Calculations And Procedure Flashcards
Gravimetric analysis
The mass of an element or compound present in a substance is determined by changing that substance into another of known chemical composition that can be readily isolated , purified and weighed
To use gravimetric analysis we need:
The equation
The reaction to proceed to completion
The accuracy of gravimetric analysis depends on…
The dexterity of the person carrying out the procedure
The two types of gravimetric analysis
Precipitation/ filtration
Heating (volatilisation)
Precipitation/ filtration
The substance undergoes a precipitation reaction which is then separated, washed and dried to constant mass. The filtrate is tested to ensure the reaction has gone to completion
Must be carried out carefully to ensure all material is transferred and all apparatus dried and weighed with extreme care
For precipitation/ filtration the product must have
Low solubility so that all the product is precipitated
Particle size large enough for filtration
Be stable at temperatures of 100-105°C so that it can be dried in an oven
Heating (volatilisation)
Involves heating to change one substance into another
E.g dehydration of a hydrated salt to calculate the number of miles of water of crystallisation
Phase equilibriums
A solute is added to two immiscible liquids and the solute is soluble in both, then some of the solute will dissolve in both liquids. It will distribute itself in a definite ratio which we call the partition co-efficient
Partition co-efficient
A measure of the distribution of a solute between two immiscible solvents and is simply a specific case of equilibrium
The size of a partition co-efficient will vary as it depends upon
The solute
The nature of the two immiscible liquids
The temperature
The partition co-efficient is independent of…
The amount of solute
The volume of solvent
Partition co-efficient can be calculated with
[solutex]solvent B(top layer)
K= —————————————
[solutex]solvent A(bottom layer)
What are the solvent requirements for phase equilibria
The solvent used must be immiscible with the liquid mixture or soloution
The solute must be more soluble in one of the layers
The solvent must be volatile which allows the solute to be obtained by evaporation of the solvent
The solvent must be interactive with the solute
Most organic solvents are less dense in water so…
Will be on the top layer (except chloroform/ dichloromethane)
Vacuum filtration
Using a Buchner, hirsch or sintered glass funnel. These methods are carried out under reduced pressure and provide a faster means of separating the precipitate from the filtrate. The choice of filtering medium depends on the quantity and nature of the precipitate
Recrystallisation
A laboratory technique to purify solids based upon solubility.
The impure compound is dissolved gently in the minimum volume of hot solvent then filtered to remove the insoluble impurities. The filtrate is allowed to cool slowly to force crystallisation. The more soluble impurities are left behind in the solvent
For recrystallisation must be carefully selected so…
The impure compound is insoluble at lower temperatures
Refluxing
Heat energy is applied to a chemical reaction mixture over a period of time. The liquid is placed in a round bottomed flask along with anti-bumping granules with a condenser at the top. The flask is heated vigorously over the course of the chemical reaction; any vapours given off are immediately returned to the reaction vessel as liquids when they reach the condenser
Melting point of a crystalline substance
A sharp melting point falling within a very small temperature range
The presence of impurities in organic compounds ….
Lowers the melting point and extends the temperature range
Since impurities lower the melting point, the techniques of mixed melting point determination can be used…
As a means of identifying the product of a reaction
Colorimetry
Uses the relationship between colour intensity of a solution and the concentration of the coloured species present.
Colorimeter Is used to measure the absorbent light of a series of solutions of known concentration. this data is then used to construct a calibration curve
Distillation
A separation technique used to separate two liquids. In organic chemistry it is a powerful tool used to be identification and purification organic compounds. The boiling point as well defined and so we can use this physical property to identify an extract compounds
Used to purify a compound my nonvolatile or less volatile compound
Chromatography
Separate substances by making use of the differences in polarity or molecular size
Depends upon the protection of substances between two phases one stationary and one mobile
The separation occurs because the substances in the mixture have different partition coefficient between the stationary and mobile phases
Paper chromatography
Involves spotting a piece of paper with the sample mixture which is to be separated
The bottom of the paper is then placed in a pool of solvent what is drawn up the paper
This is the mobile phase and the water held in the paper as the stationary phase
When the paper is removed from the solvent the various parts of the mixture it will have moved different distances along paper
In chromatography what factors does the separation rely on
Solvent
rate of flow of solvent over the paper
solubility of the compounds in the mixture and in the solvent
the partition of the components between the moving solvent and the water bound to the surface of paper
And chromatography for each of the components in the mixture the Partition coefficient remains….
Constant
It is the difference between the partition coefficient of each component that separates them
Rad value=
distance travelled by component
———————————————
Distance travelled by solvent
Thin-layer chromatography
Similar to Paper chromatography but uses a fine film of silica or aluminium oxide spread over glass or plastic
Small sample of the mixture being tested is spotted onto the baseline
The solvent used dissolves the compounds in the spot and carries them up the chromatogram
The distance travelled to depend upon how soluble compounds are in the chosen solvent and how well they adhere to the plate
What is the order of the reaction
The no of moles involved in the rare determining step
A chemical reaction is said to be in equilibrium when
the composition of the reactants and products remaining constant indefinitely
Equilibrium can only occur in
A closed system
Closed system
One which allows energy to be transferred to and from the surroundings but not from the reactants or the products
Hertogeneous equilibrium
All species are in more than one phase
Homogeneous equilibrium
All the species are in the same phase
The value of an equilibrium constant gives
And indication of the extent of reaction once it is reached equilibrium
When the value of Kc is <10^-3
Effectively no reaction
When the value of Kc is 10^-3 to 10^3
Significant quantities of the reactants and products at equilibrium
When the value of Kc is >10^3
The reaction is affectively complete
Changing the concentration of either a product or reactant and equilibria mixture will affect the
Equilibrium position but not the equilibrium constant
Changing the temperature of an equilibria mixture will affect
The equilibrium constant and the equilibrium position
For endothermic reaction is a rise in temperature causes
An increase in equilibrium constant
For an exothermic reaction arise and temperature causes
A decrease in the equilibrium constant
What is the effect of a catalyst on the equilibrium mixture
A catalyst does not affect the equilibrium position that merely affects the rate at which equilibriums achieved
It will speed up the forward and reverse reaction by the same extent until the equilibrium constant is not affected
What affect what changing pressure have on an equilibriums mixture
If the pressure of an equilibriums has changed the system will adjust itself to minimise that change and so the equilibrium constant is not effected
If they’re equal numbers of gaseous molecules products and the alteration of the pressure does not affect the equilibrium mixture
