paper 2 required practicals Flashcards
describe the method for the ‘disappearing cross reaction’ (rates of reaction):
- sodium thiosulfate solution + hydrochloric acid = sulphur (solid)
- the sulphur makes the solution go cloudy - scientists call this cloudiness turbidity. we can use this to see how long the reaction takes to finish.
- use measuring cylinder, put 10cm^3 of sodium thiosulfate solution into a conical flask.
- place conical flask onto a printed black cross.
- add 10cm^3 hydrochloric acid into the conical flask.
- swirl the solution and start a stopwatch.
- look down through the top of the flask. after a certain time, the solution will turn cloudy. stop the clock when you can no longer see the cross.
- repeat the experiment with lower concentrations of sodium thiosulfate solution.
- repeat the entire experiment, calculate mean values for each concentration of sodium thiosulfate solution.
what is the problem with the reproducibility of the ‘disappearing cross’ experiment?
- a measurement is reproducible if it can be repeated by another person or by using a different technique or equipment and still get the same result.
- the problem with this experiment is that different people have different eye sights.
- this means that some people can see the cross for longer than others, meaning they may not get the same results.
- however, because all students should use the same size printed cross, this problem shouldn’t be too great.
what are the hazards, risks, and solutions for the ‘rates of reactions’ practicals?
- hydrochloric acid - skin and eye irritation. wear goggles. cover skin.
what does the ‘rates of reaction’ experiment show us?
shows us that the greater the concentration of a chemical in a reaction, the faster the reaction takes place.
- because this result is shown by two different experiments, the finding is reproducible.
describe the method pt.1 for the chromatography required practical:
- use a ruler to draw a horizontal pencil line on the chromatography paper, around 2cm from the bottom
- mark 5 pencil spots at equal spaces along the line, leaving at least 1cm clear at each side
- use a capillary tube (very thin glass tube) to put a small spot of each of the known food colours and the unknown colour onto the pencil spots - relatively small spots, preventing colours spreading into each other
- pour water into a beaker, depth of 1cm
- attach the paper to a wooden spill balanced on the top of the beaker, lower the paper into the beaker. the bottom of the paper should dip into the water
- the water moves up the paper, the colours are carried up (don’t move the beaker)
what are the key things you must ensure in the chromatography practical?
- ensure the pencil line with the spots of ink is above the surface of the water, otherwise the water will wash the ink off of the line
- the sides of the paper must not touch the side walls of the beaker, this will interfere with the way the water moves
- put a lid on the beaker to reduce the evaporation of the solvent
describe the method pt.2 for the chromatography required practical:
- remove the paper when the water has travelled 3/4 up
- use a pencil to mark the point where the water has reached, called the solvent front
- hang the paper up to dry
what can we interpret from some chromatography required practical results?
- the unknown colour could have separated into multiple spots, telling us it’s a mixture of multiple colours
- line these spots up with the spots of the known colours on the other half of the chromatography paper
- this tells us the mixture of colours making up the unknown colour
how do you calculate the Rf values of the colours in chromatography?
- measure the distance from the pencil line to the centre of each spot
- measure the distance moved by the water from the pencil line
- divide the distance moved by the chemical by the distance moved by the solvent
does an Rf value have units?
no
describe the method for the water required practical:
- check the pH of the water - pure water has a pH of 7. place a small amount of water on a piece of universal indicator paper, which turns green if the pH is 7
- if the pH isn’t 7, then the water sample contains dissolved acid/alkali, so is not pure
- however, having a pH of 7 doesn’t guarantee it’s pure. it could still contain dissolved solids - use a balance to weigh an empty evaporating basin, record the mass
- fill the evaporating basin with the water sample, place it on a tripod and gauze
- use a bunsen burner to gently heat the water until it has all evaporated
- allow the evaporating basin to cool, and weigh it again
what can we imply from the water required practical?
- if the water sample contained any dissolved solids, then the mass of the empty evaporating basin will have increased
- as the water has evaporated, but the dissolved solids will have formed crystals on the surface of the evaporating basin
- if the water sample contains dissolved solids, it’s impure
why don’t we know for sure if water is ever completely pure?
it could still contain dissolved gases, so may still not be pure
how can you set up apparatus to purify water through distillation?
- conical flask containing the water sample. it’s on a tripod and gauze, heated by a bunsen burner
- top of the conical flask has a delivery tube, which is pointed into a test tube
- test tube is sitting in a beaker containing ice and water
how do you purify water through distillation?
- gently heat the water using a bunsen burner, as we want the water to boil gently
- the water will evaporate, and form water vapour (steam)
- the water vapour now travels along the collecting tube, and as it enters the cold test tube, it condenses back into liquid water
- this is distilled water (contains no dissolved solids, and has a pH of 7). it’s pure
