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Chemistry paper 2 > Chemical analysis > Flashcards

Chemical analysis Flashcards

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1
Q

What is a pure substance ?

A

A pure substance is something that only contains one compound or element throughout, not mixed with anything else

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2
Q

How to measure the purity of a substance

A

You can test the purity of a substance by its melting or boiling point and comparing it with the melting point or boiling point of a pure substance, which you can find from a databook. The closer your measured value is to the actual melting or boiling, the purer the sample is.

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3
Q

Boiling and melting point in pure substances

A

A chemically pure substance will boil and melt at a specific temperature.

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4
Q

Boiling and melting point in impure substances

A

Impure substances boil and melt at a range of substances. Impurities will lower the melting point and increase the boiling point

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5
Q

What is the formulation?

A

A formulation is a useful mixture that has been designed as a useful product
In a formulation, the quantity of each component is carefully measured so that the product hs the properties we need.

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6
Q

Examples of formulation

A

Fuels, cleaning products, paints, medicines, alloys, fertilisers and food.

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7
Q

why are formulations really important

A

Formulations are really important in the pharmaceutical industry. For example, by altering the formulation of a pill, chemists can make sure it delivers to the correct part of the body, has the right concentration, is consumable and has a long enough self life.

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8
Q

Is paper chromatography a physical process

A

It is a physical process, they do not involve chemical reactions and no new substances are made

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9
Q

What does paper chromatography do

A

It allows us to separate substances depending on their solubility

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10
Q

How does chromatography work

A

During a chromatography experiment, the substances in the sample constantly move between the mobile and the stationary phases, an equilibrium is formed between the two phases.
When the mobile phase moves up through the stationary phase, anything dissolved in the mobile phase moves up with it. Chemicals that spend more time in the mobile phase than the stationary phase (chemicals that are weakly attracted to the paper) will move further up the stationary phase. But chemicals that are strongly attracted to the stationary phase (spend more time in the stationary phase than the mobile phase) won’t move very far

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11
Q

Stationary phase

A

where the molecules cannot move, lie the paper

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12
Q

Mobile phase

A

where the molecules can move like a solvent

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13
Q

Pure chemicals and paper chromatography

A

a pure chemical will produce a single spot in all solvents

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14
Q

Chemicals in a mixture and paper chromatography

A

the chemicals in a mixture may separate into different spots depending on the solvent

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15
Q

Chromatography required practical

A

1) use a ruler to draw a horizontal line using a pencil, about 2 cm from the bottom of the chromatography paper
2) Mark five pencil spots at equal spaces across the line. Leave at least 1 cm clear at each side.
3) Put a small spot of the ink onto the pencil spots. Make sure the spots are small so that they do not spread into each other.
4) place water into a beaker to a depth of 1 cm
5) Place the paper into the water, the bottom of the paper should dip into the water. The pencil line with the ink should be above the surface of the water, otherwise, the water will wash the ink off the line. the ides of the paper should not touch the sides of the beaker as this might interfere with the way the water moves.
6) place a lid on the beaker to reduce evaporation of the solvent
7) At this stage the water will move up the paper and the colours will be carried up
8) Remove the paper when the water has travelled across three quarters up the paper
9) use a pencil the mark the point where the water has reached
10) hang the paper to dry

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16
Q

What is chromatography

A

chromatography is an analytical method used to separate the substances in a mixture, you can then use it to identify the substances

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17
Q

The amount of time the molecules spend on each phase depends on two things

A

how soluble they are in the solvent
How attracted they are to the paper
molecules with a higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase, and they will be carried further up the paper

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18
Q

Rf value formula

A

distance travelled by substance / distance travelled by solvent

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19
Q

How can you identify a substance using the rf value

A

We can identify the chemical using rf values. First, measure the distance from the baseline to the centre of each spot, then measure the distance travelled by the solvent from the baseline. Then using the formula calculate the rf value and using a database to tell you the identity of the chemical
HOWEVER
several chemicals may have this rf value, we may need to repeat the experiment using a different solvent to narrow it down further. If the chemical has never been analysed before, there will not be a rf value on the database, we would need to carry further analysis to identify it

20
Q

what does the rf value depend on?

A

the solvent

21
Q

Test for hydrogen gas

A

we remove the bung of a test tube containing hydrogen and insert a burning splint, you should hear a squeaky pop sound because hydrogen burns rapidly with the o2 in the air to form H20

22
Q

Test for oxygen gas

A

Place a glowing splint into a test tube of oxygen, the oxygen will relight the splint

23
Q

Test for carbon dioxide gas

A

Use an aqueous solution of calcium hydroxide like lime water - calcium hydroxide dissolved in water. Draw some of the gas using a plastic pipette, bubble the gas through the lime water if we repeat this a few times the lime water should turn cloudy

24
Q

Test for Chlorine

A

Insert damp litmus into the mouth of a test tube, chlorine bleaches litmus paper and turns it white, it may turn red for a moment at first because a solution of chloride is acidic

25
Q

Anions

A

negatively charged ions

26
Q

Tests for carbonate - carbon ions CO3^2- ions

A

Put a sample in a test tube, using a pipette add couple drops of dilute acid (HCL), the acid will react with carbonate and produce carbon dioxide, we will see effervescence. Now connect the test tube to a test tube of lime water, if carbonate ions are present tge co2 will turn lime water cloudy
Na2CO3 + 2HCL = CO2 + 2NaCl + H20

27
Q

Test for sulfates - sulfate ions SO4^2-

A

Add a couple of dilute HCL to remove any carbonate or sulfate ions, which are impurities and will react with Barium ions and give us invalid results. Then add a couple of barium chloride solution (BaCl2) to the test tube containing the sample solution. if sulfate ions are present, a white precipitate of barium sulfate will form
Ba^2+ + SO4^2- = BaSO4

28
Q

Test for Halides ions - Chloride ions (CL^-) Bromide ions (Br^-) and Iodide ions (I^-)

A

Add dilute nitric acid to get rid of carbonate and sulfite impurities. Then add a couple of drops of silver nitrate solution. The silver ions react with the halide ions to form a precipitate.
A chloride gives a white precipitate of silver chloride
Ag+ + CI- = AgCl
A bromide gives a cream precipitate of silver bromide
Ag+ + Br- = AgBr
An iodide gives a yellow precipitate of silver iodide
Ag+ + I- = Agl

29
Q

How to prepare a flame test?

A

First nlean a platinum wire loop by dipping it in some dilite HCL and then holding it in a blue flame from a bunsen burner until it burns without any colour of the flame. Then dip the loop into the sample you want to test and put it back in the flame. Record the colour of the flame. You can use these colours to detect and identify different ions. However it only works for samples that contain a single metal ion. If the sample tested contains a mixture of ions, the flame colours of some ions may be hidden by the colours of others.

30
Q

Flame test for lithium ions Li+

A

Crimson flame

31
Q

Flame test for sodium ions Na+

A

Burn with yellow flame

32
Q

Flame test for potassium ions K+

A

Burn with a lilac flame

33
Q

Flame test for calcium ions Ca2+

A

Burn with an oragne-red flame

34
Q

Flame test for copper ions Cu2+

A

Burn with a green flame

35
Q

What are cations

A

Positively charged ions

36
Q

Calcium Ca^2+ and sodium hydroxide

A

If we add sodium hydroxide to a solution of calcium ions, it will produce a white precipitate, we will need to use flame test to identify calcium
Example
Calcium nitrate +sodium hydroxide =sodium nitrate + calcium hydroxide
Ca(NO3)2 + 2NaOH = 2NaNO3 + Ca(OH)2

37
Q

Magnesium Mg^2+ and sodium hydroxide

A

If we add sodium hydroxide to a solution of magnesium ions, it will produce a white precipitate, we will need to use flame test to identify calcium
Example
magnesium nitrate + sodium hydroxide = sodium nitrate + magnesium hydroxide
Mg(NO3)2 +2NaOH = 2NaNO3 + Mg(OH)2

38
Q

Aluminium Al^3+ and sodium hydroxide

A

If we add sodium hydroxide to a solution of aluminium ions, it will produce a white precipitate, we will need to add excess sodium hydroxide, so that the aluminium precipitate re-dissolve to form a colourless solution
For example
aluminium nitrate + sodium hydroxide = sodium nitrate + aluminium hydroxide
Al(NO3)2 +3NaOH = 3NaNO3 + Al(OH)3

39
Q

Copper (II) Cu^2+ and sodium hydroxide

A

Copper II ions react with sodium hydroxide to form a blue precipitate of copper II hydroxide
Copper (II) Nitrate + sodium hydroxide = sodium nitrate + copper (II) hydroxide
Cu(NO3)2+2NaOH =2NaNO3 + Cu(OH)2

40
Q

Iron (II) Fe^3+ and sodium hydroxide

A 
Iron II ions react with sodium hydroxide to form a blue precipitate of iron II hydroxide to form a green precipitate of iron(II) hydroxide 
Iron nitrate(II) + sodium hydroxide = sodium nitrate + iron (II) hydroxide 
Fe(NO3)2 + 2NaOH = 2NaNO3 + Fe(OH)2
41
Q

Iron (III) Fe^3+ and sodium hydroxide

A 
Iron III ions react with sodium hydroxide to form a brown precipitate of iron III hydroxide to form a brown precipitate of iron(III) hydroxide 
Iron nitrate (III) + sodium hydroxide = sodium nitrate + iron (III) hydroxide 
Fe(NO3)3 + 3NaOH = 3NaNO3 + Fe(OH)3
42
Q

Problems with manual flame tests

A

Colour of flame test can be difficult to distinguish. Especially if there is a low compound of the metal compound. sometimes a sample can contain a mixture of metal ions which can mask the colour of the flame
Instead of flame tests scientist use another technique called flame emission spectroscopy.

43
Q

flame emission spectroscopy.

A

A sample of the metal ion in solution is placed into a flame.
The light given out is then passed through a machine called a spectroscope.
The spectroscope converts the light into a line spectrum. The positions of the lines in the spectrum are specific to a given metal ion. Because the combination of wavelengths emmited by an ion depends k its charge and electron arrangement, since no two ions have the same charge or electron arrangement, different ions emit different wavelength of light.
The flame emission spectroscopy can also tell the concentration of the metal ion because the lines become more intense at higher concentration

44
Q

flame emission spectroscopy is an example of what method

A

instrumental method

45
Q

Instrumental method advantages

A

they are more rapid
sensitive so flame emission spectroscopy can work out even a tiny sample of the metal compound
Accurate

Chemistry paper 2 (1 decks)

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