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Prac Flashcards

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1
Q

Why must weighing bottle and beaker be completely dry in energetics reactions?

A

Some of the solid will dissolve in water before reaction starts if either container is not completely dry. Hence calculated ∆H will be inaccurate since Q calculated is not reflective of the actual amount of of solid dissolved.

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2
Q

Energetics: Why is styrofoam cup placed in a beaker?

A

To prevent styrofoam cup from toppling over when solution is stirred. Air trapped between beaker and styrofoam cup also good insulator to minimise heat loss to surroundings.

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3
Q

Account for the presence or lack thereof anamalous results

A

None. continuous stirring was done after solid was added to ensure that heat is evenly distributed throughout the whole solution and to ensure that solid completely dissolves.

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4
Q

Why must temp readings be taken at a regular interval before the start of energetics experiment?

A

Ensure a constant initial temperature

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5
Q

Why can’t we measure temp at time when solid is added to solution?

A

Unable to get a steady/accurate reading as temperature is changing too rapidly

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6
Q

Major sources of error in energetics reactions and modifications to minimise the errors

A

Measuring cylinder not precise for measurement of volume of solution, leading to inaccuracies in calculated values of Q and ∆H; use more precise apparatus like burette or pipette to give more accurate measurement of solution.
Temp recorded not precise because of slow response time/ low precision of thermometer, leading to inaccurate calculation of ∆T, and hence Q and ∆H; use datalogger with temp probe for more accurate measurement of temp change.

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7
Q

Why solution 2 cannot be added into solution 1 from burette/pipette? (Energetics and kinetics - 2 dif answers)

A

energetics: Takes a long time leading to significant heat loss from surroundings, so temp change is less than expected.
Kinetics: Takes a long time. For kinetics experiment, reactants must be mixed in shortest time so instantaneous rate is accurate.

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8
Q

Purpose of stirring solution immediately after mixing

A
  1. Ensure that reaction is completed in shortest time possible to obtain max temp change
  2. Ensure that heat is evenly distributed throughout the whole solution
  3. Ensure that solid completely dissolves so that ∆H is reflective
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9
Q

Which experiment has the greatest total percentage error?

A

The one with smallest volume, because percentage error is inversely proportional to volume measured.

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10
Q

Percentage error of different apparatus, how to calculate (eg pipette, burette, measuring cylinder)

A

Pipette: they will tell you, make sure you choose the data for the correct volume of the pipette
Measuring cylinder: 0.5/V x 100%
Burette: (2 x 0.05)/V x 100%

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11
Q

Minimum amount of solution to ensure that bulb of thermometer is fully submerged (make sure for plannings!)

A

50cm^3

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12
Q

Kinetics: why must a dry beaker be used for kinetics experiment (initial rate)?

A

Ensure that no water is introduced so the total volume of the solution is kept constant. Presence of water affect initial conc of each of the reactants

Contrast to normal titration where it doesn’t matter if the inside of the conical flask is wet cos the number of moles that you want to react would still be the same no. of moles in the conical flask

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13
Q

Kinetics: why deionised water in each experiment different?

A

So total volume can be kept constant at 55cm^3 and initial concentration of each reactant is directly proportional to volume used

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14
Q

Kinetics: Can deionised water be mixed after FA1 and FA2 are mixed?

A

No. Initial rate will be higher than expected if it is not added before reaction starts.

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15
Q

Kinetics (obscuring printed material) : Can dif printed material be used for dif experiments?
Why is it necessary to produce the same amount of ___?

A

No. Same material must be used so that amount of sulfur that has to be produced to obscure the print is kept constant for all the experiments
So rate is dependent only on time taken, since rate = [I formed]/time —> rate directly prop 1/t

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16
Q

Things to take note of if they ask you to ‘state the relationship’

A

Cannot write =; it must be how they are directly proportional/inversely proportional!

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17
Q

Explain how you use rate vs conc graph to deduce order wrt reactant.

A

1000/t directly prop to rate, and V (FA1) directly prop to [reactant] since total volume is constant. Since graph of V (FA1) against 1000/t is straight line passing through origin, rate directly prop to [reagent] —> order wrt reactant is one

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18
Q

Kinetics: Major SOE/limitation and how to reduce error

A

Human reaction time in starting/stopping stopwatch (accuracy of time affected)
—> repeat experiment once more and find average of time recorded to reduce percentage error. Due to human reaction time and improve accuracy of results OR conduct experiment with colourimeter/light sensor to detect the instant solution turns opaque to improve accuracy of timings measured

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19
Q

Kinetics: reduce total volume, same amount, how will percentage error in measurement of reaction time change?

A

Conc of reactants double —> rate x4 (when order and stuff taken into account)
—> rate = change in conc/time
—> time for full reaction to be complete falls by 4 times but percentage error due to human reaction time same
—> since percentage error is inversely proportional to reaction time, percentage error increases

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20
Q

M2w1,2,3 done. The rest not yet

A
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21
Q

Purpose of using a white tile in titration

A

To see the colour change at end point more clearly when conical flask is placed on it

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22
Q

Why do we need to rinse pipette and burette before the start of the titration?

A

To ensure that there is no change in concentration when they are filled with the solutions to be used

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23
Q

Why do we need to rinse pipette and burette before the start of the titration?

A

To ensure that there is no change in concentration when they are filled with the solutions to be used

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24
Q

Must conical flask be completely dry before pipetting the solutions into it?

A

No. Amount of water does not affect number of moles of solution in the conical flask.

affects rate though, that’s why for kinetics experiments it has to be dry.

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25
Q

Why do we use conical flasks instead of beakers in titration?

A

Slopping sides help prevent splashes which affect number of moles of acid or alkali in the conical flask and make titrations inaccurate

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26
Q

Titration: how much indicator to add? Why?

A

Just a few drops until a clear colour can be seen.
Indicator is a weak acid/base so excessive volume added affects concentration of acid/base in conical flask and leads to inaccurate reading

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27
Q

How do indicators work and why do we observe the colour change over a range of pH/volume rather than at a specific point?

A

POE shifting for indicators, in acidic form one colour, basic form another. Usually the end point we wanna find is when it’s a mixture of the acidic form and basic form colour (eg orange for methyl orange which is red in acid and yellow in alkali)
Observed over a range because human eye not sensitive to the colour change, continue to perceive it as orange when ratio of [HIn]:[In-] ranges from 10:1 all the way until [HIn]:[In-] = 1:10 (Pka +/- 1)

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28
Q

Main considerations when choosing indicator

A

End point of indicator should lie within the region of rapid pH change at equivalence point.

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29
Q

Situation: very concentrated solution 1, very dilute solution 2. Why cannot successfully titrate? Suggest changes to enable experiment to be carried out successfully.

A

Calculate volume of solution 2 needed to titrate against solution 1
However, max volume of burette is only 50 cm^3, so cannot carry out successfully due to limitations of apparatus.

Dilute solution 1 by 10 times, by measuring 25cm^3 of solution 1 with a 25.0cm^3 pipette and transferring into a 250cm^3 volumetric flask. This will ensure that volume of solution 2 ended to reach end point in burette is about 25.0cm^3 which is now within measurement range of burette.

NB: cannot use a burette, empty then top up again. Percentage error extremely high!

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30
Q

Why is dilution needed?

A

To ensure that end point of titration is within end point of burette.

31
Q

Titrate x against y

A

X in conical flask, y in burette

32
Q

How to decide if dilution is needed when conc is unknown?

A

Estimate. 10 drops of FA2, 1 drop of indicator, count number of drops to cause a colour change

33
Q

Can any other apparatus replace volumetric flask (for dilution?)

A

No. It ensures that total volume of diluted solution is precise.

34
Q

Major SOE/limitation for titration

A
  1. Lack of precision of measuring instrument —> inaccurate titration results
  2. Excessive amount of indicator added —> inaccurate titration readings since it is a weak acid/base and will change conc.
35
Q

What is the largest mass you can deviate when measuring?

A

+/- 0.05g

36
Q

Why is it necessary to ensure that all solid is dissolved first before transferring?

A

Difficult to dissolve solid once it is in volumetric flask since stirring is not possible.

37
Q

Dark solutions, read top or bottom of meniscus? Precautions?

A

Wait for glass surface to ‘clear’ then read bottom. Dark purple —> lower meniscus cannot be seen clearly.

38
Q

How to qualitatively transfer sample?

A

Wash glass rod, filter funnel and beaker with deionised water and transfer all washing to the volumetric flask.

39
Q

Why titration results are higher when dilute HCl is used as a OA instead of dilute H2SO4

A

KMnO4 is a strong OA that can oxidise Cl- to Cl2, hence an additional volume of KMnO4 is needed.

40
Q

Why do most redox reactions not require an indicator?

A

Distinct colour change can already be observed at end point.

41
Q

Dilution factors that we can use with lab equipment

A

4, 10, 25 times

42
Q

Purpose of adding starch in iodine based titration

A

Without adding starch, colour change at end point is pale yellow to colourless which is not distinct.

43
Q

Why starch is added near end point rather than at start of titration

A

At high conc of iodine, starch molecules get trapped in complex spiral starch molecules and cannot be released even at end point, so the slow dissociation into free iodine leads to a higher than expected titre value, hence inaccurate titration results.

44
Q

After titration, slow return of blue after a while, why?

A

Atmospheric oxidation of I- back to I2.

45
Q

Dilution procedure

A

Calculation —> FA1 will require a __ times dilution
1. Using a 25cm^3 pipette, measure and transfer 25cm^3 of FA1 into a 100cm^3 volumetric flask.
2. Make up to the mark with deionised water and shake to obtain a homogenous solution.

46
Q

Gas collection, can use measuring cylinder instead of beaker/gas syringe?

A

Yeah, measuring cylinder is granulated, so it is possible to quantify volume of gas collected.

47
Q

Why must weighing bottle always be reweighed after transferring solid?

A

The weighing bottle may still contain some residue.

48
Q

Acid + carbonate, how to tell that acid is in excess without additional tests?

A

All solid fully dissolved, and no more gas evolved.

49
Q

Significant SOE of gas collection and suggestion to improve accuracy

A

Loss of gas between addition of HCl and stoppering of flask / CO2 gas sparingly soluble would mean a lower volume of gas collected

Modification: Use thistle funnel to introduce HCl into conical flask, minimising gas loss OR collect volume of CO2 gas with gas syringe, eliminating possible losses of CO2 via dissolution in water—> volume more accurate

50
Q

Thermal decomposition collection of gas procedure

A
  1. Weigh with electronic weight balance and transfer into boiling tube. Set up as shown in diagram and ensure hat gas syringe is at zero mark
  2. Heat solid mixture, gently at first then strongly. Stop heating when no more increase in volume of gas.
  3. Let set up cool to room temperature and record volume of CO2 measured (I think this one needs cooling so that we can actually calculate no. Of moles with 24000)
51
Q

Gravimetric: Why do we cool test tube before weighing?

A

Mass reading fluctuates for hot substances due to air convection

52
Q

When exposed to air, FA1likely to be contaminated with water. Explain why this is so and how it affects results.

A

Absorbs moisture from the air (hydroscopic)
Measured mass larger than actual value, so percentage by mass found is lower than actual.

53
Q

Experiment happens too fast to be observed. What can you do?

A

Add same amount of delaying agent for each experiment so measurement of time can be done.

54
Q

Kinetics: why must total volume of solution be kept constant?

A

So that [FA1] directly prop to V(FA1) added

55
Q

What does gradient of concentration time graph represent?

A

Rate of reaction wrt reactant, apparently

56
Q

Kinetics: propose another experiment…

A

Use 12.5cm^3 of FA1 and 12.5cm^3 of deionised water to halve the conc of FA1.
Plot graph of V(FA2) used against time
Find grad to determine reaction rate, which should be half of value previously calculated when 25cm^3 of FA1 is used.

57
Q

Quenching procedure

A
  1. Fill burette with 50 cm^3 of FA2
  2. Using 2 separate 50cm^3 measuring cylinders, measure 25cm^3 of FA3 and FA4 and transfer into 250cm^3 conical flask.
  3. Use another 100cm^3 measuring cylinder to measure 50cm^3 of FA1 and add into the same conical flask, starting stopwatch immediately.
  4. Using a dropper and a 10cm^3 measuring cylinder, measure 10cm^3 of the mixture in the conical flask into another small conical flask. At the 2 minute mark, add a spatula of NaHCO3 and shake to mix, ensuring that excess NaHCO3 is seen at the bottom of the beaker so that quenching is complete.
  5. Titrate this solution against FA2, and run it into conical flask until brown colour of iodine fades to light yellow
  6. Add 5 drops of starch solution and continue titrating until blue black colour just disappears, and record the volume.
  7. Withdraw further portions of 10cm^3 of the reaction mixture approximately every 3 minutes, repeating steps 4-6 at each timing till you obtain 5 more sets of data.
  8. Plot a graph of Volume of FA2 against time for each set of data, and use that to determine rate of experiment wrt FA2.
58
Q

Why all other reactants in large excess?

A

To keep the conc of products relatively invariant during reaction so rate is only affected by conc of iodine.

59
Q

Why can we use … to quench experiment?

A

Reaction is acid catalysed, can use NaHCO3 (base) to neutralise acid and stop reaction.

60
Q

How to ensure sufficient quencher added?

A

Excess white solid and no more effervescence.

61
Q

Why need to filter in QA?

A

To test for ions present in filtrate and residue separately.

62
Q

When we add sodium carbonate, what are we looking for?

A

Highly charged ions!

63
Q

How X acts as a catalyst?

A

Used up in step 1, regenerated in step 3 –> colour of solution at start and end is the same
Rate increased since effervescence observed and O2 gas produced when X is added
Alternative reaction pathway because new reactants of different colours formed

64
Q

Purpose of salt bridge

A

To maintain elecrical neutrality. Ions move from salt bridge to half cells to maintain charge balance
or act as electrical connector to complete the circuit by allowing ions to flow between the 2 half cells

65
Q

When they ask you to find percentage uncertainty, percentage uncertainty of the thing measured, (match theoretical to measured, not the other way around)

A

See M4W3 last qn

66
Q

Gravimetric analysis

A

Procedure study book!

67
Q

Energetics: assumptions

A

No heat loss to surroundings and complete combustion of hexane

68
Q

Experiment repeated by pouring off hexane and replacing with cyclohexane. Significant SOE and improvement?

A

Still have hexane in the bottle, hence cyclohexane used not pure, and loss in mass measured not accurate.
Use a new wick and bottle or clean the lamp first before using wick of same length to conduct experiment again.

69
Q

Why hexane less accurate than candle wax?

A

Hexane more volatile. Even when heat extinguished, still evaporating –> higher than expected mass loss (inaccurate)

70
Q

Why cannot have draught in energetics experiments

A

Draughts cause heat loss from flame to surroundings, reducing actual amount of heat transferred to the lamp –> more hexane burnt to produce same temp rise–> larger calcualted heat capacity of can

71
Q

Why reaction is initially slow but becomes faster as reaction progresses?

A

High Ea at the start because MnO4 - and C2O8 2- both negatively charged, repel each other.
Reaction becmes faster because of formation of catalyst Mn2+ which provides alt pathway with lower Ea.

72
Q

Common ion effect, solubility and Ksp both change?

A

Solubility yeah, Ksp no (because temp and pressure no change)

73
Q
A

Chemistry (8 decks)

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