8.3 Identification of ions by chemical and spectroscopic means (Triple)

Question 1

Describe the test for carbonate anions

Answer 1

Add dilute acid, e.g. HCl

Fizzing observed, as CO2 is released.

E.g. Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2

Question 2

Describe the test for sulfate
anions

Answer 2

Add a solution containing Ba2+ cations, e.g. a solution of BaCl2

White precipitate of BaSO4 forms

E.g. K2SO4+ BaCl2 → 2 KCl + BaSO4

Question 3

Describe the test for halide
anions

Answer 3

Add a solution of AgNO3 (acidified with HNO3)

Chlorides - white precipitate, silver chloride; Ag+ + Cl−
→ AgCl

Bromides - cream precipitate, silver bromide; Ag+ + Br−→ AgBr

Iodides - yellow precipitate, silver iodide; Ag+ + I−
→ AgI

Question 4

What are instrumental
methods?

Answer 4

They are accurate, sensitive and rapid methods which are useful when the amount of sample is very small

Question 5

Describe the flame emission
spectroscopy

Answer 5

1. Instrumental method used for identifying metal ions in solution or measuring
   their concentration
2. Spectroscope measures the exact wavelength of the light emitted by a metal ion
3. That allows for definite identification - sometimes colours are difficult to
   distinguish.
4. Concentrations are found by measuring the intensity of light emitted. The
   more intense light, the greater the concentration of the metal ion in a solution.
5. From the intensity vs concentration graph, you can read off a relevant
   concentration value at a given intensity.

Question 6

What are cations and anions?

Answer 6

Cations are the positive ions; anions are the negative ions.