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Chemistry Paper 2 > Chemical Analysis > Flashcards

Chemical Analysis Flashcards

1
Q

What is a pure substance?

A

A pure substance is a single element or compound, not mixed with any other substance

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2
Q

How are melting and boiling points used to distinguish between a pure and impure substance?

A

If a sample is impure, the melting and boiling point range will be greater than if the substance was pure.

This is because a pure substance will have a single, fixed melting and boiling point rather than a range of temperatures.

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3
Q

What is the test and result for the presence of water?

A

Test:
-add anhydrous copper sulfate
-add anhydrous cobalt chloride

Result:
-anhydrous copper sulfate turns white to blue
-anhydrous cobalt chloride turns blue to pink

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4
Q

What is the test for pure water?

A

The test for pure water is that it melts at exactly 0ºC and boils at exactly 100ºC.

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5
Q

What is a formulation?

A

A formulation is a mixture that has been designed as a useful product.

They are made by mixing the components in carefully measured quantities to ensure the product has the required properties.

Formulations include: paints, cleaning products and medicines

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6
Q

What is paper chromatography?

A

-Paper chromatography separates the components of a mixture (often inks or dyes) by their attraction to the mobile and stationary phase (how well something dissolves in a solvent).

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7
Q

What is the mobile and stationary phase?

A

-The solvent is the mobile phase because it moves up the paper.

-The paper is the stationary phase because the paper does not move.

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8
Q

What does a chromatogram show?

A

-A pure substance shows one spot

-A mixture will show more than one spot (in a vertical line).

-The more soluble (attracted) the components is in the mobile phase, the further up the stationary phase it travels.

-The more attracted to the stationary phase (paper), the less far it travels.

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9
Q

Give a method for paper chromatography.

A

-First we draw a horizontal line on the chromatography paper in pencil. The line should be around 2cm from the bottom of the paper.

-Along the baseline, add small spots of different coloured inks not too close together using a capillary tube.

-Pour the chosen solvent into a beaker to a depth of 1cm.

-Attach the chromatography paper to a glass rod. The paper is then lowered into the beaker, ensuring the solvent does not move above the baseline.

-Stop the experiment when the water is around three quarters up the paper. At this point, draw a pencil line where the solvent reached and leave the paper to dry.

-Gather the results from the chromatogram by calculating the Rf value.

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10
Q

Why should the spots in paper chromatography be small and not too close together?

A

The spots should be small and not too close together to prevent the colours from merging together when they run up the paper.

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11
Q

Why should the paper in chromatography not touch the side of the beaker?

A

-Ensure the paper does not touch the side of the beaker as it may interfere with the way the solvent moves up the paper.

-For example, the solvent at the edges may move up faster than the solvent at the centre.

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12
Q

Why should the beaker not be moved during paper chromatography?

A

-Do not move the beaker during the experiment as the solvent will wash the spots off the paper before they have a chance to move.

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13
Q

Common Mistakes Made During Paper Chromatography

A
  1. Drawing the baseline in pen rather than pencil. This is a problem as the baseline might dissolve in the solvent which could cause the baseline to move.
  2. Placing the solvent above the baseline. This is a problem as the solute will dissolve in the solvent rather than travelling up the paper.
  3. Using a solvent that the mixture is not soluble in. This is a problem as the sample will not move off the baseline.
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14
Q

What is the Rf value?

A

The Rf value is the ratio of a distance moved by a compound to the distance moved by the solvent.

The value will always be between 0 and 1.

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15
Q

How to calculate the Rf value?

A

Rf value= distance travelled by spot/ distance travelled by solvent

-Measure the distance from the middle of the spot

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16
Q

Why can Rf value not always be used to identify chemicals?

A

-Sometimes, a mix may contain chemical with the same (or very similar) Rf values. These two chemicals will run up the paper at very similar speeds.

-To solve this problem, we would need to repeat the experiment using a different solvent.

-The two chemicals may not run at the same speed in the new solvent, allowing us to identify them.

-The mix may contain a chemical which has never been analysed before. This means the chemical’s Rf value will not be on any database so we cannot look it up.

-In this case we would need to use another method to identify the chemical.

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17
Q

What is the test and positive result for Hydrogen?

A

-Insert a lit splint into a test tube containing a gas.

-If positive, the splint will go out and a squeaky pop will be heard.

18
Q

What is the test and positive result for Oxygen?

A

-Insert a glowing splint into a test tube containing a gas.

-If positive, the splint will relight.

19
Q

What is the test and positive result for Carbon Dioxide?

A

-Bubble the gas through limewater.

-If positive, the limewater will turn from colourless to cloudy.

20
Q

What is the test and positive result for chlorine?

A

-Allow the gas to pass over damp, blue litmus paper.

-If positive, the paper will bleach white.

21
Q

How to carry out a flame test.

A

-Dip a clean wire loop into a solid sample of the compound being tested.

-Put the loop into a blue Bunsen burner flame.

-Observe and record the flame colour produced. The colour can be used to identify the metal ion present.

22
Q

What are the five flame test colours for metal ions?

A

-Lithium produces a crimson flame.

-Sodium produces a yellow flame.

-Potassium produces a lilac flame.

-Calcium produces an orange-red flame.

-Copper produces a green flame.

23
Q

Describe the positives and negatives of flame tests.

A

-Flame tests are a rapid and easy way to identify metal ions.

-However, they have a low sensitivity.

-This means it can be difficult to determine the colour of the flame by eye, especially when the metal ion is present in a very low concentration.

-Secondly, they cannot be used to determine the metal ions present in a mixture.

24
Q

What is instrumental analysis?

A

(Modern) instrumental analysis is using a machine instead of a chemical test as a method of analysing a product.

25
Q

Give the advantages of modern instrumental analysis.

A

-Rapid

-Accurate

-Sensitive (can detect small amounts/ quantities)

-Quantitative (some methods)

26
Q

Give the disadvantages of modern instrumental analysis.

A

-Expensive

-May need specialist training to do

-May need to be compared to a database of known results (although this is also true for Rf values in paper chromatography)

27
Q

Describe flame emission spectroscopy.

A

-It is an example of an instrumental method.

-The sample is heated.

-Light is emitted and appears as a spectrum

-It can be compared to a database of known elements

-It can also indicate concentration (how faint or intense a line is).

28
Q

What is a precipitate?

A

A precipitate is an insoluble solid formed in a reaction between solutions.

29
Q

What colour changes happen when sodium hydroxide solution is added to certain metal ions?

A

-Copper (II) forms a blue precipitate

-Iron (II) forms a green precipitate

-Iron (III) forms a brown precipitate

-Aluminium forms a white precipitate which in excess sodium hydroxide solution dissolves

-Calcium forms a white precipitate

-Magnesium forms a white precipitate

30
Q

What are the ionic equations for the six precipitate tests?

A

Copper (II)- Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)

Iron (II)- Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)

Iron (III)- Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

Aluminium- Al3+(aq) + 3OH- → Al(OH)3(s)

Calcium- Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)

Magnesium- Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

31
Q

How to carry out a test for carbonate ions?

A

-Add any dilute acid to the sample (often nitric as it doesn’t interfere with other tests).

-If you see effervescence, collect the gas.

-Bubble the gas through limewater.

-If the limewater turns cloudy, then the gas is carbon dioxide.

32
Q

Why is effervescence not enough to prove the presence of carbonate ions?

A

-The carbonate ion is not the only chemical which produces effervescence when reacted with an acid.

-For example, when a metal is reacted with an acid, effervescence is also seen (except hydrogen is produced).

-Therefore, in order to prove that we have carbonate ions, we have to prove that any gas produced is carbon dioxide.

33
Q

What is the ionic equation for carbonate ions?

A

H+ + CO3 2+ → CO2 +H20

34
Q

How to carry out a test for sulfate ions?

A

-Add dilute hydrochloric acid to the sample (the acid removes carbonate ions).

-Add barium chloride solution.

-If sulfate ions are present, a white precipitate will be formed.

35
Q

Why can sulfuric acid not be used in the test for sulfate ions?

A

-We cannot use sulfuric acid as it already contains sulfate ions.

-This means if we used it, we would always get a positive result even if sulffate ions weren’t present in the sample.

36
Q

What is the ionic equation for sulfate ions?

A

Ba2+(aq) + SO42-(aq) → BaSO4(s)

37
Q

How to carry out a test for halide ions?

A

-Add dilute nitric acid to the sample (the acid removes carbonate ions).

-Add silver nitrate solution.

38
Q

Why can hydrochloric acid not be used in the halide test?

A

-Hydrochloric acid contains chloride ions.

-This means if it is used in the test, the chloride ions present would react with the silver nitrate solution to form a white precipitate.

-Therefore, we would always get a positive result for the chloride ion.

39
Q

What colour precipitates are formed from the halide test?

A

-Chloride ions form a white precipitate

-Bromide ions form a cream precipitate

-Iodide ions form a pale yellow precipitate

40
Q

What is the general ionic equation for halide ions?

A

Ag+ + X- → AgX

Chemistry Paper 2 (5 decks)

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