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Required Practicals Flashcards

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1
Q

Describe how you prepare the pipette to carry out a titration

A

Rinse pipette with volume of 25 cm^3 with distilled water - removes unwanted chemicals
Rinse pipette with alkali and discard alkali to remove any droplets of water that will dilute solution

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2
Q

Describe how to prepare the alkali for a titration

A

Place conical flask next to beaker containing alkali - rinse conical flask with distilled water
Place tip of pipette into alkali and draw using pipette filler - do slowly and carefully to avoid bubbles
Fill pipette past 25 cm^3 mark
Lift pipette and release drops of alkali into beaker until meniscus is at 25cm^3 mark - view meniscus at eye level
Transfer to conical flask - add few drops of indicator (e.g. four)

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3
Q

When filling the pipette with the alkali why do you do it past the 25cm^3 mark

A

Because level drops slightly as you rise the pipette above the beaker

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4
Q

With a strong acid strong base titration what indicators do you use

A

Phenolphthalein
Methyl orange

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5
Q

If we’re titrating a weak acid with a strong base what indicator do you use

A

Phenolphthalein

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6
Q

If you are titrating a strong acid with a weak base what indicator do you use

A

Methyl orange

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7
Q

Why do you only add a few drops of indicator to conical flask

A

Indicators are weak acids so if we add a lot it could give us inaccurate results

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8
Q

How do you prepare the burette to use for titration

A

Rinse burette with distilled water to remove unwanted chemicals
Rinse burette with acid to remove any traces of water
Clamp burette so that it is level
Use funnel and slowly fill burette with acid slightly above the zero line
Remove funnel
Open tap and allow acid to slowly leave the burette so that the bottom of the meniscus is on the 0cm^3 mark

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9
Q

Why do you remove the funnel from the burette after using it to fill the burette with acid

A

To prevent acid from dripping from the funnel into the burette

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10
Q

It is hard to read the meniscus sometimes
How could you solve this problem

A

Hold a piece of white paper behind the burette to make the meniscus clearer

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11
Q

What must you always do when reading the meniscus

A

Read at eye level

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12
Q

What do you do with the conical flask containing the alkali after filling the burette with acid

A

Place it onto a white tile - to make colour change of indicator easier to see accurately

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13
Q

Assuming all the preparation for the apparatus is completed how do you carry out a titration

A

Open tap of burette and slowly release acid into conical flask while swirling the conical flask at the same time
Watch colour of indicator
Stop adding acid when the indicator changes colour to show the end point
Read level of acid on the burette

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14
Q

Why do we swirl the conical flask at the same time as acid being added to it

A

Ensures that the acid and alkali mix thoroughly so they can react

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15
Q

Explain the rules with regards to reading the level of acid after carrying out titration

A

Uncertainty is considered half the scale division
So the uncertainty of a burette is 0.05 cm^3
So if the meniscus lies in between two scale divisions then we record the volume to the nearest 0.05 cm^3

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16
Q

What is the formula for the titre

A

Starting volume of acid - final volume of acid

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17
Q

What are the number of decimal places that you need to record the values of the volume of the acid

A

To 2 decimal places

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18
Q

Describe how you find the mean titre

A

First titre is considered rough titration
So we repeat titration and add the acid drop by drop as we approach the end point
Repeat until we have two concordant titres
Calculate mean of the two concordant titres
The value of that is the mean titre

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19
Q

What does it mean to have two concordant titres

A

Within 0.1cm^3 of each other

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20
Q

What is the test for alcohols (don’t give description on how to do it just give general description)

A

Add acidified potassium dichromate
If alcohols are present then the dichromate (VI) ion will be reduced to a chromate (III) ion
The observation is orange to green

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21
Q

What is the ionic equation for what happens to the dichromate ions if alcohols are present

A
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22
Q

How do you carry out the test for alcohols

A

Add 10 drops of sample and 2cm^3 of acidified potassium dichromate to a test tube
Warm the mixture gently in hot water bath
Watch for colour change

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23
Q

What is Benedict’s solution and what does it look like

A

Copper (II) ions dissolved in sodium carbonate
Blue solution

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24
Q

What is the test for aldehydes

A

Add 2 cm^3 of Fehling’s or Benedict’s solution to test tube
Add 5 drops of sample to test tube
Put test tube in hot water bath for a few minutes

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25
What is the observation for the test of aldehydes if there are only aldehydes present
Benedict's and Fehling's solution will reduce to a brick red precipitate Cu2O
26
What is the observation for the test of aldehydes or ketones if there are only ketones present
Nothing will happen Stay blue
27
What is the equation for the reaction when an aldehyde is present and Fehling's or Benedict's solution is added to it
2Cu2+(aq) 2OH-(aq) +2e- -> Cu2O(s) + H2O(l)
28
What does Tollens' reagent contain
a [Ag(NH3)2]+ complex
29
Describe the process of making Tollens' reagent
Put 2cm^3 of 0.1 mol/dm^3 silver nitrate solution in a test tube Add a few drops of dilute sodium hydroxide solution - light brown precipitate should form Add drops of dilute ammonia solution until the brown precipitate dissolves completely - this is the reagent
30
What happens to the silver ions in Tollens' reagent when warmed with aldehyde
Silver ions are reduced to silver metal
31
If you add Tollens' reagent to an aldehyde what is the result
Silver ions reduced to silver metal This metal coats the inside of the apparatus to form a silver mirror
32
When heating aldehydes and ketones why do you not use a Bunsen burner
They are flammable so it will catch alight if you use a Bunsen burner
33
What is the method to test for carboxylic acids
Add 2 cm^3 of the sample to test tube Add 1 small spatula of solid carbonate (2cm^3 of sodium carbonate solution) If solution begins to fizz, bubble the gas that it produces through limewater in a second test tube If carboxylic acid is present in sample then CO2 will be produced and the limewater will turn cloudy
34
What is the test for alkenes
Add 2 cm^3 of the sample to test tube Add 2cm^3 of bromine water to test tube Shake test tube If it goes from orange to colourless than alkene is present
35
What is the limitation to the test for carboxylic acid
Gives positive for any acid so you can only use if you're distinguishing between organic compounds and you know one of them is a carboxylic acid
36
What are the two phases in chromatography
Mobile Stationary
37
What is the mobile phase
Where the molecules can move
38
What is the state of a mobile phase
Liquid or gas
39
What is the stationary phase
Where the molecules can't move
40
What is the state of the stationary phase
Solid Liquid on solid support
41
In chromatography what does the distance each substance moves depend on
Depends on the solubility in the mobile phase and its retention by the stationary phase
42
What does TLC stand for
Thin Layer Chromatography
43
What are the mobile and stationary phases of TLC
Mobile - liquid solvent such as ethanol Stationary - thin layer of silica (silicon dioxide) or alumina (aluminium oxide) fixed to a glass or metal plate
44
Describe how you carry out TLC
Draw a line in pencil near bottom of TLC plate and put small drop of each mixture on line Allow spots to dry Place plate in a beaker with small amount of solvent - must be below the baseline Solvent will move up plate and will carry substances in mixture with it - some chemicals will be carried faster than others Leave until solvent has moved to the top of the plate Remove plate from beaker
45
Why must the solvent must be below the baseline in TLC
So it doesn't dissolve your samples away
46
Describe what you should do to the TLC plate after you remove it from the beaker
Use pencil to mark how far the solvent has travelled up plate Place the plate in a fume cupboard and leave it to dry
47
What is the line to measure how far the solvent has travelled called
The solvent front
48
Why do we place the TLC plate in a fume cupboard
It will prevent any toxic or flammable fumes from escaping into the room
49
What is the TLC called after the experiment is finished called What can it be used for
A chromatogram Can use the positions of the chemicals to identify what the chemicals are
50
What is a limitation to TLC
Some chemicals are colourless such as amino acids so hard to figure out how far it has travelled
51
What are the two ways to solve the limitation to TLC
A fluorescent dye added to silica or alumina that glows when UV light shines on it - can put TLC under UV lamp and draw how far it has travelled Expose TLC to iodine vapour - it is a locating agent
52
What is a locating agent
It sticks to the chemicals on the plate and they'll show up as purple spots
53
If you have a chromatogram how do you find out the amount of chemicals that were in a mixture
Count the amount of spots
54
What is the equation for the Rf value
Distance travelled by spot / distance travelled by the solvent
55
For reactions that happen in solution how do you carry out calorimetry
Put reactants in polystyrene beaker to reduce the amount of heat lost Use thermometer to take the temperature at regular intervals
56
What are two sources of error in calorimetry tests
Heat change because of heat added or heat lost to surroundings Incomplete combustion so less energy is given out
57
What are the five steps you can take to calculate an accurate temperature change in a combustion calorimetry test
During experiment record temperature at regular intervals beginning at a couple of minutes before you start the reaction Plot graph of results Draw two lines of best fit Extend both lines so that they both pass the time when the reaction started Distance between the two lines at the time the reaction started is the accurate temperature change
58
What are the two lines of best fit when measuring the temperature change in combustion calorimetry
One going through the points before the reaction started One going through the points after the reaction started
59
What does the shape of the lines of best fit have to be with regards to temperature change
Straight line
60
What is the apparatus needed to calculate the standard enthalpy change of combustion
61
Explain how you carry out the experiment to find the standard enthalpy change of combustion of ethanol
Use balance to measure starting mass of spirit burner and fuel Use thermometer to measure start temperature of the water Remove cap from spirit burner and light the wick Stir the water with thermometer so that the energy is distributed After a few minutes, extinguish flame by replacing the cap on the burner and read the temperature of the water Measure the final mass of the spirit burner
62
Why is it important to light the wick of the spirit burner immediately after releasing the cap
The fuel will evaporate and make our final results less accurate
63
What equation do you use to find the thermal energy transferred into water
64
What is the symbol and unit of the energy change of water
q Joules
65
What is the formula for the standard enthalpy change of combustion
66
When calculating the standard enthalpy change of combustion why is the result you will get inaccurate (4 reasons)
If unlit spirit burner is uncapped then fuel will evaporate - will make it appear that we burned more fuel than we actually did A lot of heat energy released by the fuel doesn't pass into the water - some to the air or metal calorimeter Incomplete combustion - this releases less thermal energy than complete combustion May not have been carried out in standard conditions

Chemistry (13 decks)

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