PRACTICALS Flashcards
When would heating in a crucible (ceramic) be used?
- measuring mass loss in various thermal decomposition reactions and
- for mass gain when reacting magnesium in oxygen
Why must small amounts of solid not be used when heating with crucible
percentage uncertainties in weighing will be too high
Why should large amounts of hydrated calcium sulfate e.g 50g not be used in heating with crucible
decomposition is likely to be incomplete
Why must the crucible be dry
wet crucible can give inaccurate results. it would cause mass loss to be too large as the water would be lost when heating
3.51 g of hydrated zinc sulfate were heated and 1.97 g of anhydrous zinc sulfate were obtained. Use these data to calculate the value of the integer x in ZnSO4.xH2O
When would a gas syringe be used
when volume of gas is measured
What is volume of a gas dependant on, how does this effect the experiment using gas syringe
pressure and temperature, so when recording volume it is important to note down the temp and pressure of room
How do you calculate moles of gas using syringe
use pV=nRT
What are 3 potential errors in using gas syringe
- gas escapes before bung is inserted
- syring sticks
- some gases e.g co2 are soluble in water so true amount of gas isnt measured
Draw the set up of using ga syringe to measure volume of gas being released from a reaction
How would you use a gas syringe to find mr of propane
- extract 0.2cm3 of propanone with HYPODERMIC syringe and weigh
- remove gas syringe from oven and note volume of air already in barrel
- inject propanone into SELF-SEAL cap into barrel. Plunger will move immediately
- put gas syringe back into oven
- measure mass of empty hypodermic syringe immediatly
- after few mins measure volume of gas in gas syringe, record temp of oven shelf and pressure of room
use pV=nRT to find moles
then mr= mass/moles
How do you make a volumetric solution?
- calculate mass of required substance needed to make 250cm3 of 1moldm-3
- weigh sample bottle/weighing boat on top pan balance
- with weighting boat on scale, tare to 0 and add required mass of substance with spatula
- transfer substance to beaker and reweigh the weighing boat (record difference in mass)
- add 100cm3 of distilled h20 to beaker, use glass rod to stir and dissolve solid
- pour solution into 250cm3 graduated flask via funnel
- rinse beaker and funnel and add washings from beaker and glass rod to volumetric flask
- make up to mark its distilled water using dropping pipette for last few drops
- invert flask several times to ensure uniform solution
why must the volumetic flask be shaken
ensure uniform concentration
why cant you place graduated flask near heat or put hot solutions in it
heat can cause flask to expanse and the volume would be incorrect
How is the mass measured accurately
measure on 2/3dp balance
measure weighing boat with and without mass of substance
find difference after emptying the mass into beaker
why is volumetric pipette more accurate than measuring cylinder
has a smaller uncertainty
how do you dilute a solution
pipette 25cm3 of solution and place into 250cm3 volumetric flask
make up to mark with distilled water using dropping pipette
invert flask several times to ensure uniform solution
General method for undergoing acid-base titration
rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
pipette 25 cm3 of alkali into conical flask
touch surface of alkali with pipette ( to ensure correct amount is added)
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point
*use a white tile underneath the flask to help observe the colour change
*add acid to alkali whilst swirling the mixture and add acid drop wise at end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
What are 2 indicators used in acid base titration and what are their colour changes and when should they be used
*phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH (strong alkali) is used]
*methyl orange [yellow (alkali) to red (acid): end point orange] [use if HCl (strong acid) is used]
Typically what substance is placed in the conical flask of titrations
25cm3 of unknown conc solution
Why is a conical flask preferable over beaker
easier to swirl mixture without spilling the contents
Why is it important to rinse out the burette with the substance that’ll be put in it
prevents dilution from residual water or reaction with substances left from previous titration
conc of substance will get lowered and larger titre delivered
What is an error that can be formed in the burette
jet space not filler properly prior to titration, larger titre reading than expected
Why must only a few drops of indicators be used
they are weak acids so if too much gets aded they’ll affect titration result
Why does washing with distilled water towards end of titration not affect results
water doesnt react with reagents or change number of moles of acid added
When are values considered concordant and hence acceptable to use
when 2+ values are within 0.1cm3 of each other
What are some safety precautions to take in simple acid-base reaction
Acids and alkalis are corrosive
(at low concentrations acids are irritants)
Wear eye protection and gloves
If spilled immediately wash affected parts after spillage
If substance is unknown treat it as potentially toxic and wear gloves.
Testing batches
In quality control it will be necessary to do titrations/testing on several samples as the amount/concentration of the chemical being tested may vary between samples.
Common method taken from mark schemes:
1. Weigh the sample bottle containing the solid on a (2 dp) balance.
2. Transfer solid to beaker and reweigh sample bottle.
3. Record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved. 5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3 mark with distilled water. 7. Shake flask.
What is the uncertainty in reading a burette during titration?
±0.10cm3
May add ±0.5cm3 due to uncertainty in reading the end point so can be 0.15cm3
What are 3 ways to reduce uncertainties in titration
- replace measuring cylinders with pipettes/burettes which have lower apparatus uncertainty which lowers overall error
- to reduce uncertainty in burette reading the titre volume needs to be made larger. done by : INCREASING VOLUME AND CONC of substance in conical flask or by DECREASING CONC of substance in burette
What is the uncertainty in the volumetric flask and 25cm3 pipette?
both are ± 0.1cm3
How do you calculate percentage uncertainty
uncertainty / measurement made on apparatus
When looking at a series of measurements, which one will have the highest experimental uncertainty
the experiments with the smaller readings
If the %uncertainty due to the apparatus < percentage difference between the actual value and the calculated value then there is a discrepancy in the result due to other errors.
If the %uncertainty due to the apparatus > percentage difference between the actual value and the calculated value then there is no discrepancy and all errors in the results can be explained by the sensitivity of the equipment.
What is the equation used to find energy change
q=mcT
Whats the general method in measuring enthalpy change using a CALORIMETER
- wash out equipment with solutions to be used
- dry cup
- put polystyrene cup in glass beaker for insulation and support
- measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
5.clamp thermometer into place making sure the thermometer bulb is immersed in
solution
- measure the initial temperatures of the solution or both solutions if 2 are used. Do this
every minute for 2-3 minutes - At minute 3 transfer second reagent to cup. If a solid reagent is used then add the
solution to the cup first and then add the solid weighed out on a balance. - If using a solid reagent then use ‘before and after’ weighing method
- stirs mixture (ensures that all of the solution is at the same temperature)
- Record temperature every minute after addition for several minutes
What can happen if the reaction of the enthalpy change is slow? How is the counteracted?
exact temperature rise is difficult to obtain as cooling occurs simultaneously with the reaction
to counteract: take readings at regular time intervals and extrapolate the temperature curve/line back to the time the reactants were added together.
What graph is drawn when finding enthalpy change
temperature y-axis
time in mins x-axis
what errors are found in calorimetry
- energy transfer from surroundings (loss)
- assumption of all solutions having the heat capacity of water
- neglect SHC of calorimeter (ignore any energy absorbed by apparatus)
- reaction or dissolving may be incomplete or slow
- density of solution is assumed to be same as water
How would you find enthalpy change from experimental data
use data to find energy change with q=mct
find number of moles of reactants used
energy change / moles of reactants
ensure to convert value to match the units
When are hess’s law cycles used to measure enthalpy change
for reactions that cant be measured directly by experiments
Detailed method for measuring enthalpy change of solution of anhydrous copper(II) sulfate
What do you use to experimentally find enthalpy of combustion
flame calorimetry
What are the dependant variables of flame calorimetry
need to measure:
mass of spirit burner before and after
temp change of water
volume of water in cup
What are potential errors in flame calorimetry
- energy losses from claroimetry
- incomplete combustion of fuel
- incomplete transfer of energy
- evaporation of fuel after weighing
- heat capacity of calorimeter not included
-measurements not carried out under standard conditions (e.g h20 is gas not liquid)
Describe method of investigating how temp affects ROR ( using sodium thiosulfate and HCL)
Measure 10 cm3 of 0.2 mol dm-3 hydrochloric acid and 10 cm3 of sodium thiosulfate in separate clean measuring cylinders. Put the solutions in separate boiling tubes
*Choose a temperature to investigate use water bath to get the two solutions to that temperature by placing the boiling tubes in the water bath.
*Place the flask on the centre of the large cross; first add the sodium thiosulfate to the flask. Then add the hydrochloric acid and start the stopwatch and swirl to mix the solutions.
*Stop the clock when the cross disappears and note the time.
*Repeat the experiment for four more different temperatures (maximum temperature should be 70 oC).
What graph is plotted for testing ROR changes with temp?
ln k = constant – EA/(RT)
k is proportional to the rate of reaction so ln k can be replaced by ln(rate)
From plotting a graph of ln(rate) (y-axis) against 1/T (x-axis)
activation energy can be calculated from measuring the gradient of the line
How do you test for group 2/3/ NH4+ ions
Method: adding dilute sodium hydroxide
a) Place about 10 drops of 0.1 mol dm–3 metal ion solution in a test tube.
b) Add about 10 drops of 0.6 mol dm–3 sodium hydroxide solution, mixing well.
c) Continue to add sodium hydroxide solution, dropwise with gentle shaking, until in excess
(can be used on gap 2 metals and transition metal ions)
Describe the solubility of grp 2 metal hydroxides down the group
increasing solubility down the grp
What are the results for the magnesium and calcium hydroxides
Magnesium hydroxide is classed as insoluble in water andwill appear as a white precipitate.
Simplest Ionic Equation for formation of Mg(OH)2 (s)
Mg2+ (aq) + 2OH-(aq)Mg(OH)2 (s).
Calcium hydroxide is classed as partialy soluble in water and will appear as a white precipitate (it may need more sodium hydroxide to be added before it appears compared to a magnesium solution.)
Simplest Ionic Equation for formation of Ca(OH)2 (s) Ca2+ (aq) + 2OH-(aq)Ca(OH)2 (s).
Why does strontium and barium not form a hydroxide precipitate when mixed with sodium hydroxide
their precipitate has high solubility. Solution will be highly alkaline
compare the difference in pH between magnesium and calcium hydroxide
suspension of magnesium hydroxide in water will be slightly alkaline so some OH ions must have been formed from slight dissolving
suspension of calcium hydroxide in water will be more alkaline than magnesium OH as its more soluble so more OH ions in solution
Whats the results of aluminium salts mixed with sodium hydroxide
forms white precipitate of aluminium hydroxide which dissolves in excess NaOH to form COLOURLESS solution
write the equations to show the formation of aluminium hydroxide precipitate from addition of NaOH and the dissolving of the precipitate in excess NaOH
[Al(H2O)6]3+(aq) + 3OH- (aq) Al(H2O)3(OH)3 (s) + 3H2O (l)
Al(H2O)3(OH)3(s) +OH-(aq)[Al(OH)4]-(aq)+3H2O(l)
What are the results of adding NaOH to the following transition metals copper, iron (ii), iron (iii) + equations
Copper solutions form a blue ppt
[Cu(H2O)6]2+ (aq) + 2OH- (aq) Cu(H2O)4(OH)2 (s) + 2H2O (l)
iron (II) solutions form a green ppt
[Fe(H2O)6]2+ (aq) + 2OH- (aq) Fe(H2O)4(OH)2 (s) + 2H2O (l)
iron (III) solutions form a brown ppt
[Fe(H2O)6]3+ (aq) + 3OH- (aq) Fe(H2O)3(OH)3 (s) + 3H2O (l)
How do you test for ammonium ions NH4+
Place about 10 drops of 0.1 mol dm–3 ammonium chloride in a test tube.
Add about 10 drops of 0.4 mol dm–3 sodium hydroxide solution. Shake the mixture. c) Warm the mixture in the test tube gently using a water bath.
Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube.
ammonia gas is released turning red litmus paper blue
