TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Edexcel Chemistry Unit 3B - Laboratory Techniques - Group 1 and 2 Reactions
Group 1 and 2 Reactions
Specifications
e) recall and interpret details of the chemistry of the elements and compounds listed in Units 1 and 2 of this specification
- this includes the chemistry of Groups 1, 2 and 7 and the chemistry associated with the organic compounds listed in topic 2.2
Sulphate solubility
If a solution of any sulphate is added to a solution of a group 2 metal compound then a precipitate is likely.
| Group 2 ion in solution
| Effect of adding a sulphate solution
|
| Mg2+
| No precipitate, MgSO4 is soluble
|
| Ca2+
| White precipitate of CaSO4
|
| Sr2+
| White precipitate of SrSO4
|
| Ba2+
| White precipitate of BaSO4
|
Hydroxide solubility
If sodium hydroxide is added to a solution of a group 2 compound then a precipitate is likely.
| Group 2 ion in solution
| Effect of adding a hydroxide solution
|
| Mg2+
| Faint white precipitate of Mg(OH)2
|
| Ca2+
| Faint white precipitate of Ca(OH)2
|
| Sr2+
| Faint white precipitate of Sr(OH)2 on standing
|
| Ba2+
| No precipitate, Ba(OH)2 is soluble
|
Heating carbonates and nitrates
| Substance
| Effect of heat
|
| Lithium and all group 2 carbonates
| Carbon dioxide detected
|
| Sodium and potassium carbonates
| No effect (except water of crystallisation may be given off)
|
| Sodium and potassium nitrates
| Oxygen only gas evolved
|
| Lithium and all group 2 nitrates
| Nitrogen dioxide and oxygen evolved
|
Action of heat on compounds
- Carbonates - Carbon dioxide is given off.
- Hydrogencarbonates - Carbon dioxide and water formed.
- Group 1 nitrates - Nitrite and oxygen formed.
- Group 2 nitrates - Oxide, brown fumes of nitrogen dioxide and oxygen formed.
Distinguishing between hydrocarbons
- Alkane: Burn /oxidise/combust them. They will burn with a yellow flame and form CO2 and H2O (limited supply of CO).
- Alkene: A yellow, sootier flame is produced (due to the extra carbon and higher ratio of carbon:hydrogen).
Identifying some functional groups
- Alkene - Add to orange bromine water. The alkene will decolourise it.
- Halogenalkane - Heat with sodium hydroxide solution. Acidify with dilute nitric acid and then test with silver nitrate solution as with inorganic halides.
- Alcohols or carboxylic acids containing C-OH - In a dry test tube (i.e. dry alcohol), add PCl5. Misty fumes of HCl are produced, which turns blue litmus paper red.
Distinguishing between different classes of alcohol
Primary - Add PCl5. Warm it with aqueous potassium dichromate (K2Cr2O7) and dilute H2SO4. Misty fumes are given off and the colour changed from orange to green --> aldehyde.
Secondary - Misty fumes, changes to green --> ketone.
Tertiary - Misty fumes, no colour change.
Specification
f) interpret quantitative and qualitative results
See http://www.thestudentroom.co.uk/attachment.php?attachmentid=12846 for notes on calculations.
g) devise and plan simple experiments based on the chemistry and techniques summarised in a to e above
Normally the last question on the paper. It will ask you to identify certain compounds from four. E.g. titrations, how to make a standard solution, how to titrate, identify QCO3 when heating it where Q is a group 2 metal ion.
h) evaluate error in quantitative experiments
see Appendix I for material available to assist centres in teaching this area
Percentage error = absolute uncertainty/actual value x 100%
j) comment on safety aspects of experiments based on supplied data or recall of the chemistry of the compounds listed in Units l and 2. Safety considerations should relate to specific experiments not be of a general nature
it will be assumed that students wear eye protection during all practical work.
Halogens are toxic and harmful by inhalation, although iodine is much less so than chlorine or bromine, because it is a solid. Chlorine and bromine must always be used in a fume cupboard. Liquid bromine causes serious burns an must be handled with gloves.
Ammonia is toxic. Concentrated ammonia solutions should be handled in the fume cupboard.
Concentrated mineral acids are corrosive. If spilt on the hands, washing with plenty of water is usually enough, but advice must be sought. Acid in the eye requires immediate attention and prompt professional medical attention.
Barium chloride solution and chromates and dichromates are extremely poisonous and so should be used in the fume cupboard/should not be inhaled.
Sodium or potassium hydroxide or concentrated ammonia in the eye is extremely serious and must always receive professional and immediate attention. Sodium hydroxide and other alkali metal hydroxides are amongst the most damaging of all common substances to skin and other tissue. Wear gloves, goggles and an apron when handling these solutions in high concentrations.
General safety
- Toxic/carcinogenic – use gloves, fume cupboard
- Flammable – Water baths, no naked fumes.
- Harmful gases – Use fume cupboard
- Corrosive – wear goggles/gloves
- Spillage of concentrated acid – wash with plenty of water.
Also See
Why not read these other revision notes for Edexcel Unit 3B - Laboratory Techniques?
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