PRACTICALS Flashcards
1
Q
When would heating in a crucible (ceramic) be used?
A
- measuring mass loss in various thermal decomposition reactions and
- for mass gain when reacting magnesium in oxygen
2
Q
Why must small amounts of solid not be used when heating with crucible
A
percentage uncertainties in weighing will be too high
3
Q
Why should large amounts of hydrated calcium sulfate e.g 50g not be used in heating with crucible
A
decomposition is likely to be incomplete
4
Q
Why must the crucible be dry
A
wet crucible can give inaccurate results. it would cause mass loss to be too large as the water would be lost when heating
5
Q
3.51 g of hydrated zinc sulfate were heated and 1.97 g of anhydrous zinc sulfate were obtained. Use these data to calculate the value of the integer x in ZnSO4.xH2O
A
6
Q
When would a gas syringe be used
A
when volume of gas is measured
7
Q
What is volume of a gas dependant on, how does this effect the experiment using gas syringe
A
pressure and temperature, so when recording volume it is important to note down the temp and pressure of room
8
Q
How do you calculate moles of gas using syringe
A
use pV=nRT
9
Q
What are 3 potential errors in using gas syringe
A
- gas escapes before bung is inserted
- syring sticks
- some gases e.g co2 are soluble in water so true amount of gas isnt measured
10
Q
Draw the set up of using ga syringe to measure volume of gas being released from a reaction
A
11
Q
How would you use a gas syringe to find mr of propane
A
- extract 0.2cm3 of propanone with HYPODERMIC syringe and weigh
- remove gas syringe from oven and note volume of air already in barrel
- inject propanone into SELF-SEAL cap into barrel. Plunger will move immediately
- put gas syringe back into oven
- measure mass of empty hypodermic syringe immediatly
- after few mins measure volume of gas in gas syringe, record temp of oven shelf and pressure of room
use pV=nRT to find moles
then mr= mass/moles
12
Q
How do you make a volumetric solution?
A
- calculate mass of required substance needed to make 250cm3 of 1moldm-3
- weigh sample bottle/weighing boat on top pan balance
- with weighting boat on scale, tare to 0 and add required mass of substance with spatula
- transfer substance to beaker and reweigh the weighing boat (record difference in mass)
- add 100cm3 of distilled h20 to beaker, use glass rod to stir and dissolve solid
- pour solution into 250cm3 graduated flask via funnel
- rinse beaker and funnel and add washings from beaker and glass rod to volumetric flask
- make up to mark its distilled water using dropping pipette for last few drops
- invert flask several times to ensure uniform solution
13
Q
why must the volumetic flask be shaken
A
ensure uniform concentration
14
Q
why cant you place graduated flask near heat or put hot solutions in it
A
heat can cause flask to expanse and the volume would be incorrect
15
Q
How is the mass measured accurately
A
measure on 2/3dp balance
measure weighing boat with and without mass of substance
find difference after emptying the mass into beaker
16
Q
why is volumetric pipette more accurate than measuring cylinder
A
has a smaller uncertainty
17
Q
how do you dilute a solution
A
pipette 25cm3 of solution and place into 250cm3 volumetric flask
make up to mark with distilled water using dropping pipette
invert flask several times to ensure uniform solution
18
Q
General method for undergoing acid-base titration
A
rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
pipette 25 cm3 of alkali into conical flask
touch surface of alkali with pipette ( to ensure correct amount is added)
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point
*use a white tile underneath the flask to help observe the colour change
*add acid to alkali whilst swirling the mixture and add acid drop wise at end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
19
Q
What are 2 indicators used in acid base titration and what are their colour changes and when should they be used
A
*phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH (strong alkali) is used]
*methyl orange [yellow (alkali) to red (acid): end point orange] [use if HCl (strong acid) is used]
20
Q
Typically what substance is placed in the conical flask of titrations
A
25cm3 of unknown conc solution
21
Q
Why is a conical flask preferable over beaker
A
easier to swirl mixture without spilling the contents
22
Q
Why is it important to rinse out the burette with the substance that’ll be put in it
A
prevents dilution from residual water or reaction with substances left from previous titration
conc of substance will get lowered and larger titre delivered
23
Q
What is an error that can be formed in the burette
A
jet space not filler properly prior to titration, larger titre reading than expected
24
Q
Why must only a few drops of indicators be used
A
they are weak acids so if too much gets aded they’ll affect titration result
25
Why does washing with distilled water towards end of titration not affect results
water doesnt react with reagents or change number of moles of acid added
26
When are values considered concordant and hence acceptable to use
when 2+ values are within 0.1cm3 of each other
27
What are some safety precautions to take in simple acid-base reaction
Acids and alkalis are corrosive
(at low concentrations acids are irritants)
Wear eye protection and gloves
If spilled immediately wash affected parts after spillage
If substance is unknown treat it as potentially toxic and wear gloves.
28
Testing batches
In quality control it will be necessary to do titrations/testing on several samples as the amount/concentration of the chemical being tested may vary between samples.
29
Common method taken from mark schemes:
1. Weigh the sample bottle containing the solid on a (2 dp) balance.
2. Transfer solid to beaker and reweigh sample bottle.
3. Record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved. 5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3 mark with distilled water. 7. Shake flask.
30
What is the uncertainty in reading a burette during titration?
±0.10cm3
May add ±0.5cm3 due to uncertainty in reading the end point so can be 0.15cm3
31
What are 3 ways to reduce uncertainties in titration
1. replace measuring cylinders with pipettes/burettes which have lower apparatus uncertainty which lowers overall error
2. to reduce uncertainty in burette reading the titre volume needs to be made larger. done by : INCREASING VOLUME AND CONC of substance in conical flask or by DECREASING CONC of substance in burette
32
What is the uncertainty in the volumetric flask and 25cm3 pipette?
both are ± 0.1cm3
33
How do you calculate percentage uncertainty
uncertainty / measurement made on apparatus
34
When looking at a series of measurements, which one will have the highest experimental uncertainty
the experiments with the smaller readings
35
If the %uncertainty due to the apparatus < percentage difference between the actual value and the calculated value then there is a discrepancy in the result due to other errors.
If the %uncertainty due to the apparatus > percentage difference between the actual value and the calculated value then there is no discrepancy and all errors in the results can be explained by the sensitivity of the equipment.
36
What is the equation used to find energy change
q=mcT
37
Whats the general method in measuring enthalpy change using a CALORIMETER
1. wash out equipment with solutions to be used
2. dry cup
3. put polystyrene cup in glass beaker for insulation and support
4. measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
5.clamp thermometer into place making sure the thermometer bulb is immersed in
solution
6. measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes
7. At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance.
8. If using a solid reagent then use ‘before and after’ weighing method
9. stirs mixture (ensures that all of the solution is at the same temperature)
10. Record temperature every minute after addition for several minutes
38
What can happen if the reaction of the enthalpy change is slow? How is the counteracted?
exact temperature rise is difficult to obtain as cooling occurs simultaneously with the reaction
to counteract: take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.
39
What graph is drawn when finding enthalpy change
temperature y-axis
time in mins x-axis
40
what errors are found in calorimetry
- energy transfer from surroundings (loss)
- assumption of all solutions having the heat capacity of water
- neglect SHC of calorimeter (ignore any energy absorbed by apparatus)
- reaction or dissolving may be incomplete or slow
- density of solution is assumed to be same as water
41
How would you find enthalpy change from experimental data
use data to find energy change with q=mct
find number of moles of reactants used
energy change / moles of reactants
ensure to convert value to match the units
42
When are hess's law cycles used to measure enthalpy change
for reactions that cant be measured directly by experiments
43
Detailed method for measuring enthalpy change of solution of anhydrous copper(II) sulfate
44
What do you use to experimentally find enthalpy of combustion
flame calorimetry
45
What are the dependant variables of flame calorimetry
need to measure:
mass of spirit burner before and after
temp change of water
volume of water in cup
46
What are potential errors in flame calorimetry
- energy losses from claroimetry
- incomplete combustion of fuel
- incomplete transfer of energy
- evaporation of fuel after weighing
- heat capacity of calorimeter not included
-measurements not carried out under standard conditions (e.g h20 is gas not liquid)
47
Describe method of investigating how temp affects ROR ( using sodium thiosulfate and HCL)
Measure 10 cm3 of 0.2 mol dm-3 hydrochloric acid and 10 cm3 of sodium thiosulfate in separate clean measuring cylinders. Put the solutions in separate boiling tubes
*Choose a temperature to investigate use water bath to get the two solutions to that temperature by placing the boiling tubes in the water bath.
*Place the flask on the centre of the large cross; first add the sodium thiosulfate to the flask. Then add the hydrochloric acid and start the stopwatch and swirl to mix the solutions.
*Stop the clock when the cross disappears and note the time.
*Repeat the experiment for four more different temperatures (maximum temperature should be 70 oC).
48
What graph is plotted for testing ROR changes with temp?
ln k = constant – EA/(RT)
k is proportional to the rate of reaction so ln k can be replaced by ln(rate)
From plotting a graph of ln(rate) (y-axis) against 1/T (x-axis)
activation energy can be calculated from measuring the gradient of the line
49
How do you test for group 2/3/ NH4+ ions
Method: adding dilute sodium hydroxide
a) Place about 10 drops of 0.1 mol dm–3 metal ion solution in a test tube.
b) Add about 10 drops of 0.6 mol dm–3 sodium hydroxide solution, mixing well.
c) Continue to add sodium hydroxide solution, dropwise with gentle shaking, until in excess
(can be used on gap 2 metals and transition metal ions)
50
Describe the solubility of grp 2 metal hydroxides down the group
increasing solubility down the grp
51
What are the results for the magnesium and calcium hydroxides
Magnesium hydroxide is classed as insoluble in water andwill appear as a white precipitate.
Simplest Ionic Equation for formation of Mg(OH)2 (s)
Mg2+ (aq) + 2OH-(aq)Mg(OH)2 (s).
Calcium hydroxide is classed as partialy soluble in water and will appear as a white precipitate (it may need more sodium hydroxide to be added before it appears compared to a magnesium solution.)
Simplest Ionic Equation for formation of Ca(OH)2 (s) Ca2+ (aq) + 2OH-(aq)Ca(OH)2 (s).
52
Why does strontium and barium not form a hydroxide precipitate when mixed with sodium hydroxide
their precipitate has high solubility. Solution will be highly alkaline
53
compare the difference in pH between magnesium and calcium hydroxide
suspension of magnesium hydroxide in water will be slightly alkaline so some OH ions must have been formed from slight dissolving
suspension of calcium hydroxide in water will be more alkaline than magnesium OH as its more soluble so more OH ions in solution
54
Whats the results of aluminium salts mixed with sodium hydroxide
forms white precipitate of aluminium hydroxide which dissolves in excess NaOH to form COLOURLESS solution
55
write the equations to show the formation of aluminium hydroxide precipitate from addition of NaOH and the dissolving of the precipitate in excess NaOH
[Al(H2O)6]3+(aq) + 3OH- (aq) Al(H2O)3(OH)3 (s) + 3H2O (l)
Al(H2O)3(OH)3(s) +OH-(aq)[Al(OH)4]-(aq)+3H2O(l)
56
What are the results of adding NaOH to the following transition metals copper, iron (ii), iron (iii) + equations
Copper solutions form a blue ppt
[Cu(H2O)6]2+ (aq) + 2OH- (aq) Cu(H2O)4(OH)2 (s) + 2H2O (l)
iron (II) solutions form a green ppt
[Fe(H2O)6]2+ (aq) + 2OH- (aq) Fe(H2O)4(OH)2 (s) + 2H2O (l)
iron (III) solutions form a brown ppt
[Fe(H2O)6]3+ (aq) + 3OH- (aq) Fe(H2O)3(OH)3 (s) + 3H2O (l)
57
How do you test for ammonium ions NH4+
Place about 10 drops of 0.1 mol dm–3 ammonium chloride in a test tube.
Add about 10 drops of 0.4 mol dm–3 sodium hydroxide solution. Shake the mixture. c) Warm the mixture in the test tube gently using a water bath.
Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube.
ammonia gas is released turning red litmus paper blue
58
How do you test to prove that the group 2 sulphates become LESS SOLUBLE down the group
Method: adding sulfate ions
a) Place about 10 drops of 0.1 mol dm–3 metal ion solution in a test tube.
b) Add about 10 drops of 1.0 mol dm–3 sulfuric acid ( or other soluble sulfate solution.
c) Continue to add sulfuric acid solution, dropwise with gentle shaking, until in excess
59
State the results of the group 2 sulphate test
magnesium and calcium salts will not form a sulfate precipitate on addition of sulfate ions due to their high solubility.
Strontium and barium solutions will form WHITE precipitates with addition of sulfate ions
59
State the results of the group 2 sulphate test
magnesium and calcium salts will not form a sulfate precipitate on addition of sulfate ions due to their high solubility.
Strontium and barium solutions will form WHITE precipitates with addition of sulfate ions
60
give the ionic and full equation of strontium or barium forming white precipitate when mixed with sulfuric acid
Full equation : SrCl2(aq) + Na2SO4 (aq)2NaCl (aq) + SrSO4 (s)
Ionic equation: Sr2+ (aq) + SO42-(aq)SrSO4 (s).
Ionic equation: Ba2+ (aq) + SO42-(aq)BaSO4 (s).
61
How do you test for a sulfate ion?
add BaCl2 solution acidified with HCL
will form a white precipitate of BaS04 if present
62
give the ionic equation of the testing for sulphates reaction
Simplest ionic equation
Ba2+ (aq) + SO42-(aq)BaSO4 (s).
63
Why is HCL needed when testing for sulphates and not sulfuric acid
to react with carbonate impurities often found in slats which can form while barium carbonate precipitate and give false result.
Sulfuric acid has sulfate ions and gives fake positive result
64
If a carbonate was present in a solution, how would you know as it reacts with addition of hCL
FIZZING DUE TO CO2 IS SEEN IF CARBONATE IS PRESENT
2HCl + Na2CO3 2NaCl + H2O + CO2
65
How do you test for presence of halide ions and why are the following steps done ?
- add nitric acid (react with any carbonates present to prevent formation of the precipitate Ag2CO3. This would mask the desired observations)
- add silver nitrate
Fluorides produce no precipitate
Chlorides produce a white precipitate
Ag+(aq) + Cl- (aq) ---> AgCl(s)
Bromides produce a cream precipitate
Ag+(aq) + Br- (aq)--->AgBr(s)
Iodides produce a pale yellow precipitate
Ag+(aq) + I- (aq) ----> AgI(s)
66
Colours of silver halide precipitates are similar, how do you test the difference between them
Silver chloride dissolves in dilute ammonia to form a complex ion
AgCl(s) + 2NH3(aq) ---> [Ag(NH3)2]+ (aq) + Cl- (aq)
Colourless solution
Silver bromide dissolves in concentrated ammonia to form a complex ion
AgBr(s) + 2NH3(aq) ---> [Ag(NH3)2]+ (aq) + Br - (aq) Colourless solution
Silver iodide does not react with ammonia – it is too insoluble.
67
How do you test presence of carbonate ions
Add any dilute acid and observe effervescence.
Bubble gas through limewater to test for CO2 – will turn limewater cloudy
2HCl + Na CO ----> 2NaCl + H O + CO 23 22
68
how do you test for presence of hydroxide ions
alkaline hydroxide ions turn red litmus paper blue
69
Describe the reducing power going down grp 7 and why
A reducing agent donates electrons.
The reducing power of the halides increases down group 7 as they have a greater tendency to donate electrons.
This is because as the ions get bigger it is easier for the outer electrons to be given away as the pull from the nucleus on them becomes smaller.
70
How do you demonstrate the increasing reducing power of halides down the group?
react halide salts with concentrated sulfuric acid
71
What is the 3 possible equations of the reaction of halide ions with conc sulfuric acid + observations
NaX(s) + H2SO4(l) ----> NaHSO4(s) + HX(g) [white steamy fumes of HX]
(iodide, bromide, chloride)
2HX + H2SO4 ----> X2 (s) + SO2(g) + 2H2O(l) [SO2 stinks, orange fumes of bromine]
(bromide, chloride)
6HI + H2SO4 ----> 3I2 +S(s)+4H2O(l) [I2 black solid, yellow solid of S, H2S has eggy smell]
(only iodide)
72
Why is there no redox reaction between fluoride and chloride ions with conc sulfuric acid
they arent strong enough reducing agents to reduce the S in H2SO4
(only acid base reaction happens)
73
what is the role of h2so4 in the first reaction with halide ions
it acts as an acid, proton donor
74
What are the reduction products in the reactions of halide ions and H2SO4
sulfur dioxide
sulfur
hydrogen sulfide
75
What are the half equations in reactions of halide ions and h2so4
Ox 1⁄2 equation
2I - ---->I2 + 2e-
Re1⁄2equation
H2SO4 +2H+ +2e- ---> SO2 +2H2O
Re1⁄2equation
H2SO4 +6H+ +6e- ---> S+4H2O
Re1⁄2equation
H2SO4 +8H+ +8e- ---> H2S+4H2O
76
Describe the set up of filtration via Buchner funnel
Buchner flask (thicker glass walls than normal flasks to cope with vacuum)
air outlet to water pump in Buchner flask
buchner funnel with filter paper
77
When would you use gravitation filtration or buchner funnel filtration
simple filtration if small amounts of solid are formed
Buckner filtration if larger amounts of solid are formed
78
Draw the set up of gravitational and Buckner filtration
79
How do you form aldehyde or ketone from primary/secondary alcohol
acidified potassium dichromate
aldehyde: heat and distill out
ketone: heat
80
What are hazards when it comes to dealing with potassium dichromate and sulfuric acid
solid potassium dichromate(Vl) since it is highly toxic and a category 2 carcinogen; it is also an irritant.
Avoid inhaling any dust.
Concentrated sulfuric acid is corrosive.
81
draw the distillation set up for oxidising primary alcohol to form aldehyde
82
How do you ensure that maximum yield is collected in distillation
only collect the distillate at the approximate boiling point of the desired aldehyde and not higher.
83
Why does water enter in the bottom of the Liebig condenser
so water goes against gravity and allows more efficient cooling and prevents back flow of water
84
Why is an electrical heater used in heating organic chemicals
organic chemicals are usually flammable and could be set on fire with naked flame
85
What can you do with the collection flaks to improve yield of distillate
cool it in ice
86
What is the colour change of reduction of acidified potassium dichromate
orange to green
87
Draw the set up of heating under reflux
88
When is reflux used
when heating organic reaction mixtures for long periods of time, prevents organic vapours from escaping by condensing them back to liquids
89
why should the end of the condenser never be sealed?
build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up.
90
Why are anti-bumping granules added
added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles.
ensure that the liquid doesn’t boil too vigorously as this would result in the mixture boiling over into the condenser and undesired impurities would contaminating the product.
91
How do you undergo reflux distillation to form carboxylic acid from primary alcohol
Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.
Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or three anti-bumping granules. Put a condenser on the flask for reflux, as shown in figure below.
Add 2 cm3 of concentrated sulfuric acid down the condenser in drops from a dropping pipette. While the
mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a dropping pipette. The energy released from the reaction should make the mixture boil. Add the solution a drop at a time so that the mixture continues to boil without any external heating.
When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape.
At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3 cm3 of liquid into a test tube.
92
What is fractional distillation used for
seperating liquids with different boiling points
93
How does fractional distillation work
Heat the flask, with a Bunsen burner or electric mantle
* This causes vapours of all the components in the mixture to be produced.
* Vapours pass up the fractionating column.
* The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
* The thermometer should be at or below the boiling point of the most volatile substance.
* The vapours with higher boiling points condense back
into the flask.
* Only the most volatile vapour passes into the condenser.
* The condenser cools the vapours and condenses to a
liquid and is collected.
94
Draw set up for fractional distillation
95
Where should the thermometer be inserted in distillation
in the T junction connecting to condenser to measure correct bpt
96
How do you test for an alkene
bromine water, shake and if alkene is present it will decolourise the bromine water
97
How to make tollens reagent
Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]+ .
98
How do you test for aldehyde using tollens
Place 1 cm3 of silver nitrate solution in each of two clean boiling tubes.
Then add one drop of sodium hydroxide solution to form a precipitate of silver oxide.
Add ammonia solution dropwise until a clear, colourless solution is formed.
Add a few drops of the unknown and leave in the water bath for a few minutes.
silver mirror should form
99
How do you test for aldehydes with fehlings solution
Place 1 cm3 of Fehling's A into each of two boiling tubes, and then add Fehling's B until the blue precipitate re-dissolves.
Add a few drops of the unknown and leave in the water bath for a few minutes.
red precipitate of Cu2O forms in aldehyde is present
100
How do you test for carboxylic acid
To 0.5 cm3 of your unknown solution in a test tube add a small amount of sodium carbonate solid and observe.
Result carboxylic acids will fizz with sodium carbonate due to CO2 produced
2CH3CO2H + Na2CO3 2CH3CO2-Na+ + H2O + CO2
101
What determines the reactivity of the halogenoalkane
The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane
The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction.
102
What graph is plotted when finding rates of reaction and how do we find the ROR from this graph
The initial rate can be calculated from taking the gradient of a continuous monitoring conc vs time graph at time = zero
102
What graph is plotted when finding rates of reaction and how do we find the ROR from this graph
The initial rate can be calculated from taking the gradient of a continuous monitoring conc vs time graph at time = zero
103
Initial rate can also be calculated from clock reactions where the time taken to reach a fixed concentration is measured.
104
How is hydrogen peroxide and iodide ions used as a ROR experiment
Hydrogen peroxide reacts with iodide ions to form iodine.
The thiosulfate ion then immediately reacts with iodine formed in the second reaction as shown below.
H2O2(aq) + 2H+(aq) + 2I–(aq) → I2(aq) + 2H2O(l)
2S2O32–(aq) + I2(aq) → 2I–(aq) + S4O62–(aq)
When the I2 produced has reacted with all of the limited amount of thiosulfate ions present, excess I2 remains in solution.
Reaction with the starch then suddenly forms a dark blue-black colour.
A series of experiments is carried out, in which the concentration of iodide ions is varied, while keeping the concentrations of all of the other reagents the same. In each experiment the time taken (t) for the reaction mixture to turn blue is measured.
105
Describe the process of using sodium thiosulfate, potassium iodide, hydrogen peroxide, starch, sulfuric acid and water to test ROR
Put each of the chemicals in the table in separate burettes.
In each experiment, measure out required volumes of the potassium iodide, sodium thiosulfate, starch and water into a small conical flask from the burettes.
Measure the hydrogen peroxide into a test tube.
Pour the hydrogen peroxide from the test tube into the conical flask and immediately start the timer. Stir the mixture.
Time until the first hint of blue/ black colour appears. Repeat with different concentrations of potassium iodide.
Do each experiment at same temperature
(for each experiment you keep the volume of sodium thiosulfate, hydrogen peroxide, starch and sulfuric acid the SAME) (vary only potassium iodide and water (to make the different conc of iodide ions))
106
How do you graphically plot the results for the iodine clock test
In an experiment where the concentration of one of
the reagents is changed and the reaction rate measured it is possible to calculate the order graphically
Taking rate equation Rate = k [I- ]n
Log both sides of equation
Log rate = log k + n log [Y]
Y= c+ mx
y intercept = log K
gradient = n
A graph of log rate vs log [I- ] will yield a straight line where the gradient is equal to the order n
107
draw graph of iodide clock experiment and label y=mx+c
108
Why does a ROR graph of conc against time go horizontal
The rate drops as the reactants start to get used up and their concentration drops.
The graph will eventual become horizontal and the gradient becomes zero which represents the reaction having stopped.
109
Why is it important to carefully consider quantities of reactants when using gas syringe
typical gas syringe only measures 100ml of gas so you don’t want a reaction to produce more than this volume.
110
whats the typical method for using gas syringe to measure volume of gas made in reaction of HCL and magnesium
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.
111
If a reaction has several reactants, how can you ensure that only 1 one of these relate to the ROR
In reactions where there are several reactants, if the concentration of one of the reactant is kept in a large excess then that reactant will appear not to affect rate and will be pseudo-zero order .
This is because its concentration stays virtually constant and does not affect rate.
112
graph of conc vs time, whats the gradient
rate of reaction
113
How do you measure the EMF of electrochemical cell with zinc and copper?
Clean the zinc and copper foils with emery before use. Degrease the metal using some cotton wool and propanone.
* Place the copper strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 CuSO4 solution.
* Place the zinc strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 ZnSO4 solution.
* Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge
* Connect the Cu(s)|Cu2+ (aq) and Zn(s)|Zn2+ (aq) half- cells by connecting the metals using the crocodile clips and leads provided to the voltmeter
114
Why is a salt bridge used
to connect up the circuit and allow free moving ions to conduct charge
115
Why is potassium nitrate and not potassium chloride used
The salt should be unreactive with the electrodes and electrode solutions.
E.g. potassium chloride would not be suitable for copper systems because chloride ions can form complexes with copper ions.
116
Why is a wire not used as the same bridge
A wire is not used because the metal wire would set up its own electrode system with the solutions.
117
What do you use if a half cell doesnt have a conducting metal
platinum electrode
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Why can a platinum electrode be used
A wire is not used because the metal wire would set up its own electrode system with the solutions.
119
Whats the general process for constructing a pH curve
Transfer 25cm3 of acid to a conical flask with a volumetric pipette
Measure initial pH of the acid with a pH meter
Add alkali from burette in small amounts (2cm3) noting the volume added and its pH (measured with pH meter)
when near end point add in small increments of 0.2cm3
after end point has been reached add in 2cm3 until alkali in excess
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Why must you calibrate the pH meter first and how is this done
measuring known pH of a buffer solution. This is necessary because pH meters can lose accuracy on storage.
Most pH probes are calibrated by putting probe in a set buffer (often pH 4) and pressing a calibration button/setting for that pH. Sometimes this is repeated with a second buffer at a different pH
Rinse the pH probe thoroughly with deionised (distilled) water and shake gently to remove excess water.
2. Place the probe in the standard pH 7.00 buffer solution provided. Record the pH reading.
● Ensure the bulb is fully immersed.
3. Repeat this process using the standard pH 4.00 and pH 9.20 buffer solutions. Record the pH readings.
● Rinse the pH probe thoroughly with deionised water before taking each reading.
4. Plot a graph of your recorded pH reading (x-axis) against the pH of the buffer solution (y-axis).
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How can you improve accuracy of measuring ph CURVES
maintaining a constant temp
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How can you test the electrode potential of different metals
Clean a piece of copper using emery paper or fine grade sandpaper.
2. Connect to the positive terminal of a voltmeter using a crocodile clip and one of the leads.
3. Cut a piece of filter paper to about the same area as the copper and moisten with sodium chloride solution. Place it on top of the metal.
4. Connect a second lead to the voltmeter and use the crocodile clip to connect the lead to a piece of a different metal.
5. Hold the metal against the filter paper. Record the voltage reading, including the sign.
Cell potential can be positive or negative.
6. Repeat with different metals and compare the electrode potentials.
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Why do you clean copper with emery paper or fine sandpaper
removes the oxide layer on the metal
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How do you plot graph with results when trying to find pH curve
Use the pH probe calibration graph to adjust the pH readings obtained in the experiment. These corrected pH values should be entered into a new column in the table of results.
● Plot a graph of the corrected pH values (y-axis) against volume of sodium hydroxide solution added.
● Join the points in the most appropriate way (should be a curve).
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Describe process of purifying an organic solid
Recrystallisation
1. Dissolve the impure compound in a minimum volume of hot (near boiling) solvent.
2. Hot filter solution through (fluted) filter paper quickly.
3. Cool the filtered solution by inserting beaker in ice
4. Suction filtrate with a Buchner flask and funnel to separate out crystals
5. Wash the crystals with distilled water
To remove soluble impurities
6. Dry the crystals between absorbent paper
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why must an appropriate solvent be used to dissolve organic solid
An appropriate solvent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold.
The minimum volume is used to obtain saturated solution and to enable crystallisation on cooling
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why do the reformed crystals from the dissolve organic solid no have impurities
Crystals will reform but soluble impurities will remain in solution form because they are present in small quantities so solution is not saturated. Ice will increase the yield of crystals
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Why do you wash the crystals with distilled water/cold solvent
To remove soluble impurities
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How can yield be lost in process of recrystallisation
Crystals lost when filtering or washing
Some product stays in solution after recrystallisation other side reactions occurring
130
How can yield be lost in process of recrystallisation
Crystals lost when filtering or washing
Some product stays in solution after recrystallisation other side reactions occurring
131
How do you purify an organic liquid
Separating funnel
● Put the distillate of impure product into a separating funnel
● Wash product by either:
o Sodium hydrogencarbonate solution, shaking and releasing pressure from CO2 produced - Sodium hydrogen carbonate removes acidic impurities by neutralisation (converts to H2 O , CO2 and Na2 S O4 )
o Saturated sodium chloride solution – helps separate the organic layer from the aqueous layer
● Allow layers to separate in funnel, and then run and discard the aqueous layer
● Run the organic liquid into a clean, dry conical flask and add 3 spatula loads of drying agent (anhydrous sodium sulphate – drying agent
should be insoluble in the organic liquid and not react with the organic liquid) to dry the organic liquid
● Carefully decant the liquid into the distillation flask
● Distil to collect pure product
Anhydrous calcium chloride is a drying agent and soaks up any remaining water – the liquid will remain cloudy until all the water drops have been remove
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How will impurities affect the mpt of an organic substance
If impurities are present (and this can include solvent from the recrystallisation process) the melting point will be lowered and the sample will melt over a range of several degrees.
133
What machine can be used to find mpt of a solid
electric melting point machine
or by using a practical set up where the capillary tube is strapped to a thermometer immersed in some heating oil. In both cases a small amount of the salt is put into a capillary tube. The tube is heated up and is heated slowly when near the melting point.
134
measuring bpt of liquid how
Measuring boiling point:
To determine purity of a liquid
● This can be done in a distillation set up or by simply boiling a tube of the sample in an heating oil bath.
● Pressure should be noted- changing pressure can change the boiling point of a liquid.
● Not the most accurate method of identifying a substance as several substances may have the same boiling point.
● To get a correct measure of boiling point the thermometer should be above the level of the surface of the boiling liquid and be
measuring the temperature of the saturated vapour.
135
Describe how to cary out TLC
Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line.
b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry.
c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal.
e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.
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Why are gloves work
Wear plastic gloves to prevent contamination from the hands to the plate
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Why is line drawn with pencil
will not dissolve in the solvent
138
Why must you make the drops small
too big a drop will cause different
spots to merge
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why must depth of solvent be below pencil line
– if the solvent is too deep it will dissolve the sample spots from the plate
140
why is a lid used
to prevent evaporation of toxic solvent
141
What does separation by chromatography depend on
balance between solubility in the moving phase and retention in the stationary phase.
