CPAC Flashcards
(53 cards)
1
Q
CPAC 1 : finding the molar volume of a gas
method
A
Apparatus : conical flask, bung, gas syringe
1. Place 30cm3 (excess) ethanoic acid in conical flask, add preweighed marble chips (CaCO3) and quickly place bung on
2. Measure volume of gas produced
3. Repeating for increasing masses of CaCO3
2
Q
What are some of the issues with CPAC 1 : finding molar volume of a gas
A
- some gas may escape before bung is added
- co2 is slightly soluble in water so the exact volume is not measured as some may dissolve in solution
3
Q
CPAC 1: finding the molar volume of a gas
How can we prevent gas escaping before bung is replaced ?
A
- place the solid reactant upright in a sample tube in the conical flask, tipping the tube over by moving the conical flask around to start the reaction
4
Q
CPAC2: Preparation of a standard solution and titration
How is a standard solution made ?
A
- measure, using a balance, the mass of solid required
- transfer this to a volumetric flask and rinse the remaining weighing boat content (with distilled water) into the flask so no solid is lost
- add a volume of distilled water to dissolve the solid. Swirl to mix
- add more distilled water up to the graduation line. Invert multiple times to mix
5
Q
What are concordant results ?
A
Titres that are within 0.1cm3 of eachother
6
Q
CPAC 3: titration
How do you carry out the titration?
A
- once the pipette has been used to place HCl into conical flask, fill burette with NaOH and record initial volume
- add few drops of indicator (phenolphthalein) to the conical flask
- open burette and allow NaOH to flow into conical flask, swirling it to mix contents
- close burette tap once expected colour change occurs (use white tile so easy to identify)
- record final burette volume
- repeat until you get concordant results, then calculate a mean titre
7
Q
CPAC 3 : titration
Why might phenolphthalein used in this titration turn colourless at the end point if you leave the solution to stand ?
A
Because NaOH reacts with CO2 from the air to form Na2CO3
8
Q
CPAC 4: rate of hydrolysis of halogenoalkanes
Method
A
- In 3 different test tubes add 4 drops of 1chlorobutane, 1bromobutane and 1iodobutane
- to each add 5cm3 of ethanol. Place all the east tubes in a 50 degrees water bath
- pour ~5cm3 of silver nitrate into 3 test tubes. Place them in the water bath
- when all solutions have reached 50 degrees add silver nitrate to haloalkane-ethanol solutions
- start the stop clock. Measure the time taken for each ppt to appear
9
Q
CPAC 4: hydrolysis of halogenoalkanes
How can you decrease the uncertainty in time taken for ppts to appear ?
A
Use a lower temperature to reduce the rate of reaction
This will make the time taken for ppts to appear take longer and therefore % uncertainty will be lower
10
Q
CPAC 5: oxidation of ethanol
What 2 methods can be used
A
- Distillation apparatus : pear shaped flask, Liebig condenser, still head, stopper, thermometer and collection vessel
- ethanol is distilled with acidified potassium dichromate and is oxidised to an aldehyde (ethanol) , water also produced
- orange - green colour change - Reflux apparatus : boiled in a pear shaped flask, condenses due to water in Liebig condenser and heated again to a carboxylic acid
11
Q
Why is heating under reflux used?
A
- allows heating for a long period of time
- prevented the flask from boiling dry
- prevents volatile reactants/products escaping
- ensures even heating
12
Q
CPAC 5 : oxidation of ethanol
Why are anti-bumping granules used when heating under reflux/distillation
A
To allow smooth boiling
Prevent the appearance of bubbles caused by hot vapour in the hot liquid that would splash up the sides of the flask
13
Q
CPAC 6: chlorination
How do you form a crude product?
A
- add conc. HCl and 2methylpropan2ol to a conical flask. Put the rubber bung in and swirl
- open the bung to release pressure (repeat regularly over 20mins)
- add some anhydrous calcium chloride and shake
- should be 2 distinct layers
- upper layer = organic product. Lower layer = aqueous layer
14
Q
CPAC 6: chlorination
How do you separate the organic layer form the aq layer ?
A
- transfer the contents of the flask into a separating funnel
- allow the layers to separate and remove aq layer by opening the tap
- keep the organic layer in separating funnel
15
Q
CPAC6: chlorination
How do you remove the unreacted HCl?
A
- add solution of NaHCO3. Stopper the separating funnel and shake
- invert and open the tap to release pressure. Repeat a few times
- remove stopper and run off aq layer. Then run organic layer into a clear conical flask
16
Q
CPAC6: chlorination
How do you remove any water form the organic liquid ?
A
Add some anhydrous sodium sulfate because it acts as a drying agent
Then filter
17
Q
CPAC6: chlorination
How do you purify the product ?
A
- distillation
- separates the products based on boiling point
- collect the liquid boiling between 50-52 degrees - this should be the pure product
18
Q
CPAC 8: enthalpy change using Hess’ law
How can you reduce the uncertainty in mass measurement ?
A
- use a balance with greater resolution
- use a larger mass
19
Q
CPAC 8: enthalpy change using Hess’ law
Why can the enthalpy change of a thermal decomposition reaction not be measured directly?
A
- as thermal energy must be supplied for the reaction to occur, the temperature change measured is not only due to decomposition
20
Q
CPAC 8: enthalpy change using Hess’ law
Why might an experimental value for enthalpy chnage be different to the theoretical value ?
A
- heat loss to apparatus/surroundings
- incomplete combustion
- non-standard conditions
- evaporation of alcohol/water
- we assume the shc of the solution to be that of water (4.18)
21
Q
CPAC 8: enthalpy change using Hess’ law
How do we prevent heat loss to surroundings/apparatus ?
A
- insulate by placing the reactants in polystyrene cup with a lid into the beaker
- avoid large temperature differences between surroundings and calorimeter
- use a bomb calorimeter
22
Q
CPAC 8: enthalpy change using Hess’ law
Other than preventing heat loss, how can the accuracy of this experiment be improved ?
A
- read the thermometer at eye level to avoid parallax errors
- stir the solution so the temperature is evenly distributed
- use a digital thermometer and dats logger for more accurate and faster readings
- use greater concentrations and masses, leading to a greater temperature change and thus smaller uncertainty
23
Q
CPAC 9: finding Ka
How do you calibrate a pH probe ?
A
- submerge pH probe in buffer solutions of 3 different pHs including pH7 and usually pHs 4 and 10. Each time press the calibrate button
24
Q
CPAC 9: finding Ka
How do you carry out a titration to calculate pH?
How do you calculate Ka from this ?
A
- into a conical flask add 25cm3 of 0.1moldm-3 ethanoic acid solution with few drops of indicator
- NaOH solution goes in burette
- titrate the 2 until mixture just turns colour
- add another 25cm3 of 0.1moldm-3 ethanoic acid into conical flask
- record pH of resulting solution
- in the solution only half the acid has been neutralised therefore [HA] = [A-]
- you can therefore cancel [HA] and [A-] in Ka equation, therefore Ka = [H+]
- convert the resulting solutions pH into [H+] to give a value for Ka
25
CPAC 9: finding Ka
What should we remember when using a pH meter
- test the pH meter on a buffer solution of known pH
- wash with deionised water between readings to remove ions attached to bulb
- pH meter is better than indicators because no ions are added or removed when used, it reads to 2dp which is far more accurate than interpreting colours of indicators and is not subjective
26
CPAC 10: electrochemical cells
What does salt bridge allow ?
Transfer of ions
27
CPAC 10: electrochemical cells
Why do you need to file/sand away the outer layer of the metal
It removes the oxide layer on the outside of the metal
28
CPAC 11: redox titration
How do you carry out a titration with potassium manganate (VII)
- add the potassium manganate (VII) solution to the burette
- the other reactant solution is acidified with dilute H2SO4 and placed into the conical flask
- the potassium manganate (VII) solution flows into the flask and as it reacts it becomes colourless. When there is the first trace of a permanent pale pink solution close the tap
29
CPAC 12: transition metal complex preparation
Method
- weigh out the mass of copper sulfate accurately and dissolve in water
- in fume cupboard add conc NH3. Stir the mixture and pour into ethanol. Then cool mixture in ice bath. Crystals of product will form
- filter crystals via vacuum filtration
- record mass and calc % yield
30
CPAC 12: transition metal complex preparation
How do you use lab equipment to filter under reduced pressure?
Using a Büchner funnel and Büchner flask connected by rubber tubing to vacuum source
- the funnel contains layer of filter paper
- pour substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask
- wash the solid with cold solvent
- the solid will remain on the paper
31
CPAC 12: transition metal complex preparation
How do we prevent solid being lost in vacuum filtration ?
Wash the transferring flask with solvent ie. Ethanol and pour onto the filter paper in the Büchner funnel
32
CPAC 13 : iodine reactions
Give an example of a continuous monitoring method to find rate of reaction and explain how you would analyse the results
- mix propanone with sulfuric acid and iodine in a beaker. Start a stopwatch
- using a pipette, remove a sample of the mixture and add NaHCO3. This stops the reaction. Note this time
- titrate the remaining iodine present In the sample with sodium thiosulfate(VI) solution using starch as an indicator
- repeat titrations with samples taken every 3 mins (treat with NaHCO3)
Analysis of results
- plot a graph of titre against time (conc of iodine is proportional to titre)
- by comparing the shape of the graph to known conc-time graphs determine the order with respect to I2
33
CPAC 13 : iodine reactions
Give an example of a initial rates monitoring method to find rate of reaction and explain how you would analyse the results
- the iodine clock experiment
H2O2 + 2H+ + 2I- —-> I2 + 2H2O
2S2O3^2- + I2 —-> 2I- + S4O6^2-
- the I2 produced reacts with all the thiosulfate ions present
- excess I2 remains in solution which then reacts with starch to form a blue-black solution
- time how long it takes for blue black colour to appear
- you can then vary the [I-] to then determine the order with respect to iodide ions
34
CPAC 13 : iodine reactions
What are some issues with this experiment ?
- some [I-] concentrations may take too long to react
- delayed stopwatch reactions
- concentrations may not be exact due to measuring apparatus
35
CPAC 13 : iodine reactions
In the continuous monitoring method what is the role of H2SO4 and why are excesses of both propanone and sulfuric acid used ?
H2SO4 acts as a catalyst
Large excess are used so their concentrations remain effectively constant during the reaction allowing us to measure the influence of iodine on RoR
36
CPAC 14: activation energy
What is an example method to determine the activation energy of a reaction ?
How would you analyse this data?
- repeat for various temperatures 15-75 degrees (in water baths)
- add equal volumes of bromide/bromate solution and phenol. Add methyl red indicator
- add H2SO4 solution and time how long it takes for solution to go colourless
Analysis of data:
- plot on a graph lnt against 1/T
- the gradient = Ea/R
- rearrange for Ea
37
CPAC 14: activation energy
Why sometimes is a log scale used?
- To show large range of values without compressing the scale
- to show percentage change or multiplication factors
38
CPAC 16: synthesis of aspirin
What are the main steps in producing a pure organic solid ?
1. Synthesis of the compound (usually during reflux/distillation)
2. Filtration (usually vacuum filtration)
3. Purification (recrystallisation)
39
CPAC 16: synthesis of aspirin
What compounds are heated under reflux together in the first step to create aspirin ?
- 2hydroxybenzoic acid and ethanoic anhydride with a few drops of h2so4
- filter product using vacuum filtration
40
CPAC 16: synthesis of aspirin
How do you purify a solid product ?
By recrystallisation
- add minimum amount of warm solvent to impure sample until it has dissolved
- allow to cool, crystals should form (can do this in ice bath if want more crystals)
- when no more form you can filter under reduced pressure, washing the solid with solvent to obtain a dry crystalline solid
41
CPAC 16: synthesis of aspirin
How do you determine the melting point of a substance and why can this information be useful?
- place a small amount of the solid in a capillary tube
- melt using the melting apparatus available measuring the melting temperature with a thermometer
- a pure substance will usually melt at a single temperature
- record the starting and ending points of the melting
- you can then compare melting point to known values to identify the substance
42
What is steam distillation ?
Separates an insoluble liquid from an aqueous solution
Involves passing steam into a reaction mixture that contains an aq solution and a liquid that forms a separate layer
The agitation of the liquid caused by the steam bubbling through the mixture ensures both the insoluble liquid and the aq solution are in the surface of the mixture, so can form part of the liquid that evaporates
43
What is the advantage of the process of steam distillation ?
Include an example
The insoluble liquid is removed from the reaction mixture at a temperature below its normal boiling temperature
Eg. Reaction mixture containing phenylamine separated using steam distillation = phenylamine distils at a much lower boiling temperature = less chance of decomposition
44
If a liquid moves **less far** up the paper in paper chromatography what can we say about its attractions to the 2 phases ?
Component is strongly attracted to stationary phase but weakly attracted to the mobile phase
45
If a liquid moves **further** up the paper in paper chromatography what can we say about its attractions to the 2 phases ?
Component is strongly attracted to mobile phase but weakly attracted to the stationary phase
46
What is the difference between paper chromatography and TLC ?
Instead of paper a sheet of glass or plastic, coated in a thin layer solid such as silica or aluminia
47
Describe column chromatography
Same principles as TLC
Stationary phase is aluminia or silica packed into a burette and soaked in a solvent
The mixture is packed on top of the stationary phase and more solvent (mobile phase) is added on top
When the tap is opened, the solvent drips through and the components of the mixture begin to move down the tube and separate
More solvent is added on top and eventually one component leaves the column and can be collected in a container
48
What is the advantage of column chromatography over paper chromatography ?
Much larger quantities of material can be separated
49
High performance liquid chromatography (HPLC) is a type of column chromatography.
What are the main differences ?
- the solvent is forced through a metal tube under high pressure rather than being allowed to pass through by gravity
- particle size of the stationary phase is much smaller which leads to better separation of the components
- the sample is injected into the column
- the components are detected after passing through the column, usually by their absorption of UV light
- the process is automated and the results are quickly available on a computer screen
50
What variables do the retention **time** values depend on in HPLC?
- the nature of the solvent
- the pressure used
- the temperature inside the column
51
Gas Chromatography (GC) is a type of column chromatography
In what ways does it differ ?
- the metal tube can be several meters long and can be coiled to save space
- the stationary phase is a solid of liquid coated on the inside of the tube
- the mobile phase is an inert carrier gas (often nitrogen or helium)
- the sample is injected into the column
- the components passing through the column are detected
- process is automated and results are quickly available on a computer display
52
Once the sample is injected in GC what happens ?
The components vaporise and move through the coiled tube with the carrier gas
They move at different speeds depending on attraction to stationary phase
Weaker attraction = move faster and have shorter retention times
53
What is the one disadvantage of GC compared to column chromatography
In GC components are often detected in a flame so they are destroyed during the detection process
