practicals Flashcards
(46 cards)
1
Q
heating in a crucible
mass gain by reacting Mg with O₂
method
A
- 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
2
Q
heating in a crucible
why is a lid added to oxidation of Mg reaction?
A
- improves the accuracy of the experiment, by preventing loss of solid
- should be loose fitting to ensure gases can enter and escape
3
Q
heating in a crucible
why must you repeat the heating in the oxidation of Mg reaction?
A
- when the mass is constant, signifies the reaction is fully complete
4
Q
heating in a crucible
decomposition reaction- what does it do
same method as oxidation reaction, just different reaction
A
- removes water of crystallisation as water vapour by heating
5
Q
heating in a crucible
why must the crucible be dry in a decomposition reaction?
A
- 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
6
Q
measuring gas volumes
two methods for measuring gas volumes
A
- using a gas syringe
- measuring cylinder over water
7
Q
measuring gas volumes
potential errors of using a gas syringe
A
- gas may escape before the bung is inserted
- the syringe could stick
8
Q
measuring gas volumes
measuring cylinder over water diagram
components required labels
A
- 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
9
Q
making a standard solution
method for making a standard solution from a solid
A
- 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
10
Q
what other method can a volumetric flask be used for?
apart from standard solution
A
- dilution
- pipette 25cm³ of original solution to 250cm³ flask, make up to mark with distilled water
11
Q
acid-base titration
acid-base titration
method
A
- 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)
12
Q
acid-base titration
why is a conical flask used over a beaker?
A
- easier to swirl
- less likely to spill contents
13
Q
acid-base titration
measuring using a pipette
how do you accurately take a reading (badly explained soz)
A
- make sure the bottom of the meniscus lies on the line
14
Q
acid-base titration
before using a burette, how do you ensure no contamination occurs?
A
- 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
15
Q
acid-base titrations
what indicators are used when?
i don’t think we actually need to know this?? idk
A
- 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
16
Q
manganate redox titrations
equation
there’s like nothing on this in the pag things so idek
A
MnO₄⁻₍ₐ₎+ 8H⁺₍ₐ₎+ 5Fe ²⁺₍ₐ₎–> Mn²⁺₍ₐ₎+ 4H₂O₍ₗ₎ + 5Fe³⁺₍ₐ₎
17
Q
manganate redox titration
colour change
A
- self-indicating, MnO₄⁻ is purple and Mn²⁺ is colourless
- if manganate in burette, colourless to first permanent purple colour indicates end point
18
Q
percentage uncertainty calculation
A
%= (uncertainty/measurement taken) x 100
note that if two measurements are taken, must multiply uncertainty by 2
19
Q
decreasing % uncertainties
A
- use apparatus with a greater resolution
- or increase the size of the measurement made
20
Q
enthalpy change of reaction
equipment used
calorimetric method
A
- use a polystyrene cup inside beaker (insulation AND support)
- thermometer to monitor temp change
- glass rod to stir
21
Q
enthalpy change of reaction
method
calorimetric method
A
- 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
22
Q
enthalpy change of reaction
errors from this method
calorimetric method
A
- 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
23
Q
enthalpy change of combustion
apparatus and brief method
A
- 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
24
Q
enthalpy change of combustion
errors in this method
A
- 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
25
# 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**
26
# 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**
27
# cations
test for carbonates
* add any **dilute acid**
* **effervescence** is observed
28
# 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
29
# 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**
30
# cations
test for halides- solubility
* silver **chloride** dissolves in **dilute ammonia**
* silver **bromide** dissolves in **concentrated ammonia**
* silver **iodide** is **insoluble in ammonia**
31
# 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)**
32
# 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
33
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
34
# 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
35
# 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
36
# 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
37
# thin-layer chromatography
what must the baseline be drawn in and why?
* pencil line
* will not dissolve in the solvent
38
# 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
39
# thin-layer chromatography
how do stationary phases separate?
* solid stationary phase- by adsorption
* liquid stationary phase- by relative solubility
40
# 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
41
# 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
42
# 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
43
# 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
44
# 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
45
# 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)
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