Chemistry core practicals I
Chromatography
Method
Draw a pencil line across the bottom of the chromatogaphy paper, 2cm from the bottom
Put small spots of the unknown dyes on the line
Put water in the beaker, it should be shallow
Attatch the paper to a pencil and lower it into the beaker so the bottom of the paper reaches the line, but it doesnt go over the pencil line
Remove the paper when the water (the solvent) is 3/4 of the way up the page, draw a pencil line to mark this point
Leave to dry
Results
You should see that the unknown dye has separated into a few spots, showing the different inks involved
Two inks are the same if they travelled the same distance on the page
Rf value = distance travelled by solute/ distance travelled by solvent
Important Points
We don't use pen for the line as it's soluble so would ruin the experiment
If the ink started under the water, they would wash off
Paper is the stationary phase, solvent is the mobile phase
Acid Alkali titration
Place a funnel on top of a burette, and fill it with acid
Fill the jet of the burette and record the initial reading of the burette
Use a pipette filler to rinse and fill the pipette to the 25 cm3 mark with the sodium hydroxide solution
Empty the sodium hydroxide solution into a conical flask, add a few drops of methyl orange until the solution is yellow, then place it on a white tile
Let the acid freely flow into the sodium hydroxide until the solution turns peach, record the titre volume from the rough titration give u a rough idea of the acid needed
Rinse the apparatus and repeat the titration, this time stopping the flow 2cm3 before your titre volume, then drop by drop let the acid flow until the solution becomes peach, record the reading
Repeat the steps until u have concordant results, within 0.2cm3 of eachother
Preparing crystals of sodium chloride
Repeat the titration without any indicator using the titre volume
Pour the solution into an evapourating basin and heat until crystals start to form
Leave the basin until it is cool and the rest of the water has evapourated
Important points
Rinsing equipment allows you to avoid contamination from the previous tests
The white tile makes spotting a colour change easier
The end point is where the neutralisation occurs
Temperature changes
Method
Tranfer HCl into a polystyrene cup in a beaker to reduce heat loses
Place a thermometer and record the initial temperature
A sodium hydroxide solution to the cup
Place a plastic lid with a thermometer through it
Measure the highest temperature reached
Repeat with different volumes of sodium hydroxide solution
Results
Initially more sodium hydroxide increases the temperautre
The maximum temperature decreases as there isn't enough HCl to react and the energy released is spread out over a great volume
Combustion of alcohols
Method
Measure the mass of an alcohol burner and cap and record
Place the alcohol burner in the centre of a heat resistant mat
Use a measruing cylinder to add 100 cm3 of cold water to a conical flask
measure and record the initial remperature of the water and clamp the flask above the burner
Surround the apparatus with a draught screen
Remove the cap from the burner, light the wick and all the water to heat up by around 40 degrees
Replace the cap on the burner and measure and record the final temperature of the water
Measure the mass of the alcohol burner an cap again and record the mass
Wash out the flask with cold water and repeat using another alcohol and fresh water
Safety
Wear eye protection
All alchols are flammable, keep the tops on burners when not in use
Control variables
Height of the flask
volume of water
Important points
Alcohols show regular trend in values as they're all have the same functional group a regular increase in molecular structure
Errors - parallax error, zero error, alchohol evapourating, changing the height
Draft insulation reduces energy lost to surroundings