iodide (a solubility product for silver fluoride cannot be reported because it is too soluble). Enough solid is always precipitated to lower the ionic product to the solubility product. If the product of the concentrations exceeds this value, a precipitate is formed.Įssentially, the product of the ionic concentrations is never greater than the solubility product value.If the product of the concentrations of ions is less than the solubility product, no precipitate is formed.The square brackets indicate molar concentrations, with units of mol L -1. For the silver halides, the solubility product is given by the expression: This value is known as the solubility product. A precipitate forms if the concentrations of the ions in solution in water exceed a certain value, unique to every compound. There are no absolutely insoluble ionic compounds. Precipitate is insoluble in ammonia solution of any concentration Precipitate is almost unchanged using dilute ammonia solution, but dissolves in concentrated ammonia solution to give a colorless solution Precipitate dissolves to give a colorless solution In these reactions, bond strength is the main factor deciding the relative rates of reaction.\)Ĭonfirming the precipitate using ammonia solutionĪmmonia solution is added to the precipitates. However, the fastest reaction is with an iodoalkane. The electronegativity difference between carbon and iodine is negligible. That means that there will be more attraction between a lone pair on the water and a carbon atom attached to a chlorine atom than if it was attached to an iodine atom. The slight positive charge on the carbon will be larger if it is attached to a chlorine atom than to an iodine atom. It is tempting to think that the reaction will be faster if the electronegativity difference is greater. It is slightly positive because most of the halogens are more electronegative than carbon, and so pull electrons away from the carbon. If you have looked at the mechanisms for these reactions, you will know that a lone pair on a water molecule attacks the slightly positive carbon atom attached to the halogen. In order for a halide ion to be produced, the carbon-halogen bond has to be broken. The carbon-iodine bond is the weakest and the carbon-chlorine the strongest of the three bonds. The order of reactivity reflects the strengths of the carbon-halogen bonds. A primary chloro compound probably won't give any precipitate until well after you have lost interest in the whole thing!.A primary bromo compound takes longer to give a precipitate.A primary iodo compound produces a precipitate quite quickly.But the pattern of results is always the same. Obviously, the time taken for a precipitate of silver halide to appear will depend on how much of everything you use and the temperature at which the reaction is carried out. You might, for example, compare the times taken to produce a precipitate from this series of primary halogenoalkanes: You would have to keep the type of halogenoalkane (primary, secondary or tertiary) constant, but vary the halogen. Secondary halogenoalkanes do a bit of both of these.Ĭomparing the reaction rates as you change the halogen ![]()
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