All A Level Practicals - Mechanisms And Questions Flashcards
A student prepares a standard solution of 250.0cm3 of a standard solution of 0.150moldm-3 of (Ba(OH)2) for the titration.
The student is provided with solid Ba(OH)2 and usual laboratory apparatus and equipment.
Describe how the student would prepare the standard solution, giving full details of quantities, apparatus and method.
(6 Marks)
Calculations:
- Work out mass of Ba(OH)2 using n = m/Mr
- Mass comes out as 6.42g (3s.f)
Method:
- Dissolve the 6.42g of Ba(OH)2 in 50cm3 of distilled water in a beaker (use a stirring rod)
- Transfer into a volumetric flask
- Wash beaker, stirring rod and funnel and pour washings into volumetric flask
- Make solution up to 250cm3 using distilled water
- Invert the flask several times to ensure it’s mixed
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Butanal and butanone both react with 2,4-dinitrophenylhydrazine to produce mixtures containing orange precipitates.
Outline how the mixtures containing these orange precipitates can be used to distinguish between butanal and butanone.
(3 Marks)
Recrystallise/purify the precipitates
Measure the MP of each ppt.
Compare the MPs with known values
Tollens’ reagent can be used to identify the aldehyde group in cinnamaldehyde.
• Describe how you would make Tollens’ reagent and carry out this test in the laboratory.
• Explain what happens to both the Tollens’ reagent and the cinnamaldehyde in this reaction. Identify the organic product.
(8 Marks)
What could effect the feasibility of a reaction in a labs may prevent it from occurring in a lab
Activation energy is too high - too much energy required
Reaction may be too slow
Conditions may be non-standard
Making a standard solution
Weigh sample bottle containing required mass of solid on a 2 decimal place mass balance
Transfer the solid to a beaker an reweigh sample bottle
Record difference in mass to calculate the mass of solid used
Add 100cm3 of distilled water to beaker
Use glass rod to start to help dissolve solid
Pour solution into a 250cm3 volumetric flask via a funnel
Rinse beaker and funnel with distilled water
Add washings to volumetric flask
Make up to the mark at 250cm3 using distilled water
Makeup to the meniscus using a dropping pipettes for last few drops
Slowly invert flask several times
How to ensure all of the oxygen has been achieved and produced from lead oxide solid reacting with methane (?)
Heat up to a constant mass Break up lead oxide into smaller chunks to increase surface area for reaction to occur Pass methane through a tube as it cools Bubble gas through limewater Use excess methane Bubble escaping gas through limewater
Carrying out an acid-base titration mechanism
Rinse equipment
- burette with acid
- volumetric pipettes with alkali
- Conical flask with distilled water
Pipettes 25cm3 of alkali solution into conical flask using volumetric pipette
Touch surface of alkali with pipette
Fill burette with acid
Turn on top quickly to remove any air bubbles
Record initial burette reading
Add a few drops of indicator (phenolphthalein, etc.)
Place a white tile underneath flask to help observe colour change
Carry out a rough titration first - idea of where endpoint of reaction is
Carry out titration
- add acid to alkali whilst swirling mixture
- add acid drop-wise near endpoint
- stop adding acid when colour change occurs
Repeat titration until two concordant results are obtained
Calculate mean titre from at least two concordant results
Measuring enthalpy change mechanism
Place polystyrene cup in a beaker for insulation and support
Clamp thermometer into place - make sure thermometer bulb is immersed in liquid
Measure initial temperature of solution
Transfer reagents to a cup
If solid reagent used, add solution to cup first and then add solid weighed out on a mass balance
Stir mixture
Measure the final temperature
- Q = mcAt to calculate enthalpy change
Investigating how rate changes with temperature
Use a measuring cylinder to measure 10dm3 of 0.05 Moldovan-3 sodium thiosulphate solution into a conical flask
Place on a white tile with cross drawn on it
Note initial temperature
Add 1cm3 of HCl to thiosulphate solution an start timing
Record time for cross to disappear from view
Record final temperature of the reaction mixture
Repeat using different temperatures to obtain results
Use 5 different temperatures in total
Plot a graph of temperature against time
Use average temperature for each reaction
Monitoring production of gas
H2O2 added to conical flask and bung is replaced
Initial volume of gas in gas syringe is recorded
Manganese oxide catalyst quickly added to conical flask
Bung is replaced
Use a stop clock to measure set time intervals
Volume of gas measured at regular intervals until reaction is complete
The reaction is complete when no more gas is produced
- alternatively, measuring cylinder can be used
Order of qualitative testing
Test for these ions in order
- CO3^2- (adding dilute HNO3)
- SO4^2- (Barium Nitrate)
- Halogen ions - Cl-, Br-, I- (Silver Nitrate, AgNO3)
- NH4+ (warming with NaOH)
Test for carbonate ions (CO3^2-)
Add nitric acid
If carbonate ions present:
- effervescence and limewater turns milky
Additional info: sulfate test - barium chloride added to solution also reacts with carbonates to make a white precipitate
Test for sulfates (SO4^2-)
Add Ba2+ ions (from barium chloride and HCl)
If sulfate ions present:
a white precipitate of barium sulfate (BaSO4) will form
Test for chloride ions in solid
Add conc. H2SO4
- produce steamy fumes of HCl
Test for bromide in solid
Add conc. Sulphuric acid
Produce steamy fumes of HBr and red brown bromine
Test for iodide ions in solid
Add conc. H2SO4 Production of steamy fumes - rotten egg smell (H2S) Black solid iodine Yellow solid sulphur
Test for halides
Add silver nitrate solution
Add ammonia solution to distinguish colours more
Cl: white ppt forms
Dissolves in dilute ammonia
Br: cream ppt forms
dissolves in conc. ammonia
Iodine: yellow ppt forms
Does not dissolve in ammonia
Different dehydrating agents
CaCl2, CaSO4, MgSO4
Uses for dehydrating agents in practicals/organic chemistry
Used in purifying an organic solid
- used to remove any water from organic products - drying organic products
Method of filtration/purification of an organic products
Method of determining purity of an organic product via melting point
Melting point determination - compare MP to recorded values online
- use melting point apparatus
- as soon as crystal starts melting - start recording the temperature
- when fully melted - record final temperature to give range of MP values
- very pure product has a very sharp melting point -
- if not very pure - there is a range of melting point values
Need to draw diagram for practicals - reflux, distillation, recrystallisation, etc.
Recrystallisation method (school)
Add solvent to flask - add smallest amount possible you know solute will dissolve in
Warm solvent that you know the solute will just about dissolve in
Add the solute
Then allow the solution to cool down
Filter the solution - crystals
Once filtered, crystals should be left in an oven overnight to dry
Purifying organic product method and diagram
Depends on the density of organic product compared to water
Remix mixture and repeat two or three times to remove as much water as possible from organic product
Use a drying agent at end to remove any remaining water from organic mixture
Filtration under pressure using a bucher funnel
Test for ammonium ions
Add NaOH
Gently heat
Damp red litmus paper used to detect ammonia
Ammonia gas produced
Damp red litmus paper turns blue - alkali ammonia gas
Uses of group two compounds/metals
- group two compounds are reducing agents
Used in fields to increase pH of soils
Used as antacids for acid digestion (e.g. magnesium hydroxide and calcium carbonate neutralise stomach acid
Ca(OH)2 used to neutralise acidic soil
Halide displacement reactions
Reactivity deceases down group 7
- so elements higher in group 7 displace ones lower in group 7
- e.g. Cl displaces Br
Cl2 + 2Br- —> 2Cl- + Br2
Cl2 + 2I- —> 2Cl- + I2
Br2 + 2I- —> 2Br- + I2
Test for ammonium ions
Aq sodium hydroxide, NaOH added to solution of ammonium ions
Ammonia gas produced
Mixture is warmed and ammonia gas released
Damp red litmus paper turns blue in presence of alkaline ammonia gas
Distilling (fractional distillation) a product from a reaction mechanism
Heat flask with a burden burner/heating mantle/hot water bath
Causes vapour of all components in mixture to be produced
Vapours pass up the fractionating column
Vapour of substance with lowest boiling point reaches the top of the fractionating column first
Thermometer should be at or below the BP of the substance with the lowest boiling point
Vapours with higher boiling point condense back into the flask
Only most volatile vapour passes back into condenser
Condenser cools vapours and liquid is collected in separate beaker
Test for Alkenes
Add bromine water
- bromine water decolourises - orange to colourless
Test for carboxylic acid
Add sodium carbonate - Na2CO3
Effervescence as CO2 formed
Synthesis of organic compounds practical techniques
Use of quick fit apparatus including for distillation and heating under reflux
Prep and purify organic liquid including:
Use separating funnel to remove organic layer from aqueous layer
Drying with an anhydrous salt - e.g. MgSO4, CaCl2
Redistillation to purify further
Preparation of Tollens’ reagent
Add silver nitrate - AgNO3
Add aqueous NaOH to silver nitrate until a brown ppt of silver oxide is formed - AgO
Add dilute ammonia until brown ppt dissolves to form a clear colourless solution
2, 4 DNPH - Brady’s technique to identify carbonyls
- Add Brady’s to substance
- Add a few drops of H2SO4 if no crystals form
- Yellow-orange ppt will form if test is positive
- Recrystallise the ppt
- Find melting point of compound
- Compare with data book to identify the unknown compound
Continuous monitoring method
- pros and cons
Finding: - volume of gas - colour change - using a colorimeter - mass change of reactant or product Over a given time - to measure rate of reaction Can monitor rate of reaction further
Pros:
- more accurate
Cons:
- Takes longer to obtain results
- more work to process results
Analysing by colorimetry
Iodine clock method
Manganate redox titration method
Analysing percentage purity of iron
- using ferrous sulfate tablets and reacting with manganate ions
- include relevant equations and overall equation
