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You must be hungry 😋 > Core Practicals > Flashcards

Core Practicals Flashcards

1
Q

What is a standard solution

A

A solution with known concentration

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2
Q

How do you make a standard solution

A
  • Measure mass of required solid with a balance
  • Transfer this to a volumetric flask and rinse the remaining bottle content (with distilled water) into the flask so no solid is lost
  • Add a volume of distilled water to dissolve the solid and swirl
  • Then add more distilled water up to the line on the neck of the volumetric flask and invert multiple times to mix
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3
Q

What is the standard solution for finding concentration of sodium hydroxide

A
  • Dilute sulfamic acid
  • Dissolve 2.5g of solid sulfamic acid in 100cm3 of distilled water
  • Transfer up to 250cm3 volumetric flask and fill up ti line with distilled water
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4
Q

What is the weighing by difference technique?

A
  • It is a method to weigh materials accurately

- Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred

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5
Q

What equipment is used to carry out the titration for sodium hydroxide

A
  • A pipettes and pipette filler are used to accurately measure out the volume of NaOH before transferring it to a conical flask
  • A Burette is used to add small volumes of sulfamic acid solution to the NaOH until the reaction has reached completion
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6
Q

How do you carry out a titration?

A
  • Once the pipette has been used to place NaOH into the conical flask, fill the burette with the acid solution. Record initial volume
  • Ass a few drops of methyl orange to the conical flask
  • Open burette tap and allow the sulfamic acid to flow into the conical flask, swirling it
  • Close the burette tap once the expects colour ch she occurs. Use a white tile to easily identify this
  • Record final burette volume
  • Repeat until you get concordant results then calculate a mean titre
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7
Q

Why are acid-base indicators used

A

To detect when a reaction reaches completion, usually by the presence of a colour change

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8
Q

What are concordant results?

A

Titres that are within 0.1cm3 of each other

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9
Q

How would you analyse results from sodium hydroxide titration

A
  • Calculate the concentration of the sulfamic solution (if not already known)
  • Calculate the mean titre using concordant results
  • Calculate the concentration of the burette solution
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10
Q

Bunsen burner : risk and control

A

Risk - burns

Control - Keep away from flammable chemicals and away from the edge of the desk

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11
Q

Chemicals : risk and control

A

Risk - May be an irritant or corrosive. May be toxic. May be flammable
Control - Handle with car while wearing gloves. Wear eye protection. Keep away from the edge of the desk and from and open flame and do not ingest

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12
Q

How do you make a diluted solution of hydrochloric acid?

A
  • Add 25cm3 of the hydrochloric acid solution into the volumetric flask using a pipette
  • Make the solution up to the line by adding distilled water
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13
Q

How do you analyse the results from the NaOH HCl titration

A
  • Calculate the mean titre using concordant results
  • Calculate the number of moles of NaOH in the mean titre n=cv
  • Calculate the number of moles of HCl that were present in the 250cm3 diluted solution
  • Using this to calculate the concentration of the original solution of HCl
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14
Q

Glassware (beakers, test tubes) : risk and control

A

Risk - When broken could cut you

Control - Handle with care and keep away from the edge of the desk

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15
Q

Why are acid-base indicators used?

A

To detect when a reaction reaches its equivalence point. The indicator should be chosen so that it’s end point (point of colour change) matched the equivalence point of the reaction

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16
Q

Why does a pH probe need to be calibrated?

A

So that for each pH reading, the pH value is accurate

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17
Q

How do you calibrate a pH probe?

A

Submerge the pH probe in buffer solution of three different pHs including pH 7 and usually pHs around 4 and 10. Each time pressing calibrate

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18
Q

What is accuracy

A

The more accurate the data, the closer it is to the actual value

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19
Q

How do you carry out a titration to calculate pH?

A
  • Into a conical flask, add 25cm3 of 0.1 moldm-3 ethanoic acid solution with a few drops of phenolphthalein
  • The sodium hydroxide solution goes into the burette
  • Titrate the solutions together until it just turns pale pink
  • Add another 25cm3 of 0.1 moldm-3 ethanoic acid solution into the conical flask using a pipette
  • Record the pH of the resulting solution
20
Q

How do you calculate Ka from titration to find pH

A
  • In the solution only half the acid has been titration therefore : [HA] = [A-]
  • You can then cancel [A-] and [HA] in the Ka equation as they equal the same thing so Ka = [H+]
  • Convert the resulting solutions pH to [H+] to give a value for the acid disassociation constant, Ka. [H+]=10^-pH
21
Q

What is a redox titration?

A

A titration of a reducing agent by and oxidising agent or the other way around

22
Q

How would you find the amount of iron in an iron tablet experimentally?

A
  • Titrate potassium manganate (VII) solution against acidified (H2SO4) crushed iron tablets
  • Calculate mean titre using concordant results
  • Calculate number of moles of MnO4- in the mean titre
  • Using stoichiometry calculate the number of moles of Fe2+ reacted and in the original solution
  • Using this calculate the mass of Fe in solution and mass per tablet
23
Q

How would you reduce uncertainty in a redox titration

A
  • Use a balance with greater resolution
  • Use pipettes and burette with greater resolution
  • Use greater masses and volumes so percentage uncertainty is reduced
24
Q

What is the weighting by difference technique (used to measure mass of the iron tablets) ?

A
  • It is a method to weigh materials accurately

- Mass of substance = Mass of weighing dish and substance - Mass of dish after substance has been transferred

25
Q

What is the ionic equation for the reaction between acidified iron and potassium manganate (VII) ?

A

5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O

26
Q

What is rate of reaction?

A

The change in concentration of reactants of products over time

27
Q

How can rate of reaction be measured?

A
  • Initial rates method i.e. the iodine clock reaction

- A continues monitoring method i.e. measuring the volume of gas released in a reaction over time

28
Q

What is a continuous monitoring method

A

This involves measuring the change in concentration of a reactant or product over time (or measuring the volume of gas released) as the reaction progresses

29
Q

Give an example of a continuous monitoring method

A
  • Mix propanone with sulfuric acid and iodine in a beaker and start stopwatch
  • Using a pipette, remove a sample of the mixture and add NaHCO3 stopping the reaction. Note the time when it’s added
  • Titrate the remaining iodine present in the sample with sodium thiosulfate (VI) solution, using starch as the indicator
  • Repeat titration with samples taken every 3 mins
30
Q

How would you analyse the results from this reaction?

A
  • Plot a graph of tire against time. Concentration of iodine is proportional to titre
  • By comparing the shape of the graph to known order concentration-time graphs, determine the order of the reaction with respect to I2
31
Q

What is an initial rates method

A

The method involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies

32
Q

Give an example of an initial rates method

A

Iodine clock experiment

  • The I2 produced reacts with all the thiosulfate ions present. Excess I2 remains in solution which then reacts with starch to form blue-black solution
  • Time how long it takes for this blue-black colour to appear. You can vary [I-] to then determine the order with respect to the iodide ions
33
Q

What are the issues with the iodine clock experiment

A
  • Low [I-] may take too long to react
  • Delayed stopwatch reactions
  • Concentrations may not be exact due to measuring apparatus
34
Q

Iodopropanone: risk and control

A

Risk - Strong irritant to eyes
Control - Wear eye protection and gloves. Once each measurement is complete, the reaction mixture should be immediately disposed of with lots of running water using a sink in a fume cupboard

35
Q

Propanone: risk and control

A

Risk - Irritant, highly flammable

Control - Handle with care and with gloves. Wear eye protection. Keep away from an open flame

36
Q

Sodium thiosulfate: Risk and control

A

Risk - Releases sulfur dioxide which is toxic and corrosive

Control - Ensure room is well ventilated. Don’t ingest

37
Q

What are the main steps in producing a pure organic solid?

A

1) Synthesis of the compound (usually using reflux, distillation etc.)
2) Filtration (usually vacuum filtration)
3) Purification (recrystallisation)

38
Q

How do you due lab equipment to heat under reflux

A

Quick fit apparatus is used to heat a substance under reflux

  • The substance is boiled in a pear shaped or round bottomed flask
  • As it evaporates, it is cooled by the water in the liebig condenser and so condenses back into a liquid and drips back down into the flask to be heated again
39
Q

Why is heating under reflux used?

A
  • Allows heating for a long period of time
  • Prevents the flask from boiling dry
  • Prevents volatile reactants / products escaping
  • Ensures even heating
40
Q

Why are anti bumping granules used when heating under reflux and distillation?

A

To allow smooth boiling preventing bubbles (caused by vapour) in the hot liquid from bubbling up the sides of the flask

41
Q

What compounds are heated under reflux together in the first step to create aspirin?

A
  • 2-hydroxybenzoic and ethanoic anhydride with a few drops of sulfuric acid
  • Filter the product using vacuum filtrstion
42
Q

How do you use lab equipment to filter under reduced pressure?

A

Using a Büchner funnel and Büchner flask connected by rubber tubing to the vacuum source

  • The funnel contains a layer of filter paper
  • Pour the substance onto the filter paper and the liquid will be sucked through via vacuum filtration into the flask
  • The solid will remain on the paper
43
Q

How do you purify a solid product?

A

By recrystallisation

  • Add minimum amount of warm solvent to the impure sample until it has dissolved
  • Allow to cool, crystals should form
  • When no more form you can filter under reduced pressure, washing the solid with solvent, to obtain a dry crystalline solid
44
Q

How do you determine the melting point of a substance and how is this useful

A
  • Place small sample for he solid in a capillary tube and melt, misusing the temperature with a thermometer
  • A pure substance will usually melt at a single temperature (or a very small range) but an impure substance will melt over a range of temperatures
  • Record the starting and ending points of the melting, when the first crystal can be seen to melt and when the last crystal becomes liquid respectively
  • You can then compare the melting point to known values to identify the substance
45
Q

How do you calculate percentage yield

A

(Actual yield/ theoretical yield) * 100

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