C4.2 Flashcards
What is the best place to test for gases and why?
- test them in a fume cupboard
- not much risk of gas being inhaled by people if it’s toxic or harmful
- as gas could be toxic or an irritant
What type of solution is limewater
It’s a calcium hydroxide solution
How can you detect carbon dioxide?
Limewater goes from colourless to cloudy white when C02 is bubbled through it.
Why does limewater change to cloudy white and what is the balanced symbol equation for it?
This is as the calcium hydroxide reacts with the carbon dioxide to form water and a white precipitate of calcium carbonate.
Ca(OH)2 + CO2 —> H2O + CaCO3
What are the properties of chlorine
It’s dissolves in water to form an acidic solution
It bleaches dyes and changes them from coloured to colourless
What are the two ways of detecting chlorine?
- damp blue litmus paper
- damp starch iodide paper
How can you detect chlorine using litmus paper?
- use a drop of tap water to dampen a piece of litmus paper
- hold the paper near to a container that holds the substance
- if chlorine is present, the paper turns red then white
How can you detect chlorine using starch-iodide paper?
- damp starch-iodide paper turns blue-black in the presence of chlorine
- this is as chlorine displaces iodine from the iodide ions in the damp paper
- this causes the starch in the paper to then turn blue-black
How can you detect hydrogen?
Place a lit splint in the test tube with the gas and it makes a squeaky pop sound
- sound comes from hydrogen burning in the oxygen in the air to form water
How can you detect oxygen?
A glowing splint will relight if it’s placed in a test tube of the test gas
How should you never smell substances in the lab?
- NEVER inhale directly from the container
How should you smell substances in the lab?
- with the container well away from your nose, breathe in enough air to almost fill your lungs
- hold the container a few cm away from the nose, and waft any smell towards you. Take a cautious sniff
What does holding the container away from you do?
- makes sure you don’t breathe in a lot of the substance
- if any substance escapes, it will not go up your nose
Why do metal ions emit light?
When metal ions are heated, energy is transferred to their e-. This makes their e- move into higher electron shells. When they move back to their normal shell levels, energy is transferred to the surroundings as radiation, which is seen as light.
How do you carry out a flame test?
- Get a nichrome wire loop and clean it by dipping it into concentrated HCl hold it in the Bunsen’s flame until it doesn’t change colour
- dip in the loop into test solution or power (if power = moisten wire drop of distilled water first)
- hold loop in roaring flame and note colour
What is a source of error in a flame test?
Contamination
- if there are more than one metal ions it’s wont work as the colours would be confusing
What are the flame test colours for lithium and what ion does it make?
Red
Li +
What are the flame test colours for sodium and what ion does it make?
Yellow
Na +
What are the flame test colours for potassium and what ion does it make?
Lilac
K+
What are the flame test colours for calcium and what ion does it make?
Orange-red or brick-red
Ca2+
What are the flame test colours for copper and what ion does it make?
Green-blue
Cu2+
Why do you use sodium hydroxide solution in experiments?
Group 1 metal hydroxides are soluble in water but most other hydroxides are insoluble. This is why you use sodium hydroxide solution (and not other metal hydroxides) in experiments.
What is copper hydroxide and when is it formed?
It’s an insoluble metal hydroxide formed when copper (II) sulphate solution reacts with sodium hydroxide solution.
CuS04 (aq) + 2NaOH (aq) —> Cu(OH)2 (s) + Na2SO4 (aq)
Net ionic equation = Cu2+ (aq) + OH- (aq) —> Cu(OH)2 (s)
What does sodium hydroxide and a solution with metal ions make when reacted?
How do you carry out a hydrogen precipitate test?
It’s forms a precipitate and the colour depends on the metal ions present. To carry out a test you add 2cm cubed of sodium hydroxide to a test tube with unknown solution.
What hydrogen precipitate colour does Iron (II) form and what ions are present in it?
Green
Fe2+
What hydrogen precipitate colour does Iron (III) form and what ions are present in it?
Orange brown
Fe3+
What hydrogen precipitate colour does Copper (II) form and what ions are present in it?
Blue
Ca2+
What hydrogen precipitate colour does Calcium form and what ions are present in it?
White
Ca2+
What hydrogen precipitate colour does Zinc form and what ions are present in it?
White
Zn2+
How can you differentiate if the white precipitate is Zinc or Calcium?
To find out you add more sodium hydroxide solution.
Calcium= it stays white and doesn’t dissolve
Zinc= it dissolves and becomes a colourless solution
What are anions and cations?
- anions = negative ion attracted to anode
- cations = positive ion attracted to cathode
What happens when barium reacts with sulfate ions?
Barium ions react with sulfate ions to produce white, insoluble barium sulfate.
Add dilute hcl to it to make sure it is barium sulfate not barium carbonate and BaCO3 would dissolve but BaSO4 wouldn’t
Ba^2+(aq) + SO4^2-(aq) —> BaSO4 (s)
How can you detect sulfate ions in a solution?
- add a few drops of dilute hydrochloric acid
- then add a few drops of barium chloride solution, BaCl2 (aq)
If sulphate ions are present you get a white precipitate - if u add HCl then it doesn’t dissolve as it doesn’t react with it
What do hydrogen ions from dilute acids produce when they react with carbonate ions?
They produce carbon dioxide and water
2H^+ (aq) + CO3^2- (aq) —> CO2 (g) + H2O(l)
Why is dilute HCl useful to detect carbonate ions?
This test works whether the carbonate compound is in the solid-state or in a solution. This is useful because most carbonates such as magnesium carbonate or insoluble.
How can you detect carbonate ions?
- add barium chloride which produces a white precipitate of barium carbonate
- then add a few drops of dilute HCl and If carbonate ions are present then bubbles of gas will be produced. You can check the gas is carbon dioxide by bubbling it through limewater
- should be left worn a colourless solution with Ba2+ ions
What other acid could use for detecting carbonate ions?
You could also use dilute nitric acid but hydrochloric acid is used more often
Is silver fluoride soluble or insoluble in water?
Silver fluoride is soluble in water but the other silver halides are insoluble
How can you detect halide ions?
- if solid, add distilled water to turn into solution
- if solution add a few drops of dilute nitric acid (removes other ions (carbonate) to avoid confusing precipitate) - no HCl as then you would be adding Cl ions
- then add a few drops of silver nitrate solution, AgNO3 (aq)
If one of these ions is present then you get a precipitate. Each halide ion has a different silver halide precipitate colour.
What colour of silver halide precipitate does a chloride ion form
White
Chloride = Cl
What colour of silver halide precipitate does a bromine ion form?
Cream
Bromine = Br-
What colour of silver halide precipitate does an iodide ion form?
Yellow
Iodide = I-
What does instrumental methods of analysis mean?
- it relies on a machine to carry out the analysis of a substance
What are the advantages of instrumental methods of analysis?
- sensitivity = instruments can analyse very small amounts (useful if expensive of difficult to obtain substance)
- accuracy = instruments are very accurate (calibrates using internationally accepted standards)
- speed = instruments can carry out analyses quickly and can run all the time
What does a gas chromatography represent?
Represents the different substances in a mixture
What does each peak in a chromatography represent?
A substance in the mixture
What do the areas under each peak show?
The areas under each peak show the relative amount of substance
What is the retention time?
It’s the time taken for a substance to travel through the chromatography column. It’s different for each substance. The bigger the retention time the slower the substance is.
What can a mass spectrometer measure and analyse?
It can measure the mass of atoms and molecules.
It is used to analyse the relative amounts of different isotopes of an element and the structure of molecules
Where is mass spectrometry used?
Used in many fields of science such as environmental science where scientists use it to detect toxic substances.
How does the mass spectrometer work?
- Sample enters mass spectrometer
- sample molecules are ionised to form molecular ions
- as molecular ions gain energy they break up to form fragments
- these ions then pass through the machine
- detector record the amount of each fragment present
What does each peak on the mass spectrum show?
- it represents a fragment of the molecule
Which peak represents the molecular ion?
Far right (last one)
What is mass to charge ratio of the molecular ion peak equal to?
The mass to charge ratio of the molecular ion peak is equal to the relative formula mass, Mr, of the molecules
What does a peak in an infrared spectrum represent?
peaks shows the wave numbers present within the chemical
What do the wavenumbers in an infrared spectrum show?
wave numbers correspond to the types of bonds present
What can you do when you pair an IR spectrum with a mass spectrum?
- results of mass spec = shows the Mr
- results of IR spec = shows the bonds present
- paired up with together = can be used to work out structure of the chemical
Why can magnesium and beryllium not be detected with a flame test?
- it produces no flame colour as it has a small size and very high ionisation enthalpy
- not enough energy to excite the e- to a higher energy level
