Practicals

Question 1

Reflux

Question 2

How to carry out acid-base titration

Answer 2

1. All apparatus rinsed with distilled water and then with the relevant solution.
2. Using a pipette and pipette filler, pipette 25cm3 of NaOH into a conical flask. Touch surface of solution with tip of pipette to ensure correct quantity is transferred.
3. Using a funnel, fill burette with acid and then remove funnel

• otherwise, some drops may fall through the titration after the initial reading has been taken => lower volume recorded than used.

4. Allow a small quantity of the acid to flow through the burette to ensure jet space is filled. Note initial reading on burette.

• If jet space not filled, titre value higher than volume actually added.

5. Add 2-3 drops of phenolphthalein (or another suitable indicator) to the conical flask and place it on a white tile - can see colour change better.
6. Add acid from burette into conical flask, swirling mixture during addition, until indicator changes colour (pink —> colourless).
7. During titration, rinse sides of conical flask with water —> ensures all acid is in solution.
8. Note the burette reading.
9. Repeat experiment but add acid dropwise (drop by drop) towards the end point.
10. Continue repeating until at least two concordant results obtained (within 0.10cm3).

Question 3

What things would we use mass change experiments for?

Answer 3

1. Formulae determination.
2. Water of crystallisation.
3. Decomposition.

Question 4

Outline how to carry out a mass change experiment.

Answer 4

Crucible weighed on a balance and its mass is recorded (mass 1).

2. Some sample placed in the crucible.
3. Crucible and contents reweighed on a balance and its mass recorded (mass 2).
4. Sample heated strongly in Bunsen.
5. Crucible and contents reweighed on a balance and its mass recorded.
6. The sample heated and reweighed until mass no longer changed —> ensures reaction completed.

Question 5

Why does rinsing the flask with water not affect the end point?

Answer 5

Water does not react —> doesn’t affect number of moles present.

Question 6

Purpose of conical flask

Answer 6

Can be swirled without losing any solution.

Question 7

Why is swirling necessary?

Answer 7

To ensure that solutions mix and so react completely

Question 8

Purpose of while tile

Answer 8

To see indicator colour change better.

Question 9

How can we ensure that the (mass change) experiment has gone to completion?

Answer 9

Heat until constant mass.

Question 10

Outline how to carry out a practical to measure Enthalpies of Combustion.

Answer 10

Fuel is burnt and the energy released is used to heat a set volume of water.

2. Fuel continues to burn until the temperature of water rises by a set change (20C etc.).
3. Mass of fuel used then measured using a balance.
4. Calculation.

Question 11

What are the main errors in the experimental procedure when measuring Enthalpies of Combustion?

Ways to reduce heat loss

Answer 11

Heat loss to surroundings (air/flask)

2. Incomplete combustion.
3. High temperature changes lead to evaporation of water.

=> Heat loss can be reduced by adding lagging to the flask.

Question 12

How can we limit heat loss for practicals measuring Enthalpies of Neutralisation/Displacement?

What is the main source of apparatus error - how can this be reduced?

Answer 12

Lagging in polystyrene cup.

2. Add a lid to the polystyrene cup.

Main source of apparatus error = thermometer —> can reduce this by increasing the temperature change.
—> can increase temperature change by using a lower volume but higher concentration of solution.

Question 13

How can we use a graph to calculate the temperature change (for Neutralisation/Displacement experiments)?

Answer 13

Plot a graph of temperature (y) against time (x).

2. Before the addition of reagent to solution (0-4 mins):

• Join points for first 3 minutes with a straight line.
• Extrapolate (extend) line to 4th minute.

3. After addition of reagent to solution (4-12 mins):

• Draw a line of best fit through the points between the 5th and 12th minutes and extrapolate (extend) this line to 4th minute.

=> Temperature change can be measured - distance between 2 lines. Show it clearly on graph.

Question 14

Outline how to carry out a practical to measure Enthalpies of Neutralisation/Displacement.

Answer 14

Solution is measured out and placed in a polystyrene cup.

2. Initial temperature of solution recorded for a few minutes before second reagent is added (allows temperature to equilibrate).
3. Set, known quantity (mass/volume) of second reagent is added into the solution at fourth time.
4. Temperature recorded every minute until some cooling observed.

NB=> Polystyrene cup used to reduce heat loss.

Question 15

Outline how to investigate how the Rate of Reaction changes with temperature for reactions with gas products.

Answer 15

Rate can be determined by measuring amount (volume/mass) of gas produced at regular time intervals (every 10/20s etc.).

2. Plot graph of volume/mass against time.
3. Rate calculated by determining gradient of a tangent to graph.

=> rate would have units based on the units of the axes on the graph.

Question 16

Outline how to investigate how the Rate of Reaction changes with temperature for reactions with NO gas products.

Answer 16

Rate determined by measuring time taken for a set amount of product to be formed.

(in this method Rate = 1/T —> units s-1.)

2. Equal volumes of reactants used in each experiment to ensure time recorded is for same amount of product each time.
3. If ppt formed:

• Reaction can be timed until enough ppt formed to obscure a cross on paper.

If coloured product is formed:

• Another reagent often added to reaction mixture which reacts with product as it is formed to produce a colourless substance.
• When this additional reagent runs out, a colour change is observed => reaction then timed until the colour change is observed.

=> When investigating effect of temperature, the reaction mixture is heated to required temperature - could be using a water/sand bath.

Question 17

Other control variables

Answer 17

1. Total volume
2. Concentration.
3. Surface area of any solid reactants.

Question 18

Limitations?

Answer 18

Enthalpy change of reaction will affect temperature at which reaction is completed.

2. At high temperatures, difficult to measure rate as water will begin to evaporate.
3. At high temperatures, time for reaction is very short —> increases degree of uncertainty in the timer.
4. Human reaction times will affect the number of decimal places that can be recorded accurately if the times are measured manually

Question 19

Purpose of adding HNO3 before ion tests

Answer 19

Reacts with other anions (and so removes them from the solution) —> may also produce a ppt such as CO3 2- /O 2- / OH-.

• Many carbonates, hydroxides and oxides are insoluble so would also form a ppt (except those of group 1 metal ions).

Question 20

Under what circumstances can we distil a product from a reaction?

Answer 20

Can distil a product from a reaction if it has a lower boiling point than the reactants.

Question 21

Why does a reaction mixture containing an aldehyde product need to be frequently cooled in an ice bath?

Answer 21

To prevent vaporisation of aldehyde product from the test-tube/boiling tube.

Question 22

Why are some organic reactions completed under reflux?

Answer 22

=> Can apply constant heat to a mixture without losing any of the liquid to evaporation - way of getting reaction to go to completion.

=> Many organic reactions need to be heated as this supplies the activation energy to break bonds and allow the reaction to occur.

^ Using a condenser allows this to happen whilst preventing the reagents from leaving the reaction vessel (they would otherwise evaporate).

Question 23

How does reflux work?

Answer 23

Reactants (and products) can be heated together constantly, over a period of time.

2. Components of reaction mixture don’t escape from the reaction vessel (as heating causes them to evaporate/boil).
3. Vapour formed is cooled by the condenser —> condenses then falls back into the flask.

Question 24

Draw reflux apparatus

Answer 24

Include:

• Liebig condenser.
• COLD water in (bottom right).
• COLD water out (top left)
• Pear-shaped flask.
• Anti-bumping granules.
• Open top.
• HEAT.

Question 25

Draw distillation apparatus

Question 26

Purpose of anti-bumping granules

Answer 26

-prevents bumping (sudden release of a large bubble of vapour ---> makes reaction mixture jump up the vessel). - Anti-bumping granules produce many small bubbles, rather than a large one ---> prevent uneven boiling.

Question 27

Why does reflux top need to be kept open?

Answer 27

Pressure (due to gas) can build up and could burst the glassware ---> keeping the top open allows pressure release.

Question 28

How can we heat reaction mixtures.

Answer 28

1. Bunsen Burner - although should be avoided if flammable reactants/products present. 2. Water bath - good for gentle heating but water will boil at 100C. 3. Sand bath - can achieve higher temperatures than a water bath. Often used in conjunction with a heating mantle. - Sand used to surround reaction vessel and conducts heat into vessel from sides as well => reduces need for stirring.

Question 29

Outline how to measure the Rate of Reaction by the initial rate method.

Answer 29

1. Rate determined by measuring time taken for a set amount of product to be formed. (in this method Rate = 1/T ---> units s-1.) 2. Equal volumes of reactants used in each experiment to ensure time recorded is for same amount of product each time. 3. If ppt formed: - Reaction can be timed until enough ppt formed to obscure a cross on paper. If coloured product is formed: - Another reagent often added to reaction mixture which reacts with product as it is formed to produce a colourless substance. - When this additional reagent runs out, a colour change is observed => reaction then timed until the colour change is observed. or: 1. Can measure rate by monitoring the change in concentration of the substance over time. 2. Graph plotted and initial rate determined by tangent to curve at t=0.

Question 30

Which variables should be kept constant? When would this method be often used?

Answer 30

1. Temperature. 2. Total volume. 3. Concentration. 4. Surface area of any solid reactant. => Often used to help determine the order of a reaction with respect to a reactant.

Question 31

Outline how to measure the Rate of Reaction using a continuous monitoring method - Gas product.

Answer 31

1. Rate can be determined by measuring amount (volume/mass) of gas produced at regular time intervals (every 10/20s etc.). 2. Plot graph of volume/mass against time. 3. Rate calculated by determining gradient of a tangent to graph. => rate would have units based on the units of the axes on the graph

Question 32

Outline how to measure the Rate of Reaction using a continuous monitoring method - Quenching method.

Answer 32

1. Sample removed at regular time intervals from mixture and then quenched with another reagent. 2. Other reagent may be something that reacts with the catalyst, or dilutes the solution so the rate is effectively reduced to 0. 3. The mixture is then titrated to determine the number of moles present in the sample which was removed.

Question 33

Under what circumstances would the cell EMF drop towards zero?

Answer 33

When the reaction reaches 0.

Question 34

Outline how to investigate how the pH changes when an acid reacts with a base. pH meter method.

Answer 34

1. pH meters first calibrated as after being stored for a while, the meter does not give accurate readings. 2. Using a series of buffer solutions with known pH, we record pH meter reading. 3. Simple calibration cuve plotted - pH meter reading vs pH of buffer solution. 4. Once pH measured - the calibration curve is then used to calculate the true pH. 5. pH meters can also be calibrated by using a small dial on the meter (although this can be a little fiddly).

Question 35

Outline how to investigate how the pH changes when an acid reacts with a base. Determination of Ka method.

Answer 35

1. Set volume of acid pipetted into a conical flask. 2. Alkali added 1cm3 at a time. 3. Mixture stirred between each addition. 4. pH measured and recorded after each addition. 5. As equivalence point approaches, the alkali should be added dropwise to allow for a more accurate determination of the equivalence point. 6. Once equivalence point passed, alkali is then added 1cm3 at a time until it is in excess. 7. Curve of pH (y) plotted against volume of alkali added (x) 8. Find volume added at equivalence point. 9. Halve this value to calculate the volume at half-equivalence. 10. Use the curve to read off the pH at this volume = value of pKa. ---> at this point, calibration curve used to adjust meter pH to actual pH.

Question 36

Outline practical procedure for the reaction to prepare a pure organic solid/liquid.

Answer 36

1. Reactants mixed in a suitable apparatus ---> could be a beaker, conical flask, Quick-Fit apparatus. 2. If reactants are to be cooled, ice bath used. 3. If reactants need to be heated, reflux usually used (with anti-bumping granules) ---> constant heating with no evaporation. 4. Reactions can be heated using water/sand baths or Bunsens - but they have limitations etc as previously mentioned.

Question 37

Outline how to collect a SOLID product.

Answer 37

1. Solid collected by filtration from the reaction mixture. 2. Solid then separated by filtration under reduced pressure using Buchner apparatus. 3. Once solid is isolated, washed using a suitable solvent. 4. Solvent needs to be able to wash away or dissolve any impurities on the isolated product but the residue must not dissolve in the solvent.

Question 38

Outline how to purify a SOLID product.

Answer 38

1. Purified by recrystallisation. 1. Crude product dissolved in minimum amount of hot solvent => creates a saturated solution at a high temperature - enables a high yield of product to recrystallise when solution cooled. 2. Solution filtered whilst still hot to remove any insoluble impurities (apparatus must be heated as crystallisation will occur if solution cools). 3. Saturated solution cools slowly to room temperature and then in ice ---> product then crystallises out of solution while other impurities remain dissolved. => Cooling in ice allows an increased amount of crystals to be formed. However, if saturated solution cooled too quickly, impurities crystallise out as well. 4. Solid then isolated using Buchner apparatus and then washed with some cold solvent to remove any soluble impurities which may have crystallised out of solution.

Question 39

During purification by recrystallisation, how do we know if a solvent is suitable?

Answer 39

Impurities either need to be very soluble in solvent, as they then won't recrystallise with the product. or - Insoluble in solvent as then they can be filtered and removed before cooling.

Question 40

Why may a graphite or platinum electrode be used?

Answer 40

will not react with the half cell solutions and will not affect the voltmeter readings

Question 41

How do you measure comparative electrode potentials of different metals?

Answer 41

● File a piece of copper using emery paper and connect it to the positive voltmeter terminal. ● Cut a piece of filter paper, saturate with KNO3 solution and place on top of the copper. ● Connect the voltmeter to another piece of metal. ● Hold the metal against the filter paper and record the voltmeter value. ● Repeat with different metals and record the results in a table.

Question 42

Why do you need to file/sand away the outer layer of the metal?

Answer 42

Why do you need to file/sand away the outer layer of the metal? It removes the oxide layer on the outside of the metal.

Question 43

Method for setting up electrochemical cell

Answer 43

1. clean a piece of copper and zinc using sandpaper 2. degrease them using some cotton wool and propanone 3. fold the copper over a beaker containing 50cm3 of CuSO4 solution - ensuring that the end of it is in the solution 4. fold the zinc over a beaker containing 50cm3 of ZnSO4 solution 5. soak a strip of filter paper in NaCl - this is acting as your salt bridge - and drape it over the beakers so that one end is in copper solution and the other in zinc 6. clip the metals with crocodile clips and attach to power supply, and read off voltage

Question 44

why do you degrease the metal with cotton wool and propanone?

Answer 44

grease could prevent the cell working as efficiently

Question 45

You are provided with the Daniell cell which uses both zinc and copper, including a zinc electrode of known mass. Briefly outline how you would carry out an experiment to calculate decrease in mass of the zinc electrode providing it contained a 100 cm3 of a 1.0 mol dm-3 copper(II) sulfate solution to begin with, and was allowed to produce electricity until the concentration of the copper(II) ions had decreased to 0.50 mol dm-3.

Answer 45

1. weigh the zinc electrode before and after use 2. allow the cell to discharge until [Cu2+] is 0.5 3. confirm this by using colorimetric measurement

Question 46

A suitable solution containing the acidified TiO2+(aq) ion is formed when titanium(IV) oxysulfate (TiOSO4) is dissolved in 0.50 mol dm−3 sulfuric acid to make 50 cm3 of solution. Describe an experiment the student does to show that the standard electrode potential for the TiO2+(aq) / Ti(s) electrode is −0.88 V The student is provided with: • the Cu2+(aq) / Cu(s) electrode set up ready to use (Ecell = +0.34V) • solid titanium(IV) oxysulfate (Mr = 159.9) • 0.50 mol dm−3 sulfuric acid • a strip of titanium • laboratory apparatus and chemicals. Your answer should include details of: • how to prepare the solution of acidified TiO2+(aq) • how to connect the electrodes • measurements taken • how the measurements should be used to calculate the standard electrode potential for the TiO2+(aq) / Ti(s) electrode.

Answer 46

Stage 1: Preparing solution (1a) Weigh 7.995 / 8.00 g TiOSO4 (1b) Dissolve in / add (allow react with) (0.50 mol dm-3) sulfuric acid (1c) transfer to volumetric flask and make up to the mark Stage 2: Set up cell Content can be shown in a labelled diagram (2a) piece of Ti immersed in (1 mol dm−3 acidified) TiO2+(aq) / the solution (2b) (connect solutions with) salt bridge or description (2c) (connect metals through high R) voltmeter Stage 3: Measurements and calculation (3a) record voltage/potential difference/emf of the cell (3b) Ecell = ERHS - ELHS Ecell = Ecopper - Etitanium (3c) ELHS = ERHS - Ecell OR Ecell should be +1.22 V if Cu on RHS (or −1.22 if Cu electrode on LHS)