In this lesson, we are going to identify halide ions. chloride, bromide, iodide. We study what compounds are used to identify halide ions and what are the observations we can see after halide ions testing. Some halide ions compounds dissolve in water and some form precipitates. According to the physical state and halide ion, we choose different methods and reagents to test halide ion.
Halide ion the anion of halogen atom. The charge of halide ion is -1. F-, Cl-, Br-, I- are the halide ions. These halide ions have some simialar properties and different properties.
All of those halides are in -1 oxidation state. Also these halides can be presence as solid state or solution state. Some of these halide compounds are precipitates. According to the state, we have to change the experimental method to identify the ion.
Here, we are going to discuss about 3 different methods to identify halide ions which exist such as NaCl(aq) , KI(aq) .
Here, we are going to discuss about 2 different methods to identify halides which exist as solids such as NaCl(s), KI(s)
It will give NaHSO4 and HCl gas. HCl can be identified by ammonia (NH3) gas. HCl and NH3 reacts and
give white solid fog of ammonium chloride(NH4Cl).
Manganese dioxide(MnO2) is a oxidizing agent which oxidizes chloride ions into chlorine gas. Light yellow green colour
Cl2 gas is released in the reaction.Cl2is a toxic gas.
Add K2Cr2O7, concentrated H2SO4 to solid chloride. Then heat the mixture. A red colour vapour CrO2Cl2 is formed.
Add NaOH. CrO2Cl2 and OH- react to give yellow solution CrO42-. Then add CH3COOH and Pb(CH3COO)2. A yellow colour PbCrO4 precipitated is formed. PbCrO4(s) solve in dilute HNO3.
It will give brown colour Br2 gas. H2 gas also is given.
NaCl(s) gives HBr with concentrated H3PO4.
When hot concentrated H3PO4 acid is added to solid NaCl, HBr vapour is formed.
It will give brown colour Br2 gas. MnO2 is reduced to Mn2+ ions.
Iodide ion is oxidized and it will give purple colour I2 gas. Also hydrogen gas is given.
It will give purple colour I2 gas.
HI is formed. H3PO4 is not a oxidizing acid.
Purple colour I2 is given. Also Cr3+ is formed. But red CrO2Cl2 vapour is not given.
I2 is released. I2 can be identified by farina.
All alkali metal halide compounds are high melting crystalline solids. All alkali metal halide compounds are soluble in water exception of LiF. LiF is insoluble in water due to its high lattice energy because of small cation and small anion size. Other halide ions of lithium are soluble in ethanol, acetone, ethyl acetate. LiCl is soluble in pyridine. CsI has also low solubility due to smaller hydration energy of its two ions.
Being covalent BeCl2 is soluble in organic solvents. MgCl2, MgBr2 are soluble in organic solvents. BeCl2 has low melting point and BaCl2 has higher melting points. Fluorides of alkali earth metals are sparingly soluble in water. The solubility increases slightly with increase of cation size. All alkali earth metals are ionic and soluble in water except BeCl2.
Some halide compounds have colours. Below all compounds are precipitates.
Hydrogen halides are the most useful compounds of halogens. HCl, HBr, HI are strong acids and HF is a weak acid. All of the hydrogen halides are very soluble in water. HCl, HBr, HI are almost completely dissociated in dilute solutions.
Heating a salt containing the halide ion with a nonvolatile acid is the usual way in which HF, HCl, and HBr are obtained in laboratory experiments.
But this method cannot use to prepare HI because HI is not relatively stable than other hydrogen halides.
prepare a compound containing iodine,then carry out a hydrolysis reaction.
There are two colourless aqueous solutions without labels. But students have been informed that those two solutions are sodium chloride and sodium bromide. You are advised that. Propose a method to identify these two solutions.
Sodium ion is common in two solutions. So we have to do the testing for chloride ions and bromide ion.
First add dilute nitric acid and then lead nitrate solution. You can notice that a white precipitate and light yellow precipitate form in two solutions. White precipitate is lead chloride. Now we can identify two solutions from comparing colours of two precipitates.
Both sodium bromide and sodium nitrite are white solid compounds at room temperature.
We can do concentrated sulfuric acid and manganeese dioxide to test chloride ion in the presence of bromide ion. Here how it does.
Manganeese dioxide - MnO2
Add concentrated sulfuric acid and heat the mixture. You can see a vapour of Br2 in red brown colour. Then again add concentrated sulfuric acid and MnO2. A yello green Cl2 gas emits.
When we add concentrated excess aqueous HCl to transition metal ions, complexes or coordination compounds are formed. Some 3d metal ions give coloured solutions with concentrated excess aqueous Cl- ions.
Chlorine is located under iodine and bromine in the electrochemistry series. Therefore reducing of chlorine(Cl2)
to chloride(Cl-) is
easier than Br2 to Br- and I2 to I-. Bromine is obtained by the oxidation of Br- with chlorine gas in saline water. Iodine is similarly produced by passing
chlorine gas through saline water containing I- ions.
Cl2 + 2I- → I2 + 2Cl-
Cl2 + 2Br- → Br2 + 2Cl-
If you want to release bromine gas, you have to add concentrated sulfuric acid. If you use concentrated sulfuric acid in the presence of copper, copper is oxidized while sulfuric acid is reduced to sulfur dioxide because concentrated sulfuric acid is an oxidizing acid.
All potassium halides ( KF, KCl, KBr, KI ) are soluble in water and give colourless solutions.
Only KI is soluble in HF. And KBr and KI are sparingly soluble in ethanol.
Iodide ion is easily oxidize to I2 than bromide to bromine and chloride to chlorine.