Seneca C8 (Chromatography onwards) Flashcards
What is Chromatography
Chromatography is a process that separates a mixture into its different components.
What are the two phases in chromatography
. Mobile phase
. Stationary phase
Mobile phase
Substances are picked up and carried by a mobile phase (liquid or gas).
Stationary phase
The mobile phase then moves through a stationary phase (solid or viscous liquid).
Separation of substances
A substance moves far if it’s more attracted to the mobile phase.
A substance doesn’t move far if it’s more attracted to the stationary phase.
Strength of attraction
Different components can sometimes be equally attracted to a solvent.
So, the number of spots a mixture produces can vary depending on the solvent used.
In paper chromatography, a solvent represents which phase?
Mobile phase
What are the steps of paper chromatography
. Dip the chromatography paper
. Solvent movement
. Distance travelled
Dip the chromatography paper
Dip the bottom of the chromatography paper into the solvent
Solvent movement
The solvent travels up the chromatography paper.
As the solvent moves, it picks up and transports the substances being tested up the chromatography paper.
Distance travelled
The components that travel furthest are highly soluble (dissolve easily) in the solvent. They are also minimally attracted to the chromatography paper
Chromatogram
Chromatography produces chromatograms. We use chromatograms to identify compounds in mixtures by calculating Rf values
Rf values
Rf = distance travelled by substance / distance travelled by solvent.
We can work this out for both reference substances and tested substances
Solvents
Rf values depend on the solvent. We can learn more about the identity of the components by testing any references and the unknown mixtures in a range of solvents
Reference substances
A reference substance is a pure sample that’s run next to the tested substances to see if it’s a component in the mixture. These substances provide valuable evidence, but not proof
Stages of chromatography
. The mobile phase moves through the stationary phase
. The substance is picked up by the mobile phase
The phases in paper chromatography
Mobile phase - The solvent
Stationary phase - Chromatography paper
Which state of matter can a mobile phase in chromatography not be?
Solid
In paper chromatography, the componenets that travel the furthest are
. minimally attracted to the chromatography paper
. highly soluble in the solvent
What is the process of paper chromatography in detail
- Use a pencil to draw a horizontal line near the bottom of the chromatography paper.
- Place samples of known food colorings (A-D) next to an unknown substance (X) on this ‘start line’.
- Place the paper in a beaker containing a small volume of solvent.
- Let the solvent move to a few cm below the end of the paper.
- By comparing spots produced by X with those produced by A-D, you will be able to identify the unknown substance.
Testing for hydrogen
. Place a lit splint close to the mouth of a test tube that contains a gas.
. If the gas is hydrogen, a “squeaky pop” sound will be produced.
. This noise is generated because hydrogen burns rapidly in the presence of oxygen to give H2O.
Testing for oxygen
Insert a glowing splint into a test tube that contains a gas.
If the gas is oxygen, the splint will relight.
Testing for Carbon Dioxide
Take an aqueous solution of limewater (calcium hydroxide) and bubble through the gas.
. Shake CO2 with limewater
If the gas is carbon dioxide, the limewater solution will turn from clear to cloudy.
Testing for Chlorine
Insert damp litmus paper into a test tube that contains a gas.
If the gas is chlorine, the litmus paper will bleach and change color from red to white.
What color will damp litmus paper turn if it comes into contact with chlorine?
White
Flame tests
Some metal ions can be identified using flame tests
The following positive metal ions all produce a characteristic flame color when burned:
. Copper - Green flame
. Lithium - Crimson flame
. Calcium - Orange - Red flame
. Potassium - Lilac flame
. Sodium - Yellow flame
What are the steps of a flame test
- Submerge a nichrome wire loop in dilute hydrochloric acid to make sure that it is clean.
- Place the nichrome wire loop into the sample to be tested.
- Hold the nichrome wire loop in a Bunsen burner’s blue flame.
- Observe any change in color of the Bunsen flame.
Metal ions in metal compounds
Metal ions can be found in aqueous solutions of metal compounds. These ions can precipitate out of solution when they react with a sodium hydroxide solution (NaOH). This produces insoluble solids (don’t dissolve in water) called precipitates.
Calcium hydroxide
Calcium ions (Ca2+) form the white precipitate calcium hydroxide.
Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)
Aluminium hydroxide
Aluminium ions (Al3+) form the white precipitate aluminium hydroxide.
Al3+(aq) + 3OH-(aq) → Al(OH)3(s)
Unlike the other two white precipitates, aluminium hydroxide can be dissolved in excess NaOH. This produces a colourless solution.
Magnesium hydroxide
Magnesium ions (Mg2+) form the white precipitate magnesium hydroxide.
Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
Here are some more examples of the formation of precipitates:
Copper ions (Cu2+) form the blue precipitate copper(II) hydroxide.
Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)
Iron(III) ions (Fe3+) form the brown precipitate iron(III) hydroxide.
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
Iron(III) ions (Fe3+) form the brown precipitate iron(III) hydroxide.
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
Testing for carbonates
CO32- ions are characteristic of all carbonates.
A reaction between a carbonate and a dilute acid gives a salt, carbon dioxide and water.
E.g. Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water
As carbon dioxide is produced, we can use the standard carbon dioxide test to work out if an unknown substance is a carbonate.
To recap, this test involves bubbling the gas through limewater. If the solution turns cloudy, the gas is carbon dioxide.
What are halides
Halide ions are negatively charged (they are anions)
What are the two steps to halide testing
. Add dilute nitric acid
. Add silver nitrate
Adding dilute nitric acid
Add dilute nitric acid to an unknown solution.
This removes carbonate ions. These could disrupt test results by forming a precipitate with the silver ions added in step 2.
Adding silver nitrate
dd silver nitrate to the same solution.
If halide ions are present, they will form a precipitate with the silver ions:
Ag+(aq) + Cl-(aq) → AgCl(s) (silver chloride is white).
Ag+(aq) + Br-(aq) → AgBr(s) (silver bromide is cream).
Ag+(aq) + I-(aq) → AgI(s) (silver iodide is pale yellow).
What are sulfates
Sulfates are compounds that contain SO42- ions
What are the two steps to sulfate testing
. Add dilute hydrochloric acid
. Add barium chloride
Adding dilute hydrochloric acid
Add dilute hydrochloric acid to an unknown solution.
This removes carbonate ions. These could disrupt test results by forming a precipitate with the barium ions added in step 2.
Adding barium chloride
Add barium chloride to the same solution.
If sulfates are present, the white precipitate barium sulfate will form:
Ba2+(aq) + SO42-(aq) → BaSO4(s)
What are the advantages of instrumental methods of chemical analysis
. Greater sensitivity and accuracy
. Quicker at producing results
. Able to analyze tiny samples
What are the disadvantages of instrumental methods
. The instruments are often expensive.
. You need to go through special training to operate the instruments.
. The results are often only useful when compared to data from known substances.
What is flame emission spectroscophy
Flame emission spectroscopy is an instrumental method used to work out the identity and concentration of the metal ions present in a solution
What is the process of flame emission spectroscophy
. Place a sample of the metal solution being tested into a flame. Light will be given off
. The light that is given off is captured by a spectroscope
. This instrument generates a line spectrum by distinguishing between light with different wavelengths
Metal solution
A sample of the metal solution being tested is placed into a flame resulting in the emission of light
Emitted light
The light that is given off is captured by a spectroscope
Spectroscope
This instrument generates a line spectrum by distinguishing between light with different wavelengths
Flame Emission Spectroscopy Analysis
. Identity of metal ions
. Concentration of metal ions
. Reference Spectra
Identity of metal ions
Each metal ion will generate a new line spectrum. This means that we can identify all ions present in a solution.
Concentration of metal ions
The concentration of an ion is indicated by the intensity of line spectra.
Reference spectra
Unlike flame tests, flame emission spectroscopy allows us to analyse mixtures by comparing them with reference spectra.
