C12 Flashcards
How do you calculate Rf values?
distance moved by chemical (the ink dot) / distance moved by solvent (water)
What can Rf values be used for?
Each Rf (Retention factor) value is unique, so we can use it to identify a specific substance from a database
Explain why different substances move different distances in chromatography
The different substances are attracted differently to the molecules in the paper and solvent, so they travel at different speeds/distances up the paper
The more soluble a substance is in the solvent during chromatography, the less/more rapidly/slowly it will move along the paper
The more soluble a substance is in the solvent during chromatography, the more rapidly it will move along the paper
Why is the lid closed during chromatography?
To reduce evaporation of the solvent
Flame tests for positive ions (results table)
lithium. Li. crimson
sodium. NA+. yellow
potassium. K+. lilac
calcium. Ca2+. orange/red
copper. cu2+ green
Explain why metal ions emit light when in a flame
The electrons become excited by the heat energy and jump to a higher energy level. When the electrons relax back down to a lower state, the excess energy gained is released in the form of emitted light.
How to carry out a flame test?
- The wire should be dipped in hydrochloric acid and then heated to clean it
- Dip into the metal compound that is being tested
- Hold the loop in the roaring blue flame of a bunsen burner
How can you test for a carbonate?
If you add a dilute acid to a carbonate, it fizzes and produces carbon dioxide gas.
How can you test for halide ions?
First, add dilute nitric acid and then silver nitrate solution.
If a precipitate forms, there are halide ions present.
Test for halide ions (results table for precipitates)
chloride. bromide. iodine
white. cream. yellow
How can you test an unknown compound for sulfate ions?
By adding dilute hydrochloric acid, followed by barium chloride solution.
A white precipitate tells you sulfate ions are present.
Advantages of instrumental analysis
- More accurate
- Can work with smaller samples
- Quicker
Disadvantages of instrumental analysis
- Expensive
- Equipment requires training to use
- Requires a database to compare
Why do you add 2 drops of nitric acid when testing for halides?
To remove any carbonate ions which will affect your result by reacting with the silver nitrate solution
Explain why a flame test cannot be used to identify a mixture of metals
Flame tests can only identify a single ion
Explain why flame emission spectroscopy is better than a flame test
- Faster
- Works with very small sample sizes
- More accurate
- Can measure multiple metal ions
How can you identify a metal cation?
Reacting unknown compounds with sodium hydroxide solution can help you identify some positive ions.
What positive ions form white precipitates with sodium hydroxide?
Aluminium, calcium and magnesium
Explain what the test for ammonia tells you about the properties of ammonia.
Ammonia dissolves in water to form an alkaline solution, because it turns damp red litmus paper blue.
Testing for positive ions (results)
copper blue
iron. green
iron3+. brown
Name a metal ion that would produce a green flame
Copper (Cu2+)
What color flame do sodium ions produce?
Yellow
What ion produces a lilac color flame?
Potassium (K+)
What ion produces a cream precipitate when added to silver nitrate solution and dilute nitric acid?
Bromide (Br-)
What does the Rf value of a dye depends on?
- Solubility of the dye in the solvent
- Attraction of the dye to the paper
What will produce a smaller Rf value if the solvent and paper are both changed?
If the dye is less soluble in the new solvent and it is more attracted to the new paper
What two ways can increase the distance between ink spots in chromotography?
- Use a different solvent
- Allow the solvent front to travel further
What is the compound needed to identify an aluminium ion?
Sodium hydroxide
If a brown precipitate forms when sodium hydroxide is added to a mystery solution, which metal ion was present?
Iron (III) Fe3+
What is the correct test for sulfate ions?
Add barium chloride
White precipitate forms
What happens to aluminium in excess sodium hydroxide?
The aluminium hydroxide precipitate dissolves to form a colorless solution
What happens to aluminium in excess sodium hydroxide?
The aluminium hydroxide precipitate dissolves to form a colorless solution
What gas is produced when a carbonate reacts with an acid?
Carbon dioxide
Children often do not like taking medicine. Suggest a substance that could be added to Aquamed to increase the desire for children to take it. Give a reason for your suggestion.
Sugar/flavouring so it tastes better
What do impurities do to a substance?
- Lower the shelf life of substances
- Increase the boiling point
What are the 2 phases in chromatography?
Mobile phase and stationary phase
The less/more attracted to the chromatography paper the ink is the further/lesser it will move
The less attracted to the chromatography paper the ink is the further it will travel
How many chromatography spots are produced by pure samples?
1
What does chromatography produce?
A chromatogram
What is the apparatus required for testing chlorine?
- Test tube
- Litmus paper
In flame tests, what piece of apparatus is used to transfer the sample to the Bunsen flame?
Nichrome (nickel and chromium) wire loop
What charge do anions carry?
Negative
What is the instrument used in flame emission spectroscopy?
A spectroscope
What is generated by a spectroscope in a flame emission spectroscopy?
Line spectrum
What does flame emission spectroscopy identify?
Metal ions
What must be added to the solution before silver nitrate when testing for halides?
Dilute nitric acid
Which chemical forms a white precipitate when you test for sulfate ions in solution?
Barium Sulfate
What state must a splint be in when testing for hydrogen?
Lit
What feature of a line spectrum provides information about the concentration of a metal ion?
Intensity
What safety precautions should you take for flame tests?
Safety googles and hair tied back
What is a pure substance to a chemist?
A pure substance is one that is made up of just one substance (one element or compound).
Examples of pure substances
- gold
- copper wire
- sugar
- salt
- hydrogen gas
- diamond
What are fixed points?
The melting and boiling point of an element or a compound
What is a mixture?
A mixture consists of 2 or more different substances that are not chemically joined together
How does the boiling and melting points of pure/impure substances differ?
- Pure substances have a sharp melting/boiling point ; the temperature stays the same when a pure substance changes state
- Impure substances like mixtures have melting and boiling points that vary ; the temperature changes slightly as an impure substance changes state
What is a formulation?
A formulation is a mixture that has been designed to produce a useful product
Examples of formulations
fuels, cleaning products, paints and medicines
Why do we draw our starting line in pencil during paper chromatography?
If we drew the line in pen, the pen ink would move up the paper, with the solvent.
Required Practical - Chromatography (6 marks)
- Use a ruler to draw a horizontal line (2cm from the bottom) on the chromatograhpy paper
- Mark (number of samples) spots on the pencil line of equal distance/size
- Use a capillary tube to put a small spot of each sample on the pencil spots
- Pour water to a beaker of depth 1cm
- Attach the paper to a glass rod using tape, and lower the paper into the beaker
- The water should be below the pencil line/spots and the sides of the paper should not touch the sides of the beaker
