Topic 8: Chemical Analysis Flashcards
What is a pure substance?
A pure substance is a single element or compound, not mixed with any other substance
What is a formulation and how is it made? Give examples
- A formulation is a mixture of compounds in measured quantities that has been designed as a useful product.
- Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
- E.g. alloys are mixtures of metals; they are harder than pure metals, so have a particular purpose.
Describe paper chromatography
a) A start line is drawn near the bottom of the paper. The mixture is spotted on the line.
b) A beaker is filled with small amount of solvent (it cannot touch or go above the start line when paper is placed in a beaker)
c) Paper is hung on a rod and placed in a beaker.
d) Solvent travels up the paper, thus separating the components.
e) Before solvent level reaches the end, the paper is taken out and the finish line is marked. The paper is dried.
f) The procedure works when different compounds have different affinities for the solvent/paper. Stronger attraction for the paper - travels slowly with the solvent etc.
g) Paper is called the stationary phase - it doesn’t move. Solvent is the mobile phase.
How is Rf calculated?
Distance moved by the spot / distance moved by solvent
In a paper chromatography experiment, a compound A was found to have an Rf value of 0.85 - what does it tell you about the compound?
It has a higher affinity for the solvent than for the paper.
Describe the tests for hydrogen, oxygen, carbon dioxide and chlorine
Hydrogen – pop with burning splint over gas
Oxygen – glowing splint relights
Carbon dioxide – turns limewater (Ca(OH)2) milky
Chlorine – bleaches damp litmus paper and makes it white
Describe the lithium flame test result
Lithium compounds– crimson red flame
Describe the sodium hydroxide test results and write the equations
Copper(II) forms a blue precipitate:
Cu2+ + 2OH− (aq) → Cu(OH)
Iron(II) forms a dirty green precipitate:
Fe2+ + 2OH− (aq) → Fe(OH)
iron(III) forms a brown precipitate:
Fe3+ + 3OH− (aq) → Fe(OH)3(s)
Al3+, Ca2+, Mg2+ form white precipitates but only the Al(OH)3 dissolves in excess NaOH to form a colourless solution.
Ca2 + 2OH− (aq) → Ca(OH)2(s)
Mg2+ + 2OH− (aq) → Mg(OH) (s)
Al3+ + 3OH− (aq) → Al(OH)3(s)
Describe the test for carbonate anions
- Add dilute acid, e.g. HCl
- Fizzing observed, as CO2 is released.
- E.g. Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2
Describe the test for sulfate anions
Add a solution containing Ba2+ cations, e.g. a solution of BaCl2
White precipitate of BaSO4 forms
E.g. K2SO4 + BaCl2 → 2 KCl + BaSO4
(!!!) can also be thought of a test for barium (II); add sulfates - white precipitate forms.
Describe the test for halide anions
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
What are instrumental methods?
They are accurate, sensitive and rapid methods which are useful when the amount of sample is very small
Describe the flame emission spectroscopy
a) instrumental method used for identifying metal ions in solution or measuring their concentration
b) Spectroscope measures the exact wavelength of the light emitted by a metal ion
c) That allows for definite identification - sometimes colours are difficult to distinguish.
d) 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.
e) From the intensity vs concentration graph, you can read off a relevant concentration value at a given intensity.
What are cations and anions?
Cations are the positive ions; anions are the negative ions.
Describe the sodium flame test results
Sodium compounds - yellow flame
