practicals Flashcards
heating in a crucible
mass gain by reacting Mg with O₂
method
- weigh crucible and lid
- add Mg (that has been cleaned with emery paper) and reweigh
- heat with **bunsen burner ** then allow to cool
- weigh contents, then repeat until mass is constant
heating in a crucible
why is a lid added to oxidation of Mg reaction?
- improves the accuracy of the experiment, by preventing loss of solid
- should be loose fitting to ensure gases can enter and escape
heating in a crucible
why must you repeat the heating in the oxidation of Mg reaction?
- when the mass is constant, signifies the reaction is fully complete
heating in a crucible
decomposition reaction- what does it do
same method as oxidation reaction, just different reaction
- removes water of crystallisation as water vapour by heating
heating in a crucible
why must the crucible be dry in a decomposition reaction?
- a wet crucible would cause a large mass loss, as the water would be lost when heating
- would give inaccurate results as the mass change isn’t from the water of crystallisation only
measuring gas volumes
two methods for measuring gas volumes
- using a gas syringe
- measuring cylinder over water
measuring gas volumes
potential errors of using a gas syringe
- gas may escape before the bung is inserted
- the syringe could stick
measuring gas volumes
measuring cylinder over water diagram
components required labels
- measuring cylinder upside down in a trough of water
- markings on measuring cylinder to show measurements can be taken
- tubing that goes under the water and into the measuring cylinder
making a standard solution
method for making a standard solution from a solid
- weigh required mass of solid (in bottle) then transfer to beaker
(weigh empty bottle to ensure mass of solid is correct) - add 100 cm³ of distilled water to beaker, stir with glass rod to dissolve
- pour into volumetric flask with a funnel, rinse beaker/funnel/glass rod with distilled water and add washings to flask
- make up to the mark with distilled water
- add stopper and invert flask multiple times to mix
what other method can a volumetric flask be used for?
apart from standard solution
- dilution
- pipette 25cm³ of original solution to 250cm³ flask, make up to mark with distilled water
acid-base titration
acid-base titration
method
- pipette 25 cm³ of alkali into conical flask, touching surface with pipette
- fill burette with acid, run down into ‘jet space’ and record initial volume
- add indicator to conical flask and place on a white tile
- add acid to alkali while swirling conical flask, drop-by-drop until end point reached (indicated by colour change)
- record final volume of acid and calculate titre
- repeat until concordant results obtained (within 0.1 cm³)
acid-base titration
why is a conical flask used over a beaker?
- easier to swirl
- less likely to spill contents
acid-base titration
measuring using a pipette
how do you accurately take a reading (badly explained soz)
- make sure the bottom of the meniscus lies on the line
acid-base titration
before using a burette, how do you ensure no contamination occurs?
- rinse out with the substance you’re about to put in it
- if residual water left over, the acid or alkali may be diluted
- if substances left over from a past titration, conc may be lowered
acid-base titrations
what indicators are used when?
i don’t think we actually need to know this?? idk
- phenolphthalein- if acid is added from the burette, and strong alkali in the conical flask- pink to colourless
- methyl orange- neutralisation reactions where strong acid is used- red in acid, yellow in alkali, orange at end point
manganate redox titrations
equation
there’s like nothing on this in the pag things so idek
MnO₄⁻₍ₐ₎+ 8H⁺₍ₐ₎+ 5Fe ²⁺₍ₐ₎–> Mn²⁺₍ₐ₎+ 4H₂O₍ₗ₎ + 5Fe³⁺₍ₐ₎
manganate redox titration
colour change
- self-indicating, MnO₄⁻ is purple and Mn²⁺ is colourless
- if manganate in burette, colourless to first permanent purple colour indicates end point
percentage uncertainty calculation
%= (uncertainty/measurement taken) x 100
note that if two measurements are taken, must multiply uncertainty by 2
decreasing % uncertainties
- use apparatus with a greater resolution
- or increase the size of the measurement made
enthalpy change of reaction
equipment used
calorimetric method
- use a polystyrene cup inside beaker (insulation AND support)
- thermometer to monitor temp change
- glass rod to stir
enthalpy change of reaction
method
calorimetric method
- measure out desired volumes of solutions with pipettes and transfer to insulated cup (or could used solid dissolving in one solution)
- clamp thermometer into place with bulb immersed in liquid
- measure initial temperature, then every 30 seconds
- at 3 minutes add second solution and stir, keep recording temp every 30 secs/1 min
enthalpy change of reaction
errors from this method
calorimetric method
- will be loss of heat energy to the surroundings
- reaction (or dissolving) may be slow or incomplete
- SHC assumed to be same as water for all solutions
enthalpy change of combustion
apparatus and brief method
- weigh empty spirit burner and lid, then reweigh with alcohol added
- add water to beaker/ calorimeter and clamp above spirit burner, with lid and thermometer
- measure temp change of water for reaction
- weigh spirit burner after experiment
calculate mass change, temp change
enthalpy change of combustion
errors in this method
- heat loss to the surroundings from spirit burner, wick and calorimeter
- evaporation of water
- evaporation of alcohol
- incomplete combustion of alcohol
- reaction is unlikely to occur under standard conditions
enthalpy change of combustion
improvements to reduce error
- improving draught exclusion by adding a draught shield
- minimising the distance between the flame and the calorimeter
- placing a cap on the calorimeter
- insulating the spirit burner and calorimeter
anions
test for ammonium ions
- add sample and NaOH to a test tube, and shake
- gently warm (using a water bath)
- test the fumes by holding damp red litmus paper over the test tube
- alkaline ammonia gas is produced which turns the red litmus paper blue
cations
test for carbonates
- add any dilute acid
- effervescence is observed
cations
test for sulfates
- barium chloride acidified with HCl is used as s reagent
- white precipitate of barium sulfate forms
- HCl used to react with carbonate impurities, which would form a white barium carbonate precipitate which is a false positive
cations
test for halides- precipitates
- add silver nitrate (and HNO₃ to remove carbonates)
- chloride ions form a white precipitate
- bromide ions form a cream precipitate
- iodide ions form a yellow precipitate
cations
test for halides- solubility
- silver chloride dissolves in dilute ammonia
- silver bromide dissolves in concentrated ammonia
- silver iodide is insoluble in ammonia
purifying an organic liquid
method - always in the mark scheme
- put the impure product into a separating funnel and allow the layers to separate
- upper layer is least dense, run off the lower (usually aqueous) layer
- run the organic layer into a conical flask and add anhydrous magnesium sulfate
- decant into distillation flask and distill to further purify (collect fraction at given boiling point)
purifying an organic liquid
why might a substance be refluxed before being put in a separating funnel?
reactant vapours of volatile compound are condensed and returned to the reaction mixture
when measuring a boiling point, what is the indication that the substance is impure?
- if the liquid is pure it will have the boiling point referred to in data books
- if impure the boiling point tends to be higher than the pure liquid
purifying an organic solid
recrystallisation method
- dissolve the impure compound in a minimum volume of hot solvent
- filter hot solution to remove insoluble impurities
- cool the solution
- filter when cold (using a Buchner flask so reduced pressure) and wash to remove soluble impurities
- dry crystals
purifying an organic solid
why must a minimum volume of hot solvent be used?
- lower yield (less product collected) as some of the product would remain dissolved in the hot water
purifying an organic solid
during filtration, why is it important to only wash your solid with ice-cold solvent?
- the solid would dissolve in hot water and so the yield would decrease
thin-layer chromatography
what must the baseline be drawn in and why?
- pencil line
- will not dissolve in the solvent
thin-layer chromatography
general method
- draw a baseline on TLC plate and draw a spot for each sample
- use a capillary tube to add a small drop to each spot
- add 1cm of solvent to a beaker and add TLC plate, making sure the solvent is below the baseline
- add a lid!
- remove from solvent just before the solvent front reaches the top of the plate
- allow to dry and place under UV light to develop the spots
- calculate Rf values
thin-layer chromatography
how do stationary phases separate?
- solid stationary phase- by adsorption
- liquid stationary phase- by relative solubility
electrochemical cells
diagram of zinc and copper electrodes
- Zn electrode in solution containing 1 moldm⁻³ of Zn²⁺ ions
- copper electrode in solution containing 1 moldm⁻³ of Cu²⁺ ions
- connected with a voltmeter
- salt bridge required
- 298K and 100KPa
electrochemical cells
what is the use of a salt bridge? how is it made?
- salt bridge is used to connect the circuit- the free moving ions conduct the charge
- usually made from a piece of filter paper soaked in a salt solution, which is usually potassium nitrate
measuring the rate of a reaction
what is the typical continuous monitoring method?
measurement of the change in volume of a gas- using a gas syringe
measuring the rate of reaction
method for volume of gas collected in gas syringe
- measure 50 cm³ of the 1.0 mol dm⁻³ hydrochloric acid and add to conical flask
- set up the gas syringe in the stand
- weigh 0.20 g of magnesium
- add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the timer
- record the volume of hydrogen gas collected every 15 seconds for 3 minutes
measuring the rate of a reaction
what is the typical method for initial rate
clock reaction- normally with hydrogen peroxide and iodine
* start the timer and stop when the solution turns blue/black
* repeat with different concentrations of potassium iodide
pH curves
titration/pH method
- transfer 25cm³ of acid to a conical flask with a volumetric pipette
- measure initial pH of the acid with a pH meter
- add alkali in small amounts (2cm³) noting the volume added each time (using a burette and titration method)
- stir mixture to equalise the pH as you go along
- measure and record the pH to 1 d.p.
- repeat but when approaching endpoint add in smaller volumes of alkali
- add until alkali in excess (or burette used up)
