Core Practicals Flashcards
What is a standard solution
A solution with known concentration
How do you make a standard solution
- 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
What is the standard solution for finding concentration of sodium hydroxide
- 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
What is the weighing by difference technique?
- 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
What equipment is used to carry out the titration for sodium hydroxide
- 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
How do you carry out a titration?
- 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
Why are acid-base indicators used
To detect when a reaction reaches completion, usually by the presence of a colour change
What are concordant results?
Titres that are within 0.1cm3 of each other
How would you analyse results from sodium hydroxide titration
- 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
Bunsen burner : risk and control
Risk - burns
Control - Keep away from flammable chemicals and away from the edge of the desk
Chemicals : risk and control
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
How do you make a diluted solution of hydrochloric acid?
- 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
How do you analyse the results from the NaOH HCl titration
- 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
Glassware (beakers, test tubes) : risk and control
Risk - When broken could cut you
Control - Handle with care and keep away from the edge of the desk
Why are acid-base indicators used?
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
Why does a pH probe need to be calibrated?
So that for each pH reading, the pH value is accurate
How do you calibrate a pH probe?
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
What is accuracy
The more accurate the data, the closer it is to the actual value
How do you carry out a titration to calculate pH?
- 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
How do you calculate Ka from titration to find pH
- 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
What is a redox titration?
A titration of a reducing agent by and oxidising agent or the other way around
How would you find the amount of iron in an iron tablet experimentally?
- 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
How would you reduce uncertainty in a redox titration
- Use a balance with greater resolution
- Use pipettes and burette with greater resolution
- Use greater masses and volumes so percentage uncertainty is reduced
What is the weighting by difference technique (used to measure mass of the iron tablets) ?
- 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
What is the ionic equation for the reaction between acidified iron and potassium manganate (VII) ?
5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O
What is rate of reaction?
The change in concentration of reactants of products over time
How can rate of reaction be measured?
- 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
What is a continuous monitoring method
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
Give an example of a continuous monitoring method
- 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
How would you analyse the results from this reaction?
- 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
What is an initial rates method
The method involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies
Give an example of an initial rates method
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
What are the issues with the iodine clock experiment
- Low [I-] may take too long to react
- Delayed stopwatch reactions
- Concentrations may not be exact due to measuring apparatus
Iodopropanone: risk and control
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
Propanone: risk and control
Risk - Irritant, highly flammable
Control - Handle with care and with gloves. Wear eye protection. Keep away from an open flame
Sodium thiosulfate: Risk and control
Risk - Releases sulfur dioxide which is toxic and corrosive
Control - Ensure room is well ventilated. Don’t ingest
What are the main steps in producing a pure organic solid?
1) Synthesis of the compound (usually using reflux, distillation etc.)
2) Filtration (usually vacuum filtration)
3) Purification (recrystallisation)
How do you due lab equipment to heat under reflux
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
Why is heating under reflux used?
- Allows heating for a long period of time
- Prevents the flask from boiling dry
- Prevents volatile reactants / products escaping
- Ensures even heating
Why are anti bumping granules used when heating under reflux and distillation?
To allow smooth boiling preventing bubbles (caused by vapour) in the hot liquid from bubbling up the sides of the flask
What compounds are heated under reflux together in the first step to create aspirin?
- 2-hydroxybenzoic and ethanoic anhydride with a few drops of sulfuric acid
- Filter the product using vacuum filtrstion
How do you use lab equipment to filter under reduced pressure?
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
How do you purify a solid product?
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
How do you determine the melting point of a substance and how is this useful
- 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
How do you calculate percentage yield
(Actual yield/ theoretical yield) * 100
