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Chemistry Paper 2 Practicals (Rf value self explanatory but P87 revision…
Chemistry Paper 2 Practicals (Rf value self explanatory but P87 revision guide)
Measuring rates of reaction
Precipitation and colour change
Record visual change in a reaction is the initial solution is transparent and the product is a precipitate which clouds the solution
Can observe a mark through the solution and measure how long it takes for the mark to disappear
If the reactants are coloured and the products are colourless, you can measure how long it takes for the solution to lose its colour
Results are very subjective - different people will agree on different times when the mark 'disappears'
Cannot plot a graph for the rates of reaction from the results
Change in mass
Measuring speed of a reaction that produces a gas can be carried out using a mass balance
As the gas is released, the mass disappearing is measured on the balance
If you take measurements at regular intervals you can plot a rate of reaction graph and find the rate quite easily
Most accurate method as the mass balance is accurate, but the gas is released straight into the room - use a gas syringe
Volume of gas given off
Involves use of a gas syringe to measure level of gas given off
Syringes usually give volumes accurate to the nearest cm^3, so quite accurate, and can take measurements at a regular interval so can plot a graph
If reaction is too vigorous the reaction can blow the plunger out of the end of the syringe
Rate experiments
Mg and HCI react to produce hydrogen gas
Add small volume of dilute HCI to a conical flask and place on a mass balance
Add magnesium ribbon to acid and quickly plug the flask with cotton wool, then start the stopwatch and record the mass on the balance, and take readings at regular intervals
Plot results in a table and work out mass lost with each reading
Repeat with more concentrated acids but keep most things the same eg. amount of HCI and magnesium ribbon used - higher conc should have faster rate of reaction
Sodium thiosulfate and HCI
Produce a cloudy precipitate
Both clear chemicals - produce a yellow precipitate of sulphur
Add small amount of dilute sodium thoisulphate to the conical flask and place over a black cross, then add some dilute HCI and start stopwatch
Measure time taken for the cross to be completely obscured by the precipitate
Repeat with one reactant having the same concentration and the other with a different (doesn't matter which)
Doesn't give a set of graphs but you do get a set of readings
Also subjective
Test for anions
Carbonates
Add couple drops dilute acid
Connect test tube to a tube of limewater
Bubble the gas produced through the lime water and if it goes cloudy then it contains carbonates as when carbonates react with acid CO2 is produced
Sulphates
Use dropping pipette to add few drops of dilute HCI followed by few drops of barium chloride solution
If sulphate ions white precipitate of barium sulfate will form
Halides
Add few drops of dilute nitric acid followed by few drops of silver nitrate solution
Chloride
gives
white
precipitate of silver chloride
Bromide
gives
cream
precipitate of silver bromide
Iodide
gives
yellow
precipitate of silver iodide
Test for cations
Flame tests
Clean platinum loop by dipping some dilute HCI and then hold it in a blue flame until it burns without any colour
Dip the loop into the sample and but back into flame, and record the colour it goes
Only works with mixtures containing single ions as mixture of ions some colours are hidden by others
Lithium
ions burn with a
crimson flame
Sodium
ions burn with a
yellow flame
Potassium
ions burn with a
lilac flame
Calcium
ions burn with an
orange-red flame
Copper
ions burn with a
green flame
Some metals form a coloured precipitate with NaOH
Many metal hydroxides are insoluble and precipitate out of solution when formed - some have a characteristic colour
Add few drops of NaOH solution to the solution of mystery compound
Calcium
forms a
white
precipitate
Copper (II)
forms a
blue
precipitate
Iron (II)
forms a
green
precipitate
Iron (III)
forms a
brown
precipitate
Aluminium
forms a
white
precipitate at first but then
redissolves
in excess NaOH to form a
colourless
solution
Magnesium
forms a
white
precipitate