AS PRACTICAL EXAM
Implementing
You will be asked to carry out one of the following experiments:
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An acid-base titration (Practicals 1 – 3)
An enthalpy change (Practicals 8, 9)
A series of simple inorganic or organic tests (Practicals 13-14)
Analysing and Evaluating
You will be asked to analyse and evaluate data on one of the following experiments:
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
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An acid-base titration (to determine concentration, relative molecular mass, water
of crystallisation or percentage purity) (Practicals 1 – 4) (hwks 1 – 3)
An enthalpy change determination (solution, displacement, neutralisation or
combustion) (Practicals 9 – 10) (hwk 10)
A gravimetric analysis (to find the water of crystallisation) (Practicals 6 - 7)
(hwk 8)
An experiment using gas volumes to determine mole ratios (Practical 5, hwk 6)
Planning
You will be asked to plan one of the following experiments:
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A gravimetric analysis (to find the water of crystallisation or empirical formula)
(Practicals 6, 7) (hwk 9)
An experiment using gas volumes to determine mole ratios or rates of reaction
(Practicals 5, 11) (hwk 7)
An enthalpy change of combustion (Practical 10)
An enthalpy change (solution, displacement or neutralisation) (Practicals 9, 10)
(hwks 11, 12)
An acid-base titration (this could include making up a standard solution or
diluting a solution) (Practicals 1 – 4) (hwks 4, 5)
IMPLEMENTING
General points:
- wear safety glasses
- don’t ask for additional reagents
Possible implementing exercises:
1.
acid-alkali titration (see practicals 1 - 3)
Don’t forget to:
- rinse the burette with acid
- rinse the pipette with alkali
- empty pipette under gravity
- touch surface of liquid with pipette
- remove funnel before titrating
- add only two drops of indicator
- swirl mixture during titration
- titrate dropwise near end-point
- read the burette accurately (eyes horizontal, bottom of meniscus)
- record burette reading to 2 decimal places (second 0 or 5)
- if your fifth titration is not concordant, fake it
Only stop after 2 titrations if the two readings are concordant – don’t cheat until
the fifth titration
2.
enthalpy change (see practicals 7 - 9)
Don’t forget to:
- support the polystyrene cup properly
- immerse the bulb in the solution
- swirl or stir mixture well
- don’t push the thermometer through the base of the cup
- record temperature to 1 decimal place
3.
simple chemical tests
Don’t forget to:
- place the correct amount of solution in the test-tube
- add the correct amount of each reagent
- record observations carefully and using the correct terminology
- record whether it is a precipitate or a solution
- state clearly the colour of the precipitate or the solution
- add a few drops first, then excess, and record whether a precipitate forms initially
and whether it dissolves in excess
ANALYSING
The most likely analysing exercises are:
a)
an acid-alkali titration
b)
an enthalpy change
c)
a gravimetric analysis
Don’t forget:
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

a)
Give all your answers to at least three significant figures
Make your working and terminology clear
When determining errors:
- the calculation is error/reading x 100
- don’t forget to add them up
acid-alkali titrations (see practicals 1 – 4)
Possible tasks could include:
finding the number of moles of water of crystallisation (see 2001, 2005)
finding the concentration of an acid (see 2002)
finding the relative molecular mass of an acid
finding the percentage purity of an acid or alkali
Make sure you only include concordant titrations in your average titre value
Quote your average titre value to two decimal places
b)
enthalpy changes (see practicals 8 - 10)
Possible tasks could include:
- finding the enthalpy change of a displacement reaction (see June 2003)
- finding the enthalpy change of a neutralisation reaction
- finding the enthalpy change of a solid dissolving in water
- finding the enthalpy change of combustion of an organic substance
Make sure you use q = mc∆T correctly
m = mass of solution (not mass of solid)
(in neutralisation reactions the mass of acid and alkali solutions must be added
together)
c = 4.18
Make sure you divide by the right number of moles
If the numbers of moles of the two reactants are different, use the smaller number
of moles (ie the reactant not in excess)
EVALUATING
1.
Comment on the quality of the results
- Were the titre results concordant?
- Are the temperature readings all on a straight line?
If so, the quality of the results is good. If not, it is poor
- Can you identify any anomalous results? Remember you get anomalour
results by rough titrations, or over-shooting the end-point
2.
Calculate the difference between the calculated value and the correct value and
express this as a percentage of the correct value.
- Give the difference before you express it as a percentage
3.
Comment on value of the answer in (2)
Either:
the percentage difference is less than the apparatus error
Or:
the percentage difference is greater than the apparatus error
4.
Suggest how the accuracy of the experiment could be improved and explain how
the change would improve accuracy.
-
Read through the procedure and identify an incorrect procedure, such as
Not including washings when making up a solution or not weighing by
difference or using a wet weighing bottle (this means that the mass used is
less accurately known)
Not using the correct apparatus (this means that the quantity is not being
measured accurately enough)
-
In an enthalpy change, the biggest error is always heat loss. It can be
minised by better lagging or by using a lid.
-
Look at the percentage errors for the apparatus used. For the apparatus
giving the largest error, suggest a way to make that measurement larger, such
as
Using more concentrated solutions
Using larger volumes
Using larger masses
-
If desperate, suggest repeating the experiment more times (this means
anomalous results are averaged out)
GUIDELINES FOR PLANNING EXERCISES
1.
2.
3.
4.
5.
6.
7.
gravimetric analysis (water of crystallisation or empirical formula)
gas syringe (rate of reaction or mole ratio)
enthalpy of combustion
enthalpy of solution/displacement/neutralisation
simple organic preparation
simple inorganic/organic tests (qualitative analysis)
titration
1. A gravimetric analysis – water of crystallisation or empirical formula
Use 3 – 5 g of solid
- too much and reaction may not proceed to completion
- not enough and apparatus error will be large
Apparatus – 2dp balance, crucible, Bunsen burner, tripod, clay pipe triangle
Weigh crucible (m1), add sensible mass and weigh crucible again (m2)
Heat for five minutes, allow to cool, weigh again
Repeat until mass is constant, record this mass (m3).
Repeat experiment
Deduce mass of solid at start of experiment (m2 – m1)
Deduce mass of solid at end of experiment (m3 – m1)
Deduce mass lost or gained (m3 – m2) or (m2 – m3)
Calculate moles of relevant substances (divide by mr)
Calculate mole ratio
Care with hot objects – use tongs
Compound may be toxic – wear gloves, wash affected areas after spillage
Wear eye protection
2. A gas syringe - rate of reaction or mole ratio
Max amount of gas is 100 cm – size of syringe
Volumes too small give a large apparatus error
Use pv = nrt to calculate the moles of gas, and hence moles and mass of reactant
Conical flask, bung, delivery tube, gas syringe
Weigh out reactants, add them to flask, and replace bung quickly
For mole ratio – wait until reaction is complete and record volume of gas
For rate of reaction – record volume produced after set time, eg 30 s
Repeat experiment
For mole ratio:
Calculate moles of gas produced
Calculate moles of reactant
Compare ratio
For rates of reaction:
Divide volume of gas by time taken
Reactants may be corrosive/toxic/harmful
Wear gloves/flood affected areas after spillage
Eye protection
Pipette filler if pipettes used
3. Determining the molar enthalpy of combustion
1. Pour 100 cm3 of water into a copper beaker
2. Place it above the alcohol burner and clamp it
3. Take the initial temperature
4. Place the alcohol burner on top of an accurate balance and record the mass, including lid
5. Place screens on either side of the burner to protect it from wind and heat loss
6. Light the alcohol burner
7. Put out the alcohol burner by placing the lid onto it when 2 grams (eg) have been used
8. Record the final temperature of the water
9. Repeat the experiment
10. Calculate the heat change (mc∆T) in J
11. Convert to kJ
12. Deduce the number of moles of alcohol used (mass/molar mass)
Divide the heat change by the number of moles
Fuels are flammable – avoid naked flames
Eye protection
Fuels are toxic/harmful – wear gloves, flood affected areas after spillage
4. Enthalpy of solution/displacement/neutralisation
1. Choose volume of solution which can be measured easily
2. If enthalpy of solution: use 3 – 5 g (more may not dissolve, less gives large error)
If enthalpy of neutralisation: use equal molarities of acid and alkali
If enthalpy of displacement: use excess solid
3. Pour solution into polystyrene cup, supported in beaker
4. Measure temperature of solution every minute for three minutes
(If neutralisation, also take initial temperature of other solution)
5. Add other reactant
6. Measure temperature every minute for 10 minutes
7. Stir mixture
8. Ensure bulb of thermometer is immersed in solution
9. Repeat
Plot graph of temperature against time (sketch a graph)
Extrapolate back to time at which second reactant was added
Deduce maximum temperature rise
Q = mc∆T
Convert to kJ
Divide by number of moles (smaller number if two reactants are involved)
Eye protection
Acids/alkalis toxic – use pipette filler, wear gloves, flood affected areas after spillage
Solids harmful/irritant – wear gloves, flood affected areas after spillage
5.
Carrying out a simple organic reaction
Possible reactions:
- oxidation of an alcohol (partial or complete)
- hydrolysis of a haloalkane
- elimination of a haloalkane
- dehydration of an alcohol
You will be given an outline of the experimental procedure so don’t panic
Just make sure you can describe the
Apparatus Draw reflux and distillation apparatus
Draw the apparatus to show an alcohol passing over heated ceramic pieces
and being collected over water.
Make sure your apparatus is never sealed (ie air must be able to escape)
Mention condensers and pear-shaped flasks
Reagents
Weigh out a few grams of the organic reactant, using a weighing bottle
Use excess reagent, eg NaOH or K2Cr2O7 and H2SO4
Hazards and safety precautions
Most organic compounds are flammable so avoid naked flames
They are also toxic so wear gloves
They are also volatile so use a fume cupboard
Always wear eye protection
Here is a step by step guide:
1. Work out the reagents that you need, and choose suitable quantities (remember the
reagents are generally in excess)
2. Decide whether the apparatus needs reflux or distillation apparatus
3. Describe the apparatus in detail (eg pear-shaped flask, condenser, bunsen burner with
gauze, dropping funnel) and make a sketch of the apparatus
4. You should be told how long to heat for and what temperature is needed. If the reagents
are flammable don’t use a Bunsen, use a water bath or oil bath.
5. If you used reflux, you then need to switch to distillation to separate out the products.
You should be given the boiling point of the reactants and products. Collect the substance
which distils off at the temperature close to the boiling point of the desired product.
Give safety precautions (don’t use flame if anything is flammable, wear gloves, clean
spillages immediately etc)
6.
Qualitative analysis
Although unlikely, you may be asked to plan a simple procedure to identify some simple
ions:
Just be aware of the following:
7.
how silver nitrate, nitric acid and ammonia are used to test for halide ions in
solution
how concentrated sulphuric acid can be used to test for halide ions in solid
samples
that magnesium and calcium ions give a precipitate when a few drops of sodium
hydroxide solution are added
that barium ions and sulphate ions give a precipitate of barium sulphate
that alkenes can be identified by their reaction with bromine water
that aldehydes can be identified by their reaction with Felling’s or Tollen’s
reagent
that primary alcohols, secondary alcohols and aldehydes turn acidified potassium
dichromate solution from orange to green
Making up a standard solution and titrating (acid-alkali)
1. Work out how many moles you need for a sensible titration (25 cm3)
2a. If solid:
- work out the mass you need
- weigh a weighing bottle
- add the correct amount of solid
- pour the contents of the solid into a beaker
- weigh the bottle again
- dissolve the solid in a little distilled water
- pour the solution into a volumetric flask
- wash the beaker several times, pouring the washings into the flask
- make up to the mark with distilled water, shaking repeatedly
2b. If liquid:
- pipette the required amount from the stock solution
- release it under gravity into the volumetric flask
- make up to the mark with distilled water, shaking repeatedly
3. wash out the burette with the acid
4. Pour the acid into the burette and take the reading
5. Use a pipette to take a sample of the alkali and release it into a conical flask
6. Add a few drops of indicator (phenolphthalein)
7. Add acid to alkali
8. Add alkali dropwise at the end-point
9. Until colour change from pink  colourless
10. Record volume in butette
11. Repeat until three concordant results obtained
find moles of alkali
moles of acid is the same
either – find concentration of acid (=moles/volume)
or – scale up to find moles of acid in volumetric flask
Safety issues – eye protection, acids and alkalis are corrosive so wear gloves or flood affected
areas after spillage, use pipette filler.