practical skills review. titrations acid + base – indicator will be used. strong acid + weak base...
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Practical Skills review
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TitrationsAcid + base – indicator will be used.
• Strong acid + weak base– methyl orange– endpoint is orange
• Weak acid+ strong base – Phenolphthalein– endpoint is pale pink
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Redox reactions – can be self indicating• With potassium manganate (VII):
– 5Fe2+ + 8H+ + MnO4- 5Fe3+ + Mn2+ + 4H2O
– Ethanedioic acid + MnO4‑ CO2 + Mn2+ (may need heat)
– MnO4- in burette
– Endpoint given by pink colour remaining
• With sodium thiosulphate, Na2S2O3(aq)– I2 + 2S2O3
2- 2I- + S4O62-
– thiosulphate in burette– starch added when iodine solution is pale yellow– goes blue black– endpoint when this colour goes– I2 might have been formed from I- in another reaction e.g.
analysis of bleach:2I- + OCl- + 2H+ I2 + Cl- + H2O
In all cases the balanced equations will be given, so mole ratios are available
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Performing the titration• Filling up the burette
– no need to go to zero– use a funnel, remove the funnel– run some out before you use it to fill the nozzle.
• The pipette– used to transfer fixed volumes of a solution– fill to above the line– take off the filler– control volume with your thumb– transfer into flask by removing your thumb– don’t force the solution out!!!!
• The indicator (if used)– max 5 drops– permanent (20s) colour change
• Need to get colour change with one drop of solution– Do a quick rough– Perform 2 more to get concordant results (within 0.1 cm3)
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Recording values• table will be needed• showing initial and final values a long with the titre• 2 d.p., with 0 or 5 in second d.p.
– on a line = 0 e.g. 12.40 cm3
– between lines = 5 e.g. 22.65 cm3
Keep to a max of 3 titrations – 1 rough, 2 accurate; any more and time will be an issue.
rough Expt 1 Expt 2
Final/cm3
Initial/cm3
Titre/cm3
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Titre values• the difference between 2 values e.g.
26.40 – 1.55 = 24.85 cm3
• Need to have 2 within 0.1 cm3 of each other e.g.24.85 and 24.75 cm3 for a mark.
• Take an average of your best titres– usually 2 or 3 values– rough can be used if it appears to match well with
another value– show which titre values you use.
• Error in a single reading– half the smallest unit– which is ½ x 0.1 = 0.05 cm3
– NB if 2 readings are used the error actually becomes 2 x 0.05 = 0.1 cm3
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Possible calculations:
• Find concentration of an acid or base;
• Find molar mass of a reactant or part of a reactant e.g. the metal present in a carbonate or the n(H2O) present in a hydrated solid.
Layout your calculations in neat, clear and logical steps
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NB an endpoint can also be determined by measuring the temperature rise of the reaction– small portions of acid maybe added to the base– the temperature rise is recorded; – when the reaction is complete the temperature
will fall;– 2 distinct lines/curves are plotted;– Where they intersect gives the endpoint
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Weighing of a solidNeed to know a range of masses:• mass of weighing vessel e.g. plastic cup• mass of vessel + solid• mass of vessel + any residue (not always needed)• must be tabulated• be aware that it might need to be done twice in a single
table:
Masses/g Expt 1 Expt 2
Vessel
Vessel + solid
Vessel + residue
Solid added
This might be used for an enthalpy calculation e.g., or for a dehydration of a hydrated salt e.g. Na2CO3.xH2O
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Enthalpy calculations• Solid reacting with a solution e.g. carbonate + acid.• Mass of solid determined as described.• Solution will be measured using a burette, pipette or more
likely a measuring cylinder (systematic error is on the cylinder; random error in reading is half the smallest division).
• Initial temperature will be measured by leaving the thermometer in the solution for a few minutes whilst the solid is being weighed out
• table will be needed to represent this data – again be aware that 2 trials may be required:
Temps/oC Expt 1 Expt 2
Final
Initial
∆T
Error is again half the smallest division usually, so 0.5oC.
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CalculationH = m x c x ∆T• volume of solution (cm3)will normally give
the mass, m (g);• c, specific heat capacity of water is given
in the question (4.3 Jg-1oC-1);• ∆T is the average change in T found;• Gives a value in Joules• Convert to a kJmol-1 value
– divide by the number of moles of substance not in excess (usually the solid)
– divide by 1000.
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Dehydrations
Solid, e.g. CuSO4.xH2O:– Weigh a boiling tube;– Add solid;– Reweigh boiling tube +
solid– Table required
Solid is then heated to drive off the water of crystallisation– boiling tube is reweighed– Process is repeated until
weight is constant – change is <0.05 g
– Table is required:
Mass/g
Boiling tube
Boiling tube + hydrated solid
Boiling tube + solid after heating
Anhydrous solid
Large box for several readings
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Calculation:
• Mass of water = hydrated solid – anhydrous solid
• Convert masses of water and anhydrous solid to moles
• Get simplest ratio of solid : water to find x (CuSO4:H2O)
• If x is already known it can be used to find the molar mass of other parts of the formula e.g. XCO3.5H2O
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Precipitations
• Methods, hints and ideas provided very clearly in the lab manual.
• New bit – looking at halides– need to separate off the silver halide ppt before adding
NH3 to confirm Cl, Br or I.– Reason? – if Cu2+ or Fe3+ is present a ppt of the
hydroxide would form as well as the possibility of the silver halide dissolving or not.
• Care also needed with coloured solutions– Colour of ppt vs colour of the solution– need to let ppt settle or separate it out by decanting
and washing or filtration.
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The writing
• Clear descriptions of tests to be performed are required e.g.– Add a few drops of NaOH followed by an excess– Add 1 cm depth of silver nitrate followed by an excess
of NH3.
• Clear statements of observations are needed– ppt or not– colour of ppt – including white– does it dissolve in excess reagent or not– is a gas given off? And if so how is it identified?