Chemical Analysis Flashcards
(21 cards)
What is chemical analysis?
Chemical analysis, also known as analytical chemistry, is the field of chemistry involved with the instruments and methods used to separate, identify and quantify substances.
Explain what is meant by the term ‘purity’.
Purity is defined differently in chemistry than in everyday life. In everyday life, when you refer to a substance being pure, you mean that nothing has been added to it. However, in chemistry, a ‘pure’ substance is one that only contains one compound or element throughout, and that has not been mixed with anything else.
How can you determine if substances are pure or impure?
Pure substances have specific melting and boiling points, whereas impure substances will melt and boil over a range of temperatures.
Define a formulation, and give examples of formulations.
A formulation is a mixture that has been prepared using a specific formula, and so contains different components in a fixed ratio. Examples of formulations include paint, medicines, alloys and fuels.
Each component of a formulation has a specific purpose. For example, a paint would contain a pigment (which would give the paint colour), a solvent (which would be used to dissolve the other components and alter the viscosity), a binder/resin (which would form a film that would hold the pigment in place after being painted on) and additives (which would be added to further change the physical and chemical properties of the paint).
Describe how to carry out paper chromatography.
To carry out paper chromatography:
1) Take a rectangular piece of filter paper and draw a line (the baseline) at the bottom using a ruler and pencil.
2) Fill a large beaker with a shallow volume of solvent, such as water or ethanol.
3) Place the filter paper carefully into the beaker, ensuring that the drop of ink that has been placed on the baseline is not submerged.
4) The solvent will seep up the paper, leaving a pattern of spots known as a chromatogram.
5) Before the solvent reaches the top of the paper, remove the paper from the beaker and leave it to dry.
6) The procedure should be carried out with the lid of the beaker on to prevent evaporation.
State the two phases in paper chromatography.
The ‘mobile phase’ of paper chromatography is the substance through which the test substance can move, and the ‘stationary phase’ is the substance through which the test substance cannot move.
For example, the solvent could be the mobile phase, and the paper could be the stationary phase.
Why will different substances move up the paper at different rates?
Different substances will move up the paper at different rates in paper chromatography since the test substance will have different solubilities in the solvent. This is because the test substance will continuously alternate between being in the mobile and stationary phase. More soluble substances will be less attracted to/have a lesser affinity for the paper, and so spend less time in the stationary phase, and so move up the paper faster and cover a greater distance than a less soluble substance in the same time.
Why can the distances that different substances move up the paper from the baseline not, by themselves, be used as a comparative measure between different experiments?
The distances that different substances move up the paper from the baseline be by themselves used as a comparative measure between different experiments since the time that the paper is left in the beaker for may vary.
How are Rf values calculated?
Rf value = distance travelled by the substance / distance travelled by the solvent.
Note: when looking up Rf values in a data book, you must use one that uses the same solvent and type of paper as used in your experiment.
Describe how to test for hydrogen gas.
1) Take a test tube with the unknown gas and put a lit splint near the test tube.
2) If the gas is hydrogen, you will hear a “squeaky pop”, which occurs since the heat energy provided by the flame causes the hydrogen to react with the oxygen in the air to form water:
2H2O (g) + O2 (g) -> 2H2O (l)
Describe how to test for chlorine gas.
1) Place a strip of damp, blue litmus paper in the test tube containing the hydrogen gas.
2) If the gas is chlorine, the paper will be bleached white. It may, however, turn red first, if the chlorine reacts with the water on the damp litmus paper to form hydrochloric acid.
Describe how to test for oxygen gas.
1) Place a glowing splint into a test tube that contains the unknown gas.
2) If the gas is oxygen, the splint will catch fire, since oxygen is necessary for combustion.
Describe how to test for carbon dioxide gas.
1) Bubble the unknown gas through a limewater (calcium hydroxide: Ca(OH)2) solution.
2) If the gas is carbon dioxide, the limewater will turn from clear to cloudy. This occurs since a solid white precipitate, calcium carbonate (CaCO3) is formed in the following reaction:
CO2 (g) + Ca(OH)2 (aq) -> CaCO3 (s) + H2O (l)
Describe how to carry out the flame tests to test for cations.
1) Take a platinum (or nichrome) wire loop and place it in hydrochloric acid, then rinse it with distilled water and place it into the blue (hottest) part of a lit Bunsen burner.
2) Dip the platinum wire loop into the unknown ionic compound, and then place it into the edge of the flame of the Bunsen burner.
3) The cation present will determine the colour of the flame:
K+ (Potassium): Lilac
Ca2+ (Calcium): Orange-red
Cu2+ (Copper): Green
Na+ (Sodium): Yellow
Li+ (Lithium): Crimson
Any other cations: No colour change
Describe how to test for cations using sodium hydroxide
1) Add NaOH to the unknown ionic compound.
2) The OH- anions will then react with the cation present, causing a precipitate (an insoluble solid product) to form that will cause a visible colour change:
Ca2+ (aq) + 2OH- (aq) -> Ca(OH)2 (s): white ppt.
Mg2+ (aq) + 2OH- (aq) -> Mg(OH)2 (s): white ppt.
Al3+ (aq) + 3OH- (aq) -> Al(OH)3 (s): white ppt. that will redissolve in excess NaOH
Fe2+ (aq) + 2OH- (aq) -> Fe(OH)2 (s): green ppt.
Fe3+ (aq) + 2OH-(aq) -> Fe(OH)3 (s): brown ppt.
Cu2+ (aq) + 2OH-(aq) -> Cu(OH)2 (s): blue ppt.
Describe how to test for halide anions.
1) Add dilute nitric acid to the unknown ionic compound to remove any carbonate (CO32-) or sulfite (SO32-) anions that might be present (since they could also react with the Ag+ cations to form a white ppt. that would taint the results).
2) Add silver nitrate to the unknown ionic compound.
3) The Ag+ cations will react with the anion present, causing a precipitate to form:
Cl- (aq) + Ag+ (aq) -> AgCl (s): white ppt.
Br- (aq) + Ag+ (aq) -> AgBr (s): cream ppt.
I- (aq) + Ag+ (aq) -> AgI (s): yellow ppt.
Describe how to test for carbonate anions.
1) Add dilute HCl to the unknown ionic compound.
2) The carbonate anions (if present) will react with the H+ cations released when the HCl ionises to form carbon dioxide gas. Thus, when you bubble the gas produced through limewater, if CO32- ions were originally present, the limewater will turn cloudy.
Describe how to test for sulphate anions.
1) Add dilute HCl to the unknown ionic compound to remove any carbonate (CO32-) or sulfite (SO32-) anions that might be present (since they could also react with the Ag+ cations to form a white ppt. that would taint the results).
2) Add barium chloride to the unknown ionic compound.
3) If sulphate anions are present, they will react with the Ba2+ ions:
Ba2+ (aq) + SO42- (aq) -> BaSO4 (s): white ppt.
Thus, if a white ppt. forms, there were sulphate ions present.
What are the disadvantages with the flame tests for cations?
The disadvantages with the flame tests for cations include that perception of colour is subjective, and so it can be different to differentiate between two cations that produce a similar colour flame, and that there may be more than one cation present, causing the colours to be mixed.
What is flame emission spectroscopy?
During flame emission spectroscopy, a sample is placed in a flame. As the ions heat up, their electrons become excited, and then when they drop back to their original energy levels, they transfer energy as light (an EM wave). The light passes through a spectroscope, which can detect different wavelengths of light, forming a line spectrum.
The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement. Since no two ions have the same charge and electron arrangement, each ion will produce a different pattern of wavelengths, and have a different line spectrum. Thus, the line spectrums can be used to identify ions in solution (by comparing the spectrum obtained against reference spectra) and calculate their concentration (the brighter the lines, the higher the concentration of the ion).
Flame emission spectroscopy is useful since it can analyse mixtures.
What are the advantages and disadvantages of manual methods as compared to instrumental (machine) methods of analysis?
Manual methods of chemical analysis are usually cheaper and easier to do, but instrumental methods are very sensitive, fast (and tests can be automated) and accurate.
