IB Chemistry internal assessment: Stoichiometry experiments

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IB CHEMISTRY INTERNAL ASSESSMENT
quantitative chemistry
These experiments will be used to practice/assess you on skills: DCP and CE.
A ‘2’ will be awarded if the following aspects for each criterion have met been met completely:
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DCP
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CE
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Records data which is appropriate to the investigation i.e. any measurements which are
needed to achieve the aim of the experiment. Include units and uncertainties.
Record any qualitative data (e.g. observations) which you think are useful and would
help the interpretation of the results.
Under the heading “Calculations’ process your raw data correctly including correct
significant figures.
Propagate uncertainties (HL only). SL: indicate a % error considering the apparatus
used.
Present your calculations in a logical manner.
Write a conclusion and explain how you have arrived at your conclusion i.e. how you
have interpreted the data you have collected.
Compare your value with the expected value i.e. the theoretical yields in experiments 1
and 2 and the expected concentration in experiment 3.
Evaluate the procedure and materials, identifying weaknesses and limitations and state
how they have affected your result.
Suggest any improvements to avoid the errors and others that you have listed.
Introduction
Three stoichiometry experiments have been described below. Hopefully, you have understood most
or even all of the principles of stoichiometry that have been introduced in the theory lessons.
You will now need to use your knowledge and understanding of these ideas and your practical skills
to solve these stoichiometry problems.
Data collection and processing (DCP)
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Read the aim and the method of the experiments and ensure you understand both and how the
method will achieve the aim. Within the method provided, you will now need to decide what raw
data to collect so that you are able to achieve the aim of the experiment.
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For your next step, under the heading “Measurements and Observations”, design an appropriate
results table which you will use to record all the raw data which you think is necessary. Don’t
forget to also have headings, units and uncertainties in each of your tables.
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You also need to decide how you will process this raw data to achieve the aim of the investigation.
You carry out the processing under the heading “Calculations”. Remember to show units in your
calculations and show the final answer in the correct number of significant figures.
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You will also need to propagate uncertainties.
Conclusion and evaluation (CE)
1st aspect:
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Write a conclusion (= reply to your aim) based on the experimental results.
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Indicate how you have used/interpreted the experimental results to arrive at your conclusion.
Give an explanation for your conclusion using your Chemistry knowledge.
IB Chemistry internal assessment: topic 1
page 1 of 5
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Compare your experimental results with expected value (theoretical value) or literature value and
state a % error between your experimental value and the expected value and if it is positive
(more) or negative (less). You should then compare this % error with the random error %
obtained from the propagation of uncertainties.
2nd aspect:
Evaluation of procedure (=materials/equipment + design/method) consists of:
Identification of systematic errors which are errors due to the quality of the equipment and materials,
poor experimental design and ‘incorrect’ use of the equipment. These errors cannot be calculated
and are also difficult to evaluate. However, these systematic errors can be reduced by using better
equipment/materials or improved experimental technique.
a. Evaluation of materials/equipment: Measuring tools improperly calibrated? Were the
measuring tools precise enough? Accurate enough? Incorrect concentration of reagents?
Impure reagents? Amounts of reagents used large enough?
b. Evaluation of procedure: Are there any weaknesses in the procedure which could have
caused an error greater than the % uncertainty? Did you make any errors when carrying out
the experiment e.g. did you not do some thing which you should have done or did you do it
incorrectly? Were some variables not controlled? Were readings duplicated?
c. Management of time: Did you run out of time and therefore compromised the procedure?
For each identified limitation and weakness indicate the direction of its effect on the experimental
result i.e. would it have caused your experimental result to be more or less.
Evaluation of data
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Did you collect enough data? Is the data you have collected reliable?
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Describe any limitations in the way you have interpreted your results eg have you ignored any
variables which you have not measured or could not have measured? Or have you made any
assumptions?
3rd aspect
For each suggested weakness and limitation suggest improvements.
Both the 2nd and 3rd aspect can be done using a table as shown below. There is no need to always fill
something in each row.
Limitation and
weakness
Effect on experimental result
Improvement
Materials and equipment
Procedure
Data
IB Chemistry internal assessment: topic 1
page 2 of 5
Investigation 1: Mass-mass relationship in a precipitation reaction
Aim
To compare the theoretical mass of one of the products of a double replacement reaction to the actual
yield of the same product determined by experiment.
Equipment
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balance,  0.01g
graduated cylinder, 100 cm3
2 beakers, 250 cm3
1 beaker, 100 cm3
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clamp and stand
funnel
stirring rod
Materials
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lead (II) nitrate, Pb(NO3 )2
potassium iodide, KI
filter paper
Procedure
1.
Using the balance, measure out approximately 3.30 g of lead (II) nitrate.
2.
Place the lead (II) nitrate in a clean 250 cm3 beaker and add 50 ml of water. Stir thoroughly
ensuring that all crystals are dissolved. Rinse the stirring rod.
3.
Measure out more than 4.00 g of potassium iodide and place it in a clean 100 cm3 beaker and
add 50 cm3 of water. Stir until all crystals are dissolved.
4.
Pour the potassium iodide solution into the 250 cm3 beaker containing the lead (II) nitrate
solution. Stir.
5.
Fold a piece of filter paper and place it in a funnel.
6.
Pour slowly the mixture from the 250 cm3 beaker into the funnel.
7.
Rinse the beaker with about 20 cm3 of water and pour the rinse water through the filter.
8.
Repeat step 7 until all the precipitate (lead iodide) is out of the beaker.
9.
Wash the precipitate by pouring about 10 cm3 of distilled water through the filter.
10.
Remove the filter paper and precipitate from the funnel and place it somewhere in the lab out
of the way and allow it to dry.
Precaution
Lead compounds are poisonous! Wash your hands after you have finished the practical work.
IB Chemistry internal assessment: topic 1
page 3 of 5
Investigation 2: Determining percentage yield of a neutralization reaction
Aim
To determine the percentage yield of a neutralisation reaction between sodium hydrogen carbonate
and hydrochloric acid.
Equipment
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balance, 0.01g
beaker, 100 cm3
Bunsen burner
bench mat
gauze
eye protection
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evaporating basin
dropping pipette
beaker, 250 cm3
tripod
watch glass
Materials
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hydrochloric acid, 6 mol dm-3
sodium hydrogen carbonate
Procedure
1. Weigh out approximately 2.50 g of sodium hydrogen carbonate and place it in the evaporating
dish.
2. Place an inverted watch glass over the sodium hydrogen carbonate in the evaporating dish.
3. Carefully take 6 mol dm-3 hydrochloric acid and pour it in a beaker.
4. With a dropping pipette add hydrochloric acid to the sodium hydrogen carbonate underneath the
watch glass.
5. Continue to do step 4 until the reaction has stopped and all sodium hydrogen carbonate has
reacted (the fizzing has stopped).
6. Warm the salt solution on a warm water bath with the watch glass still in place - but so that steam
can escape – to concentrate the solution.
7. When most water is gone, leave to crystallise.
8. When the salt in the evaporating dish is dry, take the necessary measurements
Safety
6 mol dm-3 hydrochloric acid solution is corrosive and can cause severe injury.
Therefore you must:
 wear safety goggles and gloves;
 inform your teacher about any spillages.
IB Chemistry internal assessment: topic 1
page 4 of 5
Investigation 3: Determine the concentration of Cl- ions in seawater
Aim To determine the concentration of chloride ions in sea water.
Equipment
 eye protection
 graduated flask, 100 cm3
 2 beakers, 100 cm3
 3 conical flasks, 100 or 250 cm3
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burette and burette stand
white tile
pipette (10 cm3) and safety filler
small filter funnel
Materials
 sea water
 potassium dichromate (VI) solution (usual bench concentration)
 0.05 mol dm-3 silver nitrate solution (8.48g in 1 L)
Background information to the reactions involved
Every litre of sea water contains 35 g of dissolved salts, though the amounts vary according to the
locality. Many different ions are present in sea water; the commonest cation is Na+ and the
commonest anion is Cl- the concentration of which you are going to determine in this experiment.
The method used to detect Cl- ions is titration with silver nitrate solution of known concentration.
Silver ions form insoluble silver chloride when added to a solution containing chloride ions:
Ag+ (aq) + Cl- (aq)  AgCl (s)
By adding silver ions until silver chloride is no longer precipitated, the amount of chloride in a solution
can be found. Potassium chromate(VI) can be used to indicate the end-point of the titration - the point
at which all chloride ions have been precipitated. Silver ions combine with chromate ions to form a red
precipitate of silver chromate (VI):
2Ag+ (aq) + CrO42- (aq)  Ag2CrO4 (s)
When both chloride ions and chromate ions are present, however, no silver chromate is precipitated
until all chloride ions are removed. The sudden appearance of red silver dichromate therefore
indicates the end-point of the titration.
Procedure
Silver nitrate is expensive and is normally used in fairly low concentrations. In this titration you will
use 0.05 mol dm-3 AgNO3 (aq). To obtain sensible results, it is therefore first necessary to dilute the
sea water ten-fold to give a concentration of chloride ions comparable to that of the silver ions.
1. Pipette 10 cm3 of sea water into a 100 cm3 graduated flask.
2. Make up to the mark with distilled water, stopper the flask and mix thoroughly.
3. Pipette 10 cm3 of the diluted sea water into a conical flask and add about 10 drops of potassium
chromate indicator.
4. Rinse a burette with a silver nitrate solution, then fill it with the solution.
5. Titrate the sea water in the conical flask against the silver nitrate solution from the burette until a
reddish tinge just begins to appear.
6. You may the find the end-point a little difficult to detect so it is best to carry out a rough titration
first and keep the result to remind you of the end-point colour when carrying out later accurate
titrations.
IB Chemistry internal assessment: topic 1
page 5 of 5