Practical Skills Flashcards
1
Q
How should each risk assessment be carried out
A
Identify the risk
A way to reduce/ eliminate the risk
2
Q
What is a standard solution
A
A solution containing a known mass of solute dissolved in a known volume of solution
It is made up in a volumetric flask (250cm3)
3
Q
How are standard solutions used
A
Use identical samples (usually 25 cm3 with a pipette) of this standard solution in a series of titrations
Meaning each titration should give the same result allowing concordant results
4
Q
What is the method of creating a standard solution
A
Weigh a mass of solute out in a weighing boat
Tip the solute into the volumetric flask, the re-weigh the boat
Water is added and the flask shaken to dissolve the solute
Once dissolved, the flask is made up to 250 cm3 with good shaking
Shaking when the flask is full is hard to shake mostly before it is too full
The bottom of the meniscus should be on the line
5
Q
What is the method for a titration
A
Use 25 cm3 of standard solution is placed in conical flask with pipette
Add suitable indicator
Solution is added from burette until the indicator changes colour. Add drop by drop near end point and wash sides of flask to make sure all reactants are in solution
Repeat titration until concordant results achieved
6
Q
What are concordant results
A
Results that are close together so is repeatable
Titres be within 0.10 cm3
Only use concordant results for the mean
7
Q
What must be remembered when doing a titration
A
Pipette and burette should be rinsed with solution so no water dilutes the solution
Conical flask should be rinsed with water, so you know the exact moles in the flask
8
Q
What are common acid-alkali indicators
A
Methyl Orange
Phenolphthalein
9
Q
What are the colours of Methyl Orange
A
Red in acid
Yellow in alkali
End point is red
10
Q
What are the colours of Phenolphthalein
A
Colourless in acid
Pink in alkali
End point is colourless
11
Q
What are back titrations
A
Some acids or bases aren’t very soluble in water so a back titration is used
Add a known amount of acid or alkali that is an excess to the sample and then titrate the leftover acid/alkali to see how much is left
12
Q
What is a common reactant in redox titrations
A
Potassium manganate (Usually with an acid)
13
Q
Why is an indicator not needed when potassium manganate is used
A
The manganate ion itself acts as an indicator (end point is colourless to first hint of pink if manganate is added from burette)
14
Q
What are common redox titrations
A
with Fe2+
with C2O4 2-
15
Q
What is the reaction between manganate ions and iron ions
A
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+
16
Q
What is the reaction between Manganate ions and ethanedioate ions
A
2MnO4- + 16H+ + 5C2O4 2- -> 2Mn2+ + 8H2O + 10CO2
17
Q
What is the manganate and ethandioate ions reaction like
A
Slow as reactive between 2 negative ions, so they repel
Mn2+ acts as catalyst to speed up the reaction once some has been formed (autocatalysis)
18
Q
What is the method of heating to constant mass
A
Some compounds are analysed by measuring mass change as they are heated
Reaction mixture is heated and mass is repeatedly measured, once the mass stops changing the reaction has stopped
This method can be used to find formula or Mr of reactant or product
E.g. Thermal decomposition of hydrated salts
19
Q
What is a volatile liquid
A
A liquid that easily turns into a gas
20
Q
How can the Mr of a volatile liquid be found
A
By vapourising a known mass at a known temperature and pressure
Volume is then measured and ideal gas equation can be used in calculation
Liquid is injected from a hypodermic syringe into a gas syringe
The mass of the liquid vapourised is found by weighing the syringe before and after use
21
Q
What are 2 common errors of finding the Mr of a volatile liquid
A
Some of the liquid evaporating from the syringe during use, and some of the liquid whose mass has been measured does not turn into a gas in the gas syringe
The apparatus may not be at the desired temperature - often a glass syringe is used in a steam jacket or oven and the glass is cooler than the temperature shown
22
Q
What does the specific heat capacity of water mean
A
The heat energy to make 1g of water 1 degree hotter
4.18 J g-1 K-1
23
Q
Where is a calorimetry reaction normally done
A
A polystyrene cup to prevent heat loss/gain
24
Q
What is the equation to find the heat given out
A
q = mcAT
25
How do you convert from kJ to kJ/mol
kJ / moles reacting
26
How do you plot the graph for a calorimetry
Temperature is recorded every minute
Plot temperature against time where the data takes up more than half the graph
The graph can be extrapolated back to the time at which the reagents were added
Temperatures before the maximum/minimum temperature should be ignored for line of best fit
27
What is the main problem with calorimetry
Heat loss
Reduced using better insulation
Other errors can be incomplete combustion of fuel/ loss of fuel to evaporation
28
What is the apparatus uncertainty equation
margin of error/quantity measured x 100
29
What is apparatus uncertainity
The small uncertainty in that measurement due to the apparatus
30
What are the types of apparatus uncertainity
Measurements with:
1 error
2 error
Special cases
31
What apparatus has 1 error
Pipette
Measuring cylinder
Temperature
Volumetric flask
32
What apparatus has 2 errors
Temperature change
Mass change
Change in volume with a burette
33
What is the special case
Measurement of volume needed to reach the end point of a titration
Start, end and end point reading
34
What is the equation for experimental error
(real answer - experimental answer)/real answer x100
35
What does it mean if the experimental error is smaller than apparatus error
Then you have an accurate result
But if the experimental error is bigger than the result is inaccurate
36
How are reactants mixed
In a beaker, conical flask or Quick-Fit flask
37
What is used if the reactants need to be cooled or heated
An ice bath is used for cooling
To be heated, reflux is usually used so the reactants boil without escaping
38
What is the point of anti-bumping granules
To form smaller bubbles when heated
Or there may be a sudden release of a large bubble of vapour that makes the reaction jump
39
How is the product of a reaction separated if it is a liquid
Simple distillation
Chemicals boiling at a range close to that of the products collected
40
How is the product of a reaction separated if it is a solid
Normally filtration
Some reactions, something like cold water has to be added to make the product solidify
Filtration can happen under reduced pressure using Buchner apparatus
41
How can liquids be purified
Separating funnel
Distillation
42
How can liquids be purifies by a separting funnel
The liquid is placed in a separating funnel
Water is added so some of the impurities dissolve or sometimes a solution is added
2 layers will form in the funnel, the denser one on the bottom and the tap can remove the bottom one
Add an insoluble drying agent to remove any water
Decant the organic liquid from the drying agent
43
How can the liquids be separated by distillation
The liquids are separated based on their boiling points
Fractional distillation for liquids with similar boiling points
44
What happens during recrystallisation
The product is dissolved in the minimum volume of a hot solvent
Remove any insoluble impurities by filtration while hot
Product crystallises as filtrate cools
A further filtration is needed to remove soluble impurities
45
What are the things to remember whilst doing a re-crystallisation
The compound should be more soluble in hot water than cold water
Filter the hot solution with hot glassware so it doesn't cool
Don't dry product by cooking or compound would melt
46
How is the purity of a liquid normally determined
Finding the boiling point of the substance
The narrower the range of BP and the closer the range is to the true value the purer the product
Record the range of temperatures the product is collected during a distillation
47
How is the purity of a solid normally determined
The melting point of the solid
Place a capillary tube strapped to a thermometer in oil
Heat oil slowly until the solid melts
The greater the purity, the narrower the range of MP's and the closer to the true value
48
What is the equation for percentage yield
Mass of product/Maximum theoretical mass of product x100
49
How is pH measured
Using a probe
Calibrated by buffer solutions or adjusting pH until it matches the true value
A calibration curve can be plotted as well of true pH and pH on probe
50
How can the rate of a reaction be found
Measuring the gradient of a concentration/time graph
51
How can a kinetics practical be improved
Using thermostatically controlled water bath
52
What 2 ways can the order of a reaction be found using graphs
Initial rates method
Continuous rates method
53
How is the initial rates method done
Do multiple reactions with different starting concentrations
Measure initial rate of each one
Plot rate/ concentration line
54
How is the continuous rates method done
Measure the rate at several points during 1 reaction
Plot rate/concentration line
55
How are both graphical methods done with a reaction which produces a gas
Initial rate - Measure volume of gas vs time a plot graph. Gradient is equal to the rate
Continuous rate - Measure the volume of gas produced vs time. Find gradient at several times = rate of reaction
56
How are both graphical methods done with a reaction which shows a colour change
Initial rate - Use a colorimeter to measure time to reach specific point. Rate is equal to 1/time
Continuous rate - Use colorimeter to measure absorbance at various times. Plot graph of absorbance vs time and find gradient = rate
57
How are both graphical methods done with a reaction which turns cloudy
Initial rate - Time how long it takes to reach a point where you can't see through the mixture. Rate = 1/time
Continuous rate - Not possible
58
How are both graphical methods done with a reaction which happens through quenching
Initial rate - Stop reaction by quenching Measure concentration of product at that point and calculate the rate by change in concentration/time
Continuous rate - At intervals take a sample of mixture and stop it by quenching it ad measure concentration. Plot concentration/time and find gradient=rate
59
What does absorbance of a mixture depend on
Concentration of solution
The higher the concentration, the more it absorbs
60
How is a colorimeter used
Some ions need to change ligand to intensify the colour
Light is chosen which will be absorbed
Plot calibration curve from data on absorbance and concentration of compound
Use absorbance to find concentration of compound with unknown concentration
61
What determines the speed of a substance in chromatography
Relative affinity to stationary and mobile phase
62
What are common stationary phases in TLC
SiO2 (silica) and Al2O3 (alumina)
Regarded as a polar powder
63
What are common mobile phases in TLC
Non-polar solvents - Alkanes
Polar solvents - Water, Alcohols
64
What is the method for TLC
Draw a pencil line 1cm from bottom of plate
Place samples along the line
Stand TLC plate upright in a solvent
Solvent soaks up TLC plate
Remove the plate when solvent reaches the top of the plate, mark top of solvent
Leave plate to dry
Measure Rf values
65
How can spots from TLC be seen
UV light
Staining the plate with ninhydrin
66
How does the solvent move up the plate
Capillary action
67
How do you measure the EMF of a cell
Join the metals together with wire (electrons)
Join solutions with a salt bridge (ions flow)
Voltmeter is often included in circuit to measure potential difference (EMF)
Standard potentials are measured against Standard Hydrogen Electrode
68
What is the salt bridge like
A piece of filter paper soaked with solution of unreactive ion
Or
Tube containing unreactive ions in an agar gel
69
What compound are normally used in salt bridges
KNO3
K+ and NO3- are quite unreactive
70
How must you write your observations
Must indicate what reagents looked like before and during mixing
State colour and what state they are in
2 different observations when reagent added dropwise until excess - 1 during dropwise and 1 during excess
Requires a observation before and after if asked to let mixture stand
71
What is the test for double carbon bond
Add a few drops of bromine water
Positive - Orange to colourless solution
Negative - No change
72
What is the test for primary, secondary alcohols and aldehydes
Warm acidified potassium dichromate
Positive - Orange to green solution
Negative - No change
73
What is the test for aldehydes
Warm Tollens reagent
Positive - Colourless solution to silver mirror
Negative - No change
Warm Fehlings solution
Positive - Blue solution to orange/red precipitate
Negative - No change
74
Test for carboxylic acids
Sodium carbonate
Positive - Bubbles/Effervescence
Negative - No change
75
Test for acyl chloride
Water
Positive - White fumes
Negative - No change
76
Test for halogenalkanes
Warm NaOH and Silver Nitrate
Positive - Colourless solution to white (Cl), cream (Br), yellow (I) precipitate
Negative - No change
77
Test for sulphate ion
Nitric acid and barium nitrate
White precipitate - Barium sulphate
78
Test for carbonate ions
Add acid and test gas by bubbling through lime water
Lime water turns cloudy
79
What is the test for magnesium ion
NaOH - White precipitate, insoluble in XS
H2SO4 - No change
80
What is the test for calcium ion
NaOH - White precipitate, insoluble in XS
H2SO4 - White precipitate
81
Test for strontium ion
NaOH - No change
H2SO4 - White precipitate
82
Test for barium ion
NaOH - No change
H2SO4 - White precipitate
83
Test for ammonium ion
Warm and gas produced turns damp red litmus paper blue
NH4+ + OH- -> NH3 + H2O
84
Test for copper ion
NaOH - Blue precipitate, insoluble in XS
NH3 - Blue precipitate, soluble in XS to give deep blue solution
Na2CO3 - Blue/green precipitate
H2SO4 - No change
HCl - Green-yellow solution
85
Test for iron 2+
NaOH - Green precipitate, insoluble in XS
NH3 - Green precipitate, insolble in XS
Na2CO3 - Green precipitate
H2SO4 - No change
HCl - Yellow solution
86
Test for iron 3+
NaOH - Brown precipitate, insoluble in XS
NH3 - Brown precipitate, insoluble in XS
Na2CO3 - Green precipitate and effervescence
H2SO4 - Yellow to pale violet solution
HCl - Yellow solution
87
Test for aluminium ion
NaOH - White precipitate, soluble in XS
NH3 - White precipitate, insoluble in XS
Na2CO3 - White precipitate and bubbles
H2SO4 - No change
HCl - Colourless solution
88
How can the yield from a distillation be maximised
Correct Temperature
Cool the distillate
89
How does the water flow in a condenser jacket
The water flows in at the bottom and out at the top
90
How do you collect data for the determination of enthalpy change
Weigh out in a dry stoppered weighing bottle, keeping the stock of solid in a closed container during weighing.
Construct a table of results to record temperatures at minute intervals up to 15 minutes.
Place 25 cm3 of deionised water into a polystyrene cup and record its temperature at the beginning, start the timer and then record the temperature again every minute, stirring the liquid continuously
At the fourth minute, add the powdered anhydrous copper(II) sulfate rapidly to the water in the polystyrene cup and continue to stir, but do not record the temperature. At the fifth minute and for every minute up to 15 minutes, stir and record the temperature of the solution in the polystyrene cup.
Plot temperature against time. Draw two separate best fit lines, extrapolating both lines to the fourth minute.
f) Use your graph to determine the temperature change at the fourth minute
91
Whats the method for determining the change in the rate of reaction with temperature
Add hydrochloric acid or sulfuric acid to the
‘acid’ tube. Place this tube into the hole in the plastic container without the cross under it
Use a measuring cylinder to add sodium thiosulfate solution to the second tube. Place this tube into the hole in the plastic container with the cross under it and place a thermometer in this tube.
Note the start temperature and then add 1 cm3 of the acid to the thiosulfate solution and start timing.
Look down and record the time for the cross to disappear
Record the temperature of the reaction mixture. Pour the cloudy contents of the vial into the sodium carbonate solution (the ‘stop bath’).
Now change the temperature of the water
g) Measure another sodium thiosulfate solution into a clean tube.
Leave the tube to warm up for about 3 minutes. Repeat to obtain results for at least 5 different temperatures in total.
92
Whats the method for a dehydration and distillation
Pour cyclohexanol into a weighed 50 cm3 pear-shaped flask. Reweigh the flask and record the mass of cyclohexanol.
b) Using a pipette, add to the flask concentrated phosphoric acid. Add a few anti-bumping granules to the flask
and place in distillation apparatus
d) Pour the distillate into a separating funnel and add saturated sodium chloride solution allowing the 2 layers to form
e) Carefully run off the lower layer into a beaker and then transfer the upper layer, which contains the crude cyclohexene, into a small conical flask.
f) Add a few lumps of anhydrous calcium chloride to the crude cyclohexene to remove water. Shake the contents and allow this to stand until the liquid becomes clear.
93
Whats the method for a alcohol to an aldehyde
Using a 25 cm3 measuring cylinder, measure acidified sodium dichromate(VI). Pour this oxidising agent into a boiling tube.
b) Cool the boiling tube in cold water in a beaker.
c) Using a 10 cm3 measuring cylinder, carefully measure out 2 cm3 of ethanol.
d) Using a teat pipette, slowly add the 2 cm3 of ethanol dropwise, to the oxidising agent in the cooled boiling tube
e) After the addition of ethanol, add a few anti-bumping granules to the boiling tube and attach to it a bung fitted with a right-angled glass delivery tube.
f) Clamp the boiling tube at about 45° in a beaker of water. Heat this beaker of water gently and slowly distil off approximately 5 cm3 of liquid distillate into a test tube which is immersed in cold water in a beaker. Keep the test tube cool to avoid loss of the volatile ethanal.
94
Measuring the rate of reaction by an initial rate method
Rinse a 50 cm3 burette fill with potassium iodide solution.
b) Transfer 10.0 cm3 of hydrogen peroxide solution from the shared burette provided to a clean, dry 100 cm3 beaker.
c) Use a 50 cm3 measuring cylinder to add sulfuric acid to a clean, dry beaker.
d) Use a 25 cm3 measuring cylinder to add distilled or deionised water into the 250 cm3beaker.
e) Use a pipette to add starch solution to this beaker.
f) Use your burette to add 5.0 cm3 of potassium iodide solution to the mixture in the 250 cm3 beaker.
g) Finally, add 5.0 cm3 of sodium thiosulfate solution from the shared burette provided to the mixture in the 250 cm3 beaker.
h) Stir the mixture in the 250 cm3 beaker. Pour the hydrogen peroxide solution from the 100 cm3 beaker into the 250 cm3 beaker and immediately start the timer. Stir the mixture.
i) Stop the timer when the mixture in the 250 cm3 beaker turns blue-black.
95
Measuring the EMF of an electrochemical cell
Clean a piece of copper using emery paper or fine grade sandpaper.
Connect the positive terminal of the voltmeter to the copper using a crocodile clip and one of the leads.
c) Cut a piece of filter paper to about the same area as the copper, moisten the filter paper with the sodium chloride solution and place on top of the copper.
d) Connect the second lead to the voltmeter and use the crocodile clip on the other end of the lead to grip a piece of another metal.
e) Hold the metal against the filter paper and note the voltage reading and sign.
96
Investigation of how the pH of a solution of ethanoic acid changes as sodium hydroxide solution is added.
Fill the burette with this ethanoic acid, ensuring that it is filled below the tap.
b) Use the burette to transfer exactly 20.0 cm3 of ethanoic acid to a clean 100 cm3 beaker.
c) Fill this second burette with this NaOH, ensuring that it is filled below the tap.
d) Rinse the pH probe with distilled or deionised water and clamp it so that its bulb is fully immersed in the ethanoic acid solution in the beaker. Use a rod to stir the solution gently and record the pH reading in a suitable table.
Add the NaOH solution in 2.0 cm3 portions from the second burette to the ethanoic acid in the beaker until 18 cm3 of the NaOH solution have been added. Take a pH reading after each addition of NaOH solution
Then add the NaOH solution in 0.20 cm3 portions until 22.0 cm3 is reached.
Then add the NaOH solution in 2.0 cm3 portions again until 40 cm3 have been added.
97
How to calibrate the pH meter
Rinse the pH probe thoroughly with deionised water, and shake it gently to remove excess water. Place the probe in the standard pH 7.00 buffer solution provided, ensuring that the bulb is fully immersed. Record the pH reading in a suitable table.
b) Repeat this process using the standard pH 4.00 and 9.20 buffer solutions. Rinse the pH probe thoroughly with deionised water before taking each reading. Record the pH readings in your table.
c) Plot a graph of your recorded pH reading (x-axis) against the pH of the buffer solution.
98
Purification of aspirin
Using a 25 cm3 measuring cylinder, measure out 15 cm3 of ethanol into a boiling tube.
b) Prepare a beaker half-filled with hot water at a temperature of approximately 75 °C. the beaker until the temperature is at approximately 75 °C.
c) Use a spatula to add the crude aspirin to the boiling tube and place the tube in the beaker of hot water. Do not scrape the filter paper.
d) Stir the contents of the boiling tube until all of the aspirin dissolves into the ethanol.
e) Pour the hot solution containing dissolved aspirin into approximately 40 cm3 of water in a 100 cm3 conical flask.
f) Allow the conical flask to cool slowly and white needles of aspirin should separate.
h) Filter off the purified solid under reduced pressure and allow it to dry on filter paper.
i) Record the mass of the dry purified solid.
99
Testing the purity of a compound
Powder a sample of the organic solid by crushing it gently with a spatula onto the surface of a filter paper.
b) Fill three melting point tubes with the organic solid to a depth of approximately 0.5 cm.
c) Set up the melting point apparatus provided and mount one of the melting point tubes ready for taking a measurement.
d) Heat the apparatus gently and observe the temperature at which the solid collapses into a liquid.
e) Allow the melting point apparatus to cool and repeat the measurement of the melting point of the solid with the other two samples.
f) On the basis of the three measurements that you have taken, record the melting point of the organic solid.
g) Compare the data book value with your own.
100
Making an ester
Put a few anti-bumping granules in a 50 cm3 pear-shaped flask.
b) In a fume-cupboard, add 10 cm3 ethanol, 12 cm3 glacial ethanoic acid and 15 drops of concentrated sulfuric acid to the flask.
c) Place a 250 cm3 beaker containing some water on a tripod and gauze over a Bunsen burner.
e) Add a condenser so that it is set up for heating with reflux.
f) Light the Bunsen burner to heat the hot water bath. Raise the temperature of the hot water until the mixture in the flask is gently boiling. Continue the gentle boil of the reaction mixture for about 15 minutes. Cool the mixture
g) Prepare a saturated solution of sodium carbonate by combining 4.5 g of sodium carbonate with 15 cm3 of distilled water in a 100 cm3 beaker.
h) In a fume cupboard, transfer the reaction mixture from the pear-shaped flask to the beaker and stir.
i) Transfer the mixture to a separating funnel. Stopper it and turn it upside down gently and then open the stopcock to vent the system. Invert at least 15–20 times, opening the stopcock each time.
j) Allow the two layers to separate. Ethyl ethanoate is less dense than water, therefore the top layer is ethyl ethanoate.
k) Remove the stopper, open the stopcock and slowly drain off the waste aqueous layer into a 50 cm3 waste beaker, then close the stopcock.
l) Transfer the remaining ethyl ethanoate into a dry boiling tube containing about 1 g of anhydrous sodium sulfate. Agitate the tube so that any water is absorbed into the anhydrous solid.
101
Chromatography
Carefully use a pencil to draw a faint line 1 cm above the bottom of a TLC plate and mark five spots, equally spaced along this line.
b) Use a capillary tube to apply a tiny drop of each solution to a different origin spot and allow the plate to air dry.
c) Add approximately 10 cm3 of ethyl acetate to a development chamber
d) Place the TLC plate into the developing chamber, making sure that the level of the solvent is below the spotting line. Replace the lid and make sure it is a tight seal.
When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent front with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to visualise the spots. Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.
102
How do you prepare a sample for chromatography
Use a pestle and mortar to crush the aspirin tablet and transfer to a weighing boat or bottle.
b) Dissolve approximately 0.1 g of the powdered tablet in 0.5 cm3 of ethanol.
c) Repeat steps (a) and (b) with the ibuprofen tablet and the paracetamol tablet.
d) Use a pestle and mortar to crush the caffeine tablet and transfer to a weighing boat or bottle.
e) Dissolve approximately 0.1 g of the powdered tablet in 7.0 cm3 of ethanol.
103
Why do you need to open the tap of the separating funnel occasionally
avoid pressure build-up
