IB Chem2tr 1 Resources Prac GPR

7/18/2019 IB Chem2tr 1 Resources Prac GPR

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Chemistry for the IB Diploma

Guidance for Topic 1 – Practical 1

Methane rockets, a demonstration

Safety

Although great care has been taken in checking the accuracy of the information provided in this

guidance, Cambridge University Press shall not be responsible for any errors, omissions or

inaccuracies.

eachers and technicians should al!ays follo! their school and departmental safety policies. "ou

must ensure that you consult your employer#s model risk assessments and modify them as appropriate

to meet local circumstances before starting any practical !ork. $isk assessments !ill depend on your

o!n skills and e%perience, the skills and e%perience of your students, and the facilities available to

you. &veryone has a responsibility for his or her o!n safety and for the safety of others. he notes

belo! should not be regarded as a risk assessment.

"ou should carry out the practical yourself before presenting it to students. 'ake sure you are

comfortable !ith the procedures, and can anticipate any difficulties any of your students mayencounter.

Guidance

his demonstration sho!s the importance of stoichiometry.

he reaction bet!een methane and o%ygen is demonstrated using t!o different ratios of methane to

o%ygen. he mi%ture !hich corresponds to the correct stoichiometric ratio should give the more

spectacular reaction.

Apparatus and materials

&ach pair !ill need(

• plastic bottles )*++ cm-

• *++ cm measuring cylinder

• o%ygen

• rubber bung

Supporting the practical

Be careful !here the rockets are aimed. Check beforehand to see ho! far the rockets !ill fly.

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Determination of the empirical formula of magnesium oxide

Safety



inaccuracies.









Guidance

his practical e%ercise has been designed as a simple task to illustrate some of the points re1uired to

meet the IB internal assessment criteria. his practical cannot be used for assessment of data

collection and processing as the students have been told ho! to record their data and been given a

data table to fill in.

he practical can be used to introduce the ideas of uncertainties in measurements and in recording

data to the appropriate number of significant figures. 2or e%ample, for measuring masses using a

d.p. balance, the mass of magnesium should be recorded as +./+ 3 +.+/ g.

he results of this e%periment could also be compared !ith another e%periment, such as one using a

much smaller mass of magnesium, so that the uncertainty in the mass recorded by the balance

becomes much more significant.

4tudents should be encouraged to make detailed observations during practical !ork and to record

1ualitative as !ell as 1uantitative data.

Possible observations that could be made during this practical include the follo!ing(

• the crucible !as dirty

• the magnesium ribbon !as dull grey )indicating o%idation-

• the ribbon glo!ed bright !hite as it burned

•some !hite po!der escaped from the crucible as the lid !as lifted

• the final product !as a !hite po!der.

Possible sources of error in this practical include(

• formation of magnesium nitride by reaction !ith nitrogen in the air

• some of the magnesium o%ide escaped so the final mass !as lo!er than e%pected

• some of the magnesium had already been o%idised

• random errors due to the measuring apparatus

• if the crucible !as dirty the substance inside it could have decomposed or reacted !ith

the air.

2or the systematic errors, students should consider !hether they !ould cause the ratio of the elements

to be too high or too lo! )too much magnesium or too much o%ygen- and link this to their results.

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4tudents should suggest improvements to the procedure to reduce the errors. hese should be

both specific and possible to carry out in a school laboratory, e.g. clean the magnesium !ith

sandpaper prior to !eighing.

he formation of magnesium nitride is a significant source of error. An alternative procedure involves

adding a fe! drops of !ater after the magnesium has completely reacted. his converts the

magnesium nitride to magnesium hydro%ide and ammonia. 5eating the mi%ture strongly !ill then

decompose the magnesium hydro%ide to magnesium o%ide and !ill drive off any e%cess !ater.



• access to !eighing scales

• crucible !ith lid

• strip of magnesium, appro%imately +./+ g

• tripod

• clay triangle

• Bunsen burner )or other strong heat source-

• tongs


6hen heating the magnesium in the crucible the lid !ill need to be lifted in order to get sufficient air

to react. 'ake sure the students are careful !ith this as the !hite magnesium o%ide po!der can

escape easily and it !ill affect the students# results. 6ait for the crucible to cool do!n before

!eighing the sample again.

Clearing up

'agnesium o%ide can be placed in the bin once cooled.

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Determination of the value of x in MgSO 4·xH 2O

Safety



inaccuracies.









Guidance

4tudents !ill be able to see the change in mass !hen the magnesium sulfate is heated. he decrease in

mass is the amount of !ater that has been evaporated.



• crucible

• !eighing scales

• Bunsen burner

• tripod

• clay triangle

• hydrated magnesium sulfate

Setting up the practical

'ake sure that the crucible fits onto the clay triangle so that it is less likely to fall over. ake care

!hen heating as the crucible !ill get very hot.


It is important that the magnesium sulfate is heated until there is no more change in mass, asother!ise the results !ill not be very accurate.

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Determination of the Ar of lithium

Safety



inaccuracies.









Guidance

If this practical is to be used for assessment, the minimum amount of information must be given to

students. hey must not be told ho! to present the data or ho! to carry out the calculation.



• distilled !ater

• lithium

• conical flask

• measuring cylinders )/++ cm and *+ cm-

• bung !ith delivery tube and rubber tubing

Clearing up

All solutions can be !ashed do!n the sink !ith lots of !ater.

Data collection and processing

It !ould be e%pected that students !ill present the data in a table such as the one belo!.

Volume of water / cm3 /++ 3 /

Mass of lithium / g +./+ 3 +.+/

Volume of gas / cm3 /7* 3 /

Temperature / C 3 /

Pressure / mmHg 8*7 3 /

he students should also record 1ualitative data.

hey should carry out the calculation as follo!s, and !ill be e%pected to propagate errors to arrive at

a suitable number of significant figures for the final ans!er.

he number of moles of gas produced can be calculated using PV

9nRT

.

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Convert to an appropriate set of units(

• pressure 9 8*7 × /.0 9 /++ :*+ Pa

• temperature 9 .+ ; 8 9 <7 =

• volume 9 /7* × /+ >7 m

PV n

RT =

7/++:*+ /7* /+

:.,/ <7n

−

× ×=

×

n 9 7.87* × /+ > mol

he e1uation for the reaction is( ?i ; 5@ → ?i@5 ; 5

he number of moles of ?i is thus t!ice the number of moles of 5 ( +.+/* mol

massrelative atomic mass

number of moles=

r

+./+

+.+/,*, A =

Ar 9 8.</

Error analysis

&rror analysis must no! be carried out to determine the number of significant figures that can be

1uoted.

4ince the calculation involves multiplying and dividing 1uantities, percentage errors must be

calculated.

Unit conversion Percentage uncertainty

Mass of lithium +./+ 3 +.+/ g /+

Volume of gas /7* 3 / cm /7* × /+ >7 3 / × /+ >7 m +.7/

Temperature 3 / °C <7 3 / = +.0

Pressure 8*7 3 / mm5g /++ :*+ 3 /+ Pa +./

he conversion of °C to = involves adding !hat is considered to be a pure number and so the

absolute uncertainty stays the same.

6hen the units are converted for pressure the percentage uncertainty remains the same, thus(

the percentage uncertainty of the pressure in mm5g is

//++

8*7×

9 +./

he percentage uncertainty on the number of moles of hydrogen is calculated by adding the

percentage uncertainties for volume, temperature and pressure(

the percentage uncertainty for n is +.7/ ; +.0 ; +./ 9 /./

his percentage uncertainty stays the same !hen n is multiplied by )as is a pure number it does

not have an uncertainty-(

the percentage uncertainty on the number of moles of ?i is thus /./.

he percentage uncertainty on the relative atomic mass of ?i is the sum of the percentage uncertaintyon the number of moles and the percentage uncertainty on the mass, i.e. //./.









his is then converted to an absolute uncertainty by multiplying by the value of Ar ,

so absolute uncertainty 9

//./ 8.,</+.:

/++

×=

, to one significant figure.

he uncertainty is in the first decimal place and therefore the final value can be 1uoted as 8.0 3 +.:.

Further ideashe practical can be e%tended by measuring the volume of !ater in the conical flask more precisely,

!ithdra!ing /+.+ cm samples using a pipette and then titrating these against +./+ mol dm >

hydrochloric acid.

he practical procedure can also be adapted to determine the relative atomic mass of magnesium by

using +.+ g magnesium instead of lithium and /++ cm of +.*+ mol dm > hydrochloric acid in the

conical flask.

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Determination of vitamin concentration in a solution

Safety



inaccuracies.









Guidance

4tudents !ill be able to see the effect that temperature has on the rate of a reaction. hey !ill also be

able to calculate rates and dra! graphs. his practical activity is looking to sho! that as the

temperature of the reaction increases by /+ °C then the rate of the reaction should double. his should

also link to collision theory from the syllabus.



• sodium thiosulfate solution )+.+8+ mol dm >-

• vitamin C solution

• ethanedioic acid

• potassium iodide

• sulfuric acid )+. mol dm >-

• potassium iodate solution )+.+/+ mol dm >-

• starch solution

• conical flask

• * cm pipette

• burette

Setting up the practical

&very pair should make sure that the burette is fitted tightly to the clamp stand. 6hen the students fill

the burette !ith sodium thiosulfate they should place the clamp stand on the floor and pour in the

sodium thiosulfate using a small funnel, making sure the burette tap is closed. @nce full they should

then move the burette back to the desk.

Clearing up

4olutions can be poured do!n the sink !ith lots of !ater.

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Calculation

he calculation here can be 1uite tricky. If the practical is not to be used for assessment, then

!eaker students could be given e%tra help by the use of the follo!ing guiding 1uestions(

1 Calculate the number of moles of thiosulfate used.

2 6ork out the number of moles of iodine that the thiosulfate has reacted !ith.

3 Use the volume and concentration of potassium iodate added to calculate the number of moles

of molecular iodine produced.

Calculate the number of moles of iodine that reacted !ith the vitamin C.

! Calculate the number of moles of vitamin C in *.+ cm of the solution and hence the

concentration of the vitamin C solution.

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S!nthesis of alum, "Al#SO 4 $ 2·%2H 2O

Safety



inaccuracies.









Guidance

4tudents should carry out this e%periment in a fume hood.


&ach group !ill need(

• aluminium foil

• potassium hydro%ide ) mol dm-

•sulfuric acid ) mol dm

-• ethanol

• measuring cylinders )* cm and *+ cm-

• !eighing scales

• *+ cm beaker

• glass rod

• tripod

• gaue

• Bunsen burner

• !ater bath

• Buchner funnel

• filter paper


6hen boiling the solution be careful to not evaporate too much !ater from it. 6hen transferring the

chemicals the students should try to transfer as much as possible as this !ill affect their results.

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Documents

IB Chem2tr 1 Resources Prac GPR