Chapter 12 Flashcards
Give three advantages and three disadvantages of instrumental methods? (Chemistry only)
Advantages
- quick
- accurate
- only need a small sample
Disadvantage
- expensive
- need specialist training
- results can often only be interpreted by comparison to a reference
What is paper chromatography?
Separation of two or more compounds in a mixture.
How do you calculate the Rf value?
Rf = distance moved by the compound ÷ distance moved by the solvent
What is the mobile phase?
The solvent.
What is the stationary phase?
The paper or TLC plate.
What is a precipitate?
An insoluble solid formed by a reaction taking place in solution.
What test the test for a double bond?
Bromine water goes from orange/brown to colourless if a double bond is present
What is the test for a carbonate?
Add an acid, collect the gas and test it using limewater - limewater will turn cloudy if CO2 is present
What is a pure substance?
In chemistry, a pure element means a substance whose atoms all have the same atomic number or number of protons.
How would you determine if something is pure?
To determine if a substance is pure you can check the substance’s melting or boiling points. A pure solid has a constant/fixed melting point. ie. it will melt completely at one temperature only.
You could also use chromatography. A pure solid has a constant Rf value.
What are the flame colours for these +ve elements? (Chemistry only)
Barium
Calcium
Copper
Lithium
Sodium
Potassium
Barium - Pale green
Calcium - Yellow-red
Copper - Green-blue
Lithium - Red
Sodium - Orange
Potassium - Lilac
What conclusions can you draw from this chromatogram?
The purple and brown inks contain two of the same components
The brown ink contains three separate components
The purple ink contains two components
The green and yellow inks only contain one component
Brown ink shares a component with the yellow ink
How can you use the Rf value?
Compare the Rf value to a reference library to identify the component.
Which dot has a greater affinity for the mobile phase?
The dot that has travelled the furthest
What is the test for +ve transitional metal ions? (Chemistry only)
If sodium hydroxide solution is added, a transition metal hydroxide is formed. Transition metal hydroxides are insoluble so they form solid precipitates.
Different transition metals form different coloured precipitates:
Metal ion Colour
Iron(II), Fe2+ Green - turns orange-brown when left standing
Iron(III), Fe3+ Orange-brown
Copper(II), Cu2+ Blue
What is the test for sulphate ions? (Chemistry only)
You can test to see if a solution contains sulfate ions by using barium chloride. If barium chloride solution is added to a sample of water containing sulfate ions, barium sulfate is formed. Barium sulfate is insoluble in water and will be seen as a white precipitate.
What is the test for -ve Halide ions? (Chemistry only)
You can test to see if a solution contains chloride, bromide or iodide ions by using silver nitrate.
The results look like this:
Silver chloride (AgCl) is a white precipitate
Silver bromide (AgBr) is a cream precipitate
Silver iodide (AgI) is a pale yellow precipitate
What do insoluble salts form?
A precipitate
Higher Chemistry only -
Soluble Insoluble
All nitrates None
Most sulfates Lead sulfate, barium sulfate and calcium sulfate
Most chlorides, bromides and iodides Silver chloride, silver bromide, silver iodide, lead
NaC03, KCO3 Most other carbonates
NaOH, KOH Most other hydroxides
What is flame emission spectroscopy? (Chemistry only)
How it works
- The sample is heated in a flame
- The thermal energy provided excites electrons in the metal ions. This makes them jump into higher energy levels or electron shells.
- When they fall back into the lower shells, they release light energy
- The wavelengths of light emitted are analysed instrumentally to identify the metal ions
- This is similar to the flame tests to identify positive metal ions in compounds with a bunsen burner.
It is more accurate!
What is the test for chlorine?
Bleaches damp litmus paper
What is the test for hydrogen?
Squeaky pop test
What is the test for oxygen?
Relights a smouldering splint
What is the test for carbon dioxide?
- Turns limewater cloudy
- Puts out a light splint
