C4.2 - Identifying The Products Of Chemical Reactions

Question 1

How can you detect carbon dioxide and what is the change caused by?

Answer 1

• You can test for carbon dioxide using limewater
  (calcium hydroxide solution).
• Limewater turns cloudy white when carbon
  dioxide is bubbled through it.
• The change is caused by calcium hydroxide
  reacting with carbon dioxide to form water and
  a white precipitate of calcium carbonate.

Question 2

How can you detect chlorine gas?

Answer 2

• Chlorine dissolves in water to form an acidic
  solution.
• Chlorine bleaches dyes.
• Use a drop of tap water to dampen a piece of blue
  litmus paper.
• Hold the paper near to a container that holds the
  substance.
• If chlorine is present, the paper turns red then
  white.

Question 3

How do you smell substances in the laboratory?

Answer 3

• With the container well away from your nose,
  breathe in enough air to almost fill your lungs.
• Hold the container a few centimetres away from
  your nose, and waft any smell towards you.
• Take a cautious sniff.

Question 4

How can you detect hydrogen and oxygen?

Answer 4

Hydrogen - Place a lighted splint near the mouth of the container of gas - If hydrogen is present, a squeaky pop noise will occur.

Oxygen - Place a glowing splint near the mouth of the container of gas - If oxygen is present, the splint should relight.

Question 5

What is the basis of a flame test?

Answer 5

• Metal ions are heated, energy is transferred to
  their electrons.
• This makes their electrons move into higher
  electron shells.
• When the electrons move back to their normal
  electron shells, energy is transferred to the
  surroundings as radiation, which you see as light.

Question 6

What are the steps for carrying out a flame test?

Answer 6

• Clean a wire loop attached to a handle several
  times by dipping it into hydrochloric acid, then
  rinsing it with distilled water.
• Once it is clean, there will be no change in the
  colour of the Bunsen burner flame when you hold
  the loop in it.
• Dip the clean loop into the test powder or solution.
• Use the handle to hold the loop in the edge of a
  roaring blue flame.
• Record the flame colour.

Question 7

What is the purpose of cleaning the wire loop between each flame test?

Answer 7

• To ensure it is not contaminated with the
  previous sample.
• So you will not confuse flame colours or mix
  them.

Question 8

What are the flame test colours?

Answer 8

• Lithium - Li+ - Red
• Sodium - Na+ - Yellow
• Potassium - K+ - Lilac
• Calcium - Ca2+ - Orange/Red
• Copper - Cu2+ - Green/Blue

Question 9

What are hydroxide precipitate tests?

Answer 9

• Different metals produce different coloured
  precipitates.
• Add a few drops of sodium hydroxide solution to a
  solution containing metal ions.
• Note the colour of any precipitate formed.

Question 10

What are the different hydroxide precipitate test colours?

Answer 10

Iron(II) -  Green
Iron(III) - Orange/Brown
Copper(II) - Brown
Calcium - White
Zinc - White

Question 11

How can you tell whether the metal ion is zinc or calcium in a hydroxide precipitate test?

Answer 11

• Add excess sodium hydroxide solution.
• Zinc hydroxide dissolves to form a colourless
  solution.
• Calcium hydroxide does not dissolve.

Question 12

How do you detect sulphate ions?

Answer 12

• Add a few drops of hydrochloric acid.
• Then add a few drops of barium chloride solution.
• If sulphate ions are present you would get a white
  precipitate of barium sulphate.

Question 13

How do you detect carbonate ions?

Answer 13

• Hydrogen ions from dilute acids react with
  carbonate ions to produce carbon dioxide and
  water.
• Add a few drop of hydrochloric acid.
• If carbonate ions are present, bubbles of gas
  will be produced.
• You can confirm the gas is carbon dioxide by
  bubbling it through limewater.

Question 14

How do you detect halide ions?

Answer 14

• Add a few drops of nitric acid.
• Then add a few drops of silver nitrate solution.
• Chloride ions - Silver chloride - White
• Bromide ions - Silver bromide - Cream
• Iodide ions - Silver iodide - Yellow

Question 15

Why does the halide ion test not work for fluoride ions?

Answer 15

Silver fluoride is soluble in water.

Question 16

Why is the carbonate ions test useful?

Answer 16

• It works whether the carbonate compound is in the
  solid state or in a solution.
• This is useful because most carbonates are
  insoluble.

Question 17

What are the half equations for a positive sulphate ion and carbonate ion test?

Answer 17

Ba2+ (aq) + SO42- (aq) -> BaSO4 (S)

2H+ (aq) + CO32- (aq) -> CO2 (g) + H2O (l)

Question 18

What are instrumental methods of analysis?

Answer 18

A technique for analysing a substance that depends upon a machine.

Question 19

What are some advantages of instrumental methods of analysis compared to chemical methods?

Answer 19

• Sensitivity - Instruments can analyse very small
  amounts of substances. Useful if the substance is
  difficult or expensive to obtain.
• Accuracy - Instruments are very accurate. Can be
  calibrated using internationally accepted
  standards.
• Speed - Instruments can carry out analysis quickly
  and they can run all the time.

Question 20

What is a gas chromatogram and how do you interpret them?

Answer 20

• A gas chromatogram is a chart that represents
  the different substances in a mixture.
• Each peak represents a substance present in
  the mixture.
• The areas under each peak show the relative
  amount of each substance in the mixture.
• The retention time, the time taken for each
  substance to travel through the chromatography
  column, is different for different substances.

Question 21

What is a mass spectrometer?

Answer 21

• A machine that can measure masses of atoms and
  molecules.
• Used to analyse the relative amounts of different
  isotopes of an element and the structure of
  molecules.

Question 22

What are the four steps that occur in a mass spectrometer?

Answer 22

1) Ionisation
- Atoms bombarded with electrons.
- The electrons collide with the atoms to give a 1+
charge which makes the fragments unstable.

2) Acceleration
- All the fragments are made to have the same KE.

3) Deflection
- The fragments are passed through a magnetic
field and deflect proportionate to their mass and
charge.

4) Detection
- The beams of fragments are detection
electronically.
- It detects how much of the mass is deflected and
whether the sample is a pure substance.

Question 23

How do you interpret a mass spectrum?

Answer 23

• Relative abundance against the mass to charge
  ratio.
• Each peak represents a fragment of the molecule.
• The peak on the far right represents the molecular
  ion.
• The mass to charge ratio of the molecular ion
  peak is equal to the relative formula mass, Mr, of
  the molecule.