4 Researching Chemistry Flashcards
Unit 4
What are the 4 different types of titrations that can be assessed?
-Acid-base
-Redox
-Complexometric
-Back titration
What properties must a primary standard?
-Have a high state of purity
-Be stable when solid and in solution
-Be soluble (usually in water)
-Have a reasonably high formula mass
What are some examples of primary standards?
-Sodium carbonate
-Hydrated oxalic acid
-Potassium hydrogen phthalate
-Silver nitrate
-Potassium iodate
-Potassium dichromate
What is a standard solution?
One of accurately known concentration and is prepared directly from a primary standard to ensure its conc remains accurate
How do you prepare a standard solution?
Accuratley weigh the mass of the standard, add the standard to a beaker and dissolve with a small amount of deionised water. Rinise the weighing boat and add that to beaker, transfer this solution into a volumetric flask, rinse beaker and add the rinsings. Make up to the graduation mark with deionised water and stopper and invert
How do you dilute a standard solution?
Use a volumetric pipette to transfer a fixed volume of the solution to a volumetric flask. Add deionised water and make up to the mark and stopper and invert
What is a complexometric titration?
This is used to determine the concentration of metal ions in solution
How are complexes formed?
When a metal ion reacts with a ligand
What is a ligand?
A molecule or negative ion with one or more non-bonding pairs of electrons
What is the most common used ligand in complexometric titrations?
EDTA - wraps around metal using the 6 dative bonds - a hexadentate ligand
How is the end point of a EDTA complexometric titration determined?
Using a metal ion indicator - an organic dye that changes colour when it binds to metal ions e.g. murexide
When are back titrations used?
-The analyte is insoluble or has low solubility
-No suitable indicator is available for a direct titration
-The end point of the back titration is clearer than the direct titration
-The reaction between the standard reagent and analyte is slow
What does a back titration involve?
Adding a known quanitiy of one standard reagent in excess, to a known mass of the substance to be determined (the analyte)
What is gravimetric anaylsis based on?
This is based on the measurement of mass of a substance
What is weighing accurately approximately?
This term means that an approximate mass of a substance is weighed, but the mass that is actually used in the experiment is known accurately, and is recorded
What are the two main types of gravimetric analysis?
Volatilisation and precipitation methods
What is volatilisation?
A sample of the substance is weighed and then heated to release the volatile product (often water) by evaporation. The product is then collected and weighed, a mass is determined from the loss in mass of the original sample
What is the purpose of a desiccator?
Prevents the re absorption of moisture
How do you heat to constant mass?
-The substance is weighed by difference
-It is then heated, cooled in a desiccator and then reweighed
-this is repeated until two consecutive mass readings are recorded
-The final mass is recorded
Why is yield not always 100% when using precipitation?
-Loss of product through transfer
-Side reactions may occur reducing the yield of the main product
-The reaction may be reversible and will never achieve 100% conversion
What kind of funnels can be using in vacuum filtration?
Buchner, Hirsch or sintered glass funnels
Why and how would a filtrate be tested?
A filtrate will be tested to ensure the reaction has gone to completion. It is tested by adding the precipitating substance, if it goes cloudy then the reaction is not complete and more substance should be added and filtration repeated
How are crystals purified?
recrystallisation
What happens during recrystallisation?
The impure solid crystals are heated in the minimum volume of a suitable solvent, the hot saturated solution is filtered and then cooled, the pure crystals are then filtered off leaving the soluble impurities in the filtrate
For successful recrystalisation the substance must:
-Not react with the solvent
-Be very soluble in the hot solvent
-Be insoluble in the cold solvent
What is the hot solution filtered through?
A fluted filter paper
What does colorimetry determine?
The concentration of colored ions in a solution or the concentration of ions that can be converted into coloured species
What occurs to the absorption of light if in a more concentrated solution?
A more concentrated solution will have a more intense color, so will absorb more light
What does a colorimeter do?
It passes light of the complementary color through the solution, and measures the light that is absorbed or transmitted
What is Beer’s Law?
The colorimeter compares the intensity of the incident light with the intensity of the transmitted light
Why and how is a colorimeter calibrated?
This is done to ensure that the readings are more accurate.
A “blank” is used, normally a sample of deionised water as it does not absorb light (colourless)
What are the 5 steps involved in organic synthesis?
Preparation, isolation, purification, identification and percentage yield calculated
What occurs in preperation in organic synthesis?
The main reaction is carried out and a crude sample of the desired product is produced
Why is heating under reflux used?
-It allows a reaction to proceed for a long time
-Prevents the loss of volatile reactants or products on heating
Why are anti-bumping granules added when heating under reflux?
They are small coarse stones to make substances boil more evenly, without sudden, violent bubbling
What occurs in the isolation stage of organic synthesis?
The crude sample is separated from the reaction mixture, the product is separated from any other reactants that were in excess, other products, products of side reactions and the limiting reactant if the reaction was a reversible one
How does distillation isolate a substance?
It isolates a substance on the basis of its boiling point
When is isolation by solvent extraction used?
If the product is in a aqueous solution
Why is purification used if you have already isolated the product?
Isolation methods are unlikely to remove all impurities so these must be removed from the impure product sample using purification techniques
When is fractional distillation used?
If the impurities in the crude liquid sample are volatile
How can you tell if a substance if pure or not from its melting point?
It will entirely within a range of 1c - it has a definite and sharp melting point. The presence of impurities in a substance it lowers its melting point and broadens its melting point range
