Required Practicals & Practical Qs

Question 1

how can % uncertainty of a burette be reduced & why?

Answer 1

decrease conc. of solution in burette
so a larger vol. is used so larger titre reading

Question 2

how can % uncertainty for temp. change be reduced?

Answer 2

increase conc. of solution causing temp. change
so greater readings for change in temp.

Question 3

how can the enthalpy change experiment (2) be improved? (6 marker)

Answer 3

1. apparatus
   1a. use pipette instead of measuring cylinder
   1b. use polystyrene cup instead of beaker or insulate beaker
   1c. reweigh the watchglass after adding the solid
   1d. use powdered solid
2. temp. measurements
   2a. measure initial temp. every minute before adding solid
   2b. measure temp. every minute for 10 mins after the addition
3. analysis
   3a. plot graph of time vs temp.
   3b. extrapolate graph to the time of addition
   3c. find temp. change at the time of addition

Question 4

1a. describe a method to make up a volumetric solution

Answer 4

1. put weighing boat on mass balance & tare
2. add a known mass of solid & pour into beaker
3. reweigh mass of weighing boat to calculate the actual mass of the solid added (& add to beaker)
4. add 100cm^3 distilled water & stir with a glass rod until all solid has dissolved
5. transfer to a volumetric flask. rinse beaker & add washings to flask
6. add water up to the 250cm^3 line & invert the flask 20 times

Question 5

1b. describe the method for a simple acid-base titration

Answer 5

1. fill a burette the standard/volumetric solution of a known concentration
2. using a pipette filler, measure 25cm^3 of the solution with an unknown conc. & put it into a conical flask
3. add 2-3 drops phenolphthalein indicator to the solution in the conical flask
4. record the initial burette reading
5. do a rough titration in which you add solution from the burette until the indicator changes colour, record the final reading & calculate the titre volume (change in vol. in burette)
6. repeat, but adding dropwise near the end
7. repeat until you get 2 concordant results

Question 6

why do you rinse burette with the solution that will go in it?

Answer 6

to avoid diluting it/changing its conc.

Question 7

2. describe a method to investigate enthalpy change (using copper sulfate)

Answer 7

1. weigh out b/w 3.90 & 4.10g of anhydrous copper sulfate in a weighing boat & record the precise mass
2. make a table of results to record temperatures at minute intervals over 15 mins
3. using a measuring cylinder, measure 25cm^3 distilled water into a polystyrene cup
4. record the initial temp. of the water & start the time & record temp. every minute for 3 mins
5. at the 4th min, add the anhydrous copper sulfate to the water & stir but do not record the temp.
6. at the 5th min, continue to record the temp. every min up to 15mins, constantly stirring
7. repeat with 6.20-6.30g of hydrated copper sulfate & record precise mass
8. plot a graph of time vs. temp. for each set of results, draw 2 straight lines of best fit 1 before & 1 after the addition. Extrapolate both lines to the 4th minute.
9. use the graph to calculate the change in temp. at the time of the addition & use this to calculate energy change

Question 8

3. describe a method to investigate how temperature affects the rate of reaction

Answer 8

1. add 10cm^3 of 1moldm^-3 HCl to a small glass tube & place it in the plastic container
2. use a measuring cylinder to add 10cm^3 of 0.05moldm^-3 sodium thiosulfate to another glass tube ‘reaction tube’, put a thermometer in it & place it in the plastic container above the cross
3. record the start temp. in the reaction tube then add 1cm^3 HCl to sodium thiosulfate & start the stop clock
4. watch the tube from above & record the time taken for the cross to disappear
5. record the final temp. in the reaction tube then pour the contents of the reaction tube into a stop bath in the fume cupboard (sodium carbonate)
6. add hot water, cold water or ice to the plastic container to get the desired temp.
7. measure out another 10cm^3 sodium thiosulfate into reaction tube & leave both tubes in the water for 5 mins
8. repeat for a range of temp. from 5C-50C

Question 9

4a. describe a method to test for group 2 ions with sodium hydroxide

Answer 9

1. place 10 drops of the metal ion solution in a test tube
2. add 12 drops NaOH solution & swirl
3. continue adding NaOH dropwise until in excess
4. record observations
   Mg2+ white ppt
   Ca2+ white ppt
   Sr2+ solution stays colourless
   Ba2+ solution stays colourless

Question 10

4b. describe a method to test for group 2 ions with sulfuric acid

Answer 10

1. place 10 drops of the metal ion solution in a test tube
2. add 12 drops H2SO4 solution & swirl
3. continue adding H2SO4 dropwise until in excess
4. record observations
   Mg2+ solution stays colourless
   Ca2+ slight white ppt
   Sr2+ white ppt
   Ba2+ white ppt

Question 11

4c. describe a method to test for ammonium ions

Answer 11

1. place 10 drops of the ammonium chloride solution in a test tube
2. add 8 drops NaOH solution & swirl
3. warm in a hot water bath
4. use tongs to hold damp red litmus paper over the mouth of the test tube
5. record observations
   damp red litmus paper turns blue

Question 12

4d. describe 2 methods to test for hydroxide ions

Answer 12

first method:
1. place a few drops of sodium hydroxide on a watch glass
2. test the solution with a piece of red litmus paper & record observations
red litmus paper turns blue

second method:
1. place a few drops of ammonia solution on a piece of filter paper in petri dish
2. dampen red litmus paper & put on lid of petri dish
3. replace the lid & leave for a few minutes & record observations
damp red litmus paper turns blue

Question 13

4e. describe a method to test for carbonate ions

Answer 13

1. put 2cm^3 lime water in a test tube
2. in a different test tube, add a pipette-full of sodium carbonate solution, & prepare a delivery tube
3. add a pipette-full of hydrochloric acid to the second test tube & quickly put in the delivery tube with the open end in the lime water
4. Record any observations
   limewater turns from colourless to milky & white

Question 14

4f. describe a method to test for sulfate ions

Answer 14

1. put about 1cm^3 zinc sulfate solution in a test tube
2. add about 1cm^3 hydrochloric acid, then 1cm^3 barium chloride solution
3. record any observations
   white precipitate forms

Question 15

4g. describe a method to test for aqueous halide ions

Answer 15

1. put about 10 drops of the required halide ion solution in a test tube
2. add 5 drops of nitric acid & swirl
3. add 10 drops of silver nitrate solution
4. add excess of dilute ammonia solution & mix
5. repeat steps 1-3 with the same halide, then (working in the fume cupboard) add an excess of concentrated ammonia solution
6. record any observations
   repeat steps 1-7 using the other two halides
   Cl- white ppt then solution turns colourless w dilute ammonia
   Br- cream ppt then no visible change w dilute ammonia then solution turns colourless w conc. ammonia
   I- yellow ppt then no visible change w dilute or conc. ammonia

Question 16

4h. describe a method to test for solid halide ions

Answer 16

1. place a small sample of the required halide solid into a boiling tube
2. slowly add 5 drops of conc. sulfuric acid
3. test any gases evolved as follows:
   a. for potassium chloride: moist blue litmus paper turns red
   b. for potassium bromide: orange fumes
   c. for potassium iodide: purple fumes & black solid

Question 17

5. describe a method to distil ethanol to ethanal

Answer 17

1. pour 20cm^3 ethanol into a pear-shaped flask that has been weighed & reweigh to calculate the mass of ethanol
2. add 8cm^3 conc. phosphoric acid catalyst/acidified potassium dichromate to the flask
3. add a small spatula of anti-bumping granules to the flask so the ethanol boils more smoothly
4. heat the flask using a heating mantle, distilling the ethanol, which boils under 100C - have thermometer at top
5. distil about 5cm^3 of the liquid distillate into a test tube immersed in a beaker of ice-cold water to reduce evaporation of ethanal
6. test for ethanal using Tollen’s reagent

Question 18

5. do reflux method & apparatus diagram

Question 19

6a. describe a method to test for alcohol

Answer 19

1. add a piece of sodium metal to 1cm^3 ethanol
2. record observations
   fizzing, sodium sinks, solution stays colourless - sodium disappears

Question 20

6b. describe 2 methods to test for an aldehyde

Answer 20

1. add Tollen’s reagent
   silver mirror forms
2. add Fehling’s reagent & warm in a hot water bath for 5mins
   blue solution turns to red ppt

Question 21

6c. describe a method to test for an alkene

Answer 21

1. put 1cm^3 bromine water in a test tube
2. add 2 drops alkene
3. put bung on test tube & shake
   solution will turn from orange to colourless

Question 22

6d. describe a method to test for a carboxylic acid

Answer 22

1. put a spatula of sodium hydrogen carbonate in a test tube
2. slowly add ethanoic acid to the test tube
   bubbling –> solution stays colourless

Question 23

6e. describe a method to test for a haloalkane

Answer 23

1. put 1cm^3 sodium hydroxide solution in a test tube & add 5 drops of 1-bromobutane
2. warm in a hot water bath for 5 mins
3. add nitric acid & silver nitrate
4. record observations
   cream precipitate

Question 24

7a. describe a method to measure the rate of reaction by an initial rate method - iodine clock

Answer 24

1. fill a burette with potassium iodide solution
2. transfer 10cm3 hydrogen peroxide from burette to a beaker
3. use 50cm3 measuring cylinder to add 25cm3 sulfuric acid to beaker
4. use 25cm3 measuring cylinder to add 20cm3 distilled water & dropping pipette to add 1m3 starch solution into beaker with sulfuric acid
5. use burettes to add 5cm3 potassium iodide solution & 5cm3 sodium thiosulfate solution to this mixture
6. stir this mixture, pour hydrogen peroxide into beaker & immediately start stopclock
7. stop timer when the mixture turns blue-black & record the time
8. rinse & dry beaker then repeat 5 times with different concentrations of potassium iodide
9. plot a graph of concentration (x) vs initial rate (y) to determine the order

Question 25

iodine clock experiment in general terms (to determine to order of reaction with respect to A)

Answer 25

measure known vol. & conc. of reactants A & B add known conc. & vol. of X (reacts with E/iodine produced) measure known conc. & vol. of C in separate beaker add C to beaker of A, B & X & start timer record time taken for blue-black colour to appear (X all used up so E remains) repeat with different conc. of A keep conc. & vol. of B, C & X the same keep temperature at start of reaction the same using a water bath calculate rate by 1/t plot graph of rate vs conc. of A & deduce the order from the graph - draw graphs for 0, 1st & 2nd order

Question 26

what are the equations for the iodine clock experiment

Answer 26

hydrogen peroxide oxidises I- to I2 H2O2 + 2H+ + 2I- --> I2 + 2H2O thiosulfate ions immediately react with this iodine, reducing it to I- 2S2O3 2- + I2 --> 2I- + S4O6 2-

Question 27

7b. describe a method to measure the rate of reaction by a continuous monitoring method

Answer 27

1. measure 50cm3 of 0.8 mol dm-3 hydrochloric acid into a conical flask. 2. set up a 100cm3 gas syringe to collect & measure the gas given off 3. add 6cm of magnesium ribbon to the conical flask 4. record the volume of gas given off at regular intervals for approximately 3 minutes 5. repeat steps 2-5 using 0.4 mol dm-3 hydrochloric acid 6. plot graph of time (x) vs volume of gas collected (y) calculate rate of reaction by vol./time

Question 28

8. describe a method to measure the EMF of an electrochemical cell

Answer 28

1. clean copper & zinc pieces using sandpaper 2. degrease metal pieces with cotton wool & propanone 3. place copper into beaker with 1.0moldm3 CuSO4 solution 4. place each end of a string/filter paper soaked in KNO3 (or NaCl) in each beaker = salt bridge 5. connect the Cu/Cu2+ & Zn/Zn2+ half-cells by connecting the metals with croc. clips & wires & include voltmeter in the circuit to read the voltage see PMT for diagram

Question 29

9. describe a method to investigate how pH changes when a weak acid reacts with a strong base & a strong acid reacts with a weak base

Answer 29

1. rinse then fill burette with 0.1moldm-3 solution of ethanoic acid 2. use burette to transfer 20cm3 ethanoic acid to clean beaker 3. rinse then fill second burette with 0.1moldm-3 NaOH solution 4. rinse pH probe w distilled water then immerse it in ethanoic acid solution 5. stir solution with rod & record pH 6. add NaOH at in 2cm3 increments. stir & record pH for each volume 7. at end-point add 1.2cm3 NaOH each time 8. continue adding NaOH until it is in excess

Question 30

describe how to calibrate a pH meter/probe

Answer 30

1. rinse pH probe with distilled water 2. place probe in standard pH 7 buffer solution & record pH reading 3. repeat this process using standard pH 4 & pH 9.2 buffer solutions 4. plot a graph of recorded pH reading (x) vs pH of buffer solution (y)

Question 31

10. 'describe a method to prepare pure organic solid, test its purity & prepare pure organic liquid' to purify organic liquid

Answer 31

see RP 5 for liquid to purify organic liquid: 1. put distillate into separating funnel 2. wash with sodium hydrogen carbonate solution 3. run organic liquid into conical flask & add drying agent 4. decant liquid into distillation flask 5. distil to collect pure product

Question 32

describe how to test the purity of the aspirin

Answer 32

if impure: mp will be lower than data book value for pure aspirin it will melt over a range of temperatures if pure: mp same as value in data book it will melt at a sharp temp./very narrow range of temperatures

Question 33

describe the method used to measure melting point

Question 34

10. describe the recrystallisation process used to purify aspirin

Answer 34

1. dissolve impure compound in minimum volume of hot solvent 2. hot filter solution through filter paper (to remove insoluble impurities) 3. cool filtered solution by putting beaker in ice 4. filter under reduced pressure with Buchner flask to separate crystals 5. wash crystals with distilled water 6. dry crystals b/w absorbent paper

Question 35

why is reflux used?

Answer 35

allows organic reaction mixture to be heated without using any reactants or products

Question 36

11.

Question 37

12. describe a method to separate species by thin-layer chromatography

Answer 37

1. draw pencil line 1cm from bottom of TLC plate & mark 5 spots equally spaced along this line 2. use capillary tube to add a tiny drop of each solution to each spot & allow plate to dry - if req. repeat to get conc. spots 3. add solvent in sealed container 4. put TLC plate into the solvent with the spots above the line of the solvent. replace the lid to ensure sealed 5. when level of solvent reaches near top of the plate, remove the plate & mark the solvent front w a pencil. allow plate to dry in fume cupboard (bc solvent is toxic) 6. place plate under UV lamp to see the spots & draw around in pencil 7. calculate Rf values of the observed spots

Question 38

why does calibrating a pH meter improve the accuracy of the pH measurement?

Answer 38

over time/after storage the meter does not give accurate readings

Question 39

in rates experiment, how do you ensure other factors (apart from the IV) do not influence the rate of reaction?

Answer 39

use large amounts/concentrations so their surface area/mass/concentration are not proportionally changed much so do not affect the rate e.g. if reacting 2 solutions & IV is conc. of 1 solution, use a very high concentration of the other solution

Question 40

in XS CaCO3 & HCl rate experiment, what variables can be measured?

Answer 40

vol. of CO2 produced pH change conc. HCl

Question 41

why add NaOH dropwise around the equivalence point?

Answer 41

small addition of NaOH causes large pH change

Question 42

why is ___ not a suitable indicator

Answer 42

it does not change colour at the equivalence point

Question 43

why is universal indicator not a suitable indicator for a titration?

Answer 43

contains a mixture of indicators so colour changes gradually, not distinct colour change

Question 44

what 2 practical steps must be done before measuring initial burette reading?

Answer 44

1. remove funnel 2. open tap to allow solution to fill whole burette including jet space

Question 45

what safety precaution must be taking when filling a burette?

Answer 45

fill below eye level

Question 46

what are properties of a suitable drying agent?

Answer 46

reacts to remove water does not react with most organic compounds