PRACTICAL METHODS/KEY POINTS

Question 1

Describe the method for heating in a crucible?
- oxidation example

Answer 1

The mass gain by reacting magnesium in
oxygen can be measured by the following method
Mg (s) + O2(g) → MgO(s)
*Weigh an** empty clean dry crucible and lid **. *Add 2g of coiled magnesium ribbon (that has been cleaned
with emery paper) to the crucible and weigh again
Heat strongly with a Bunsen for a couple of minutes
**Occasionally lift lid with tongs to allow in some oxygen **but taking care not to let magnesium oxide powder escape *Allow to cool *Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant
mass ( do this to ensure reaction is complete).

Lid improves accuracy - prevents loss of solid from the crucible (should be loose enough to allow gas to escape)

Question 2

Describe the method for heating in a crucible
- decomposition example

Answer 2

The water of crystallisation in calcium sulphate crystals can be removed as water vapour by heating as shown in the following equation.
CaSO4.xH2O(s) → CaSO4**(s) + xH2O(g)
Method. *Weigh an empty clean dry crucible and lid . *Add 2g of hydrated calcium sulphate to the crucible and weigh again
*Heat strongly with a Bunsen for a couple of minutes *Allow to cool *Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant mass ( do this to ensure reaction is complete).

large amounts of hydrated calcium sulphate (eg 50g) should not be used as the reaction is likely to be incomplete

Question 3

Why is it important the crucible is dry before heating?

Answer 3

would cause mass loss to be too large, as water would be lost when heating

Question 4

Why should small amounts of solid not be used when heating in a crucible?

Answer 4

percentage uncertainty in weighing would be too high

Question 5

Describe how to make a volumetric solution

Answer 5

• weigh out known mass of solid using a balance
• add to beaker, and add the washings
• add distilled water to dissolve the solid - stir with a glass rod
• pour into a graduated flask using a funnel
• Rinse beaker and funnel and add washings from the beaker
  and glass rod to the volumetric flask.
• make up to the mark with distilled water using a dropping
  pipette for last few drops - bottom of the meniscus should sit on the line
• Invert flask several times to ensure uniform solution.

Question 6

Describe the most accurate way to measure mass

Answer 6

1. Measure mass on 2 or 3d.p. balance of a weighing bottle
   with the required quantity of solid in it
2. Empty mass into reaction vessel/flask
3. Reweigh the now empty weighing bottle
4. Subtract the mass of the empty weighing bottle from the
   first reading to give exact of mass actually added.

Question 7

Describe how to dilute a solution

Answer 7

*Pipette 25cm3 of original solution into a 250cm3 volumetric
flask
*make up to the mark with distilled water using a dropping
pipette for last few drops. * Invert flask several times to ensure uniform solution.

Question 8

Why is a volumetric pipette more accurate than a measuring cylinder ?

Answer 8

it has a smaller uncertainty

Question 9

Describe how to carry out a general titration

Answer 9

*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)
*add 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 *phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH is used] *methyl orange [yellow (alkali) to red (acid): end point orange] [use if HCl is used] *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

Question 10

Why does the burette need to be rinsed before use?

Answer 10

should be rinsed out with **substance that will be
put in it. **If it is not rinsed out the **acid or alkali added may be diluted by residual water in the burette or may react with substances left from a previous titration. **This would lead to the concentration of the substance being lowered and a **larger titre **being delivered.

Question 11

Why is it important to ensure the jet space of the burette is filled and all air bubbles are removed?

Answer 11

it will fill during the titration, leading to a larger than expected titre reading

Question 12

phenolphthalein colour changes

Answer 12

PINK in ALKALI
COLOURLESS in ACID
(use with titrations using strong alkalis)

Question 13

Methyl orange colour change

Answer 13

YELLOW IN ALKALI
RED IN ACID
ORANGE AT END POINT
(use in titrations with strong acids)

Question 14

When should you add water to the conical flask in titration?

Answer 14

• to wash the sides of the flask so that all the acid on the side is washed into the reaction mixture to react with the alkali.
• It does not affect the titration reading as water does not react with the reagents or change the number of moles of acid added.

Question 15

What is the equation for the manganate redox titration?
what is the colour change?

Answer 15

MnO4-(aq) + 8H+(aq) + 5Fe2+ (aq) TO Mn2+ (aq) + 4H2O (l) +5Fe3+ (aq)
COLOUR CHANGE - Purple to colourless
- self indicating due to the clear colour change

If the manganate is in the burette then the end point of the titration will be the first permanent pink colour.
Colourless  purple

Question 16

Which acid should be used in manganate titrations?

Answer 16

dilute sulfuric acid

ethanoic acid is too weak and would not supply enough H+ - same thing will occur if an insufficient volume of sulfuric acid is used
MnO2 would be produced instead of Mn2+ - brown colour - will mask the colour change and lead to a greater (inaccurate) volume of Manganate being used in
the titration.

Question 17

Reading vs measurement

Answer 17

Reading - the values found from a single judgement when using a piece of equipment
Measurement - the values taken as the difference between the judgements of two values

Question 18

How to reduce apparatus uncertainty?

Answer 18

• use apparatus with a greater resolution
• increase the size of the measurment made (experiments with the smallest readings will have the highest experimental uncertainties)

Question 19

How to reduce uncertainty in a titration?

Answer 19

• Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the error.
• Make the titre a larger volume. This could be done by: increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette.

Question 20

How to reduce uncertainty in measuring mass?

Answer 20

• Using a more accurate balance or a larger mass will reduce the
  uncertainty in weighing a solid
• Weighing sample before and after addition and then
  calculating difference will ensure a more accurate
  measurement of the mass added.

Question 21

Describe the solution calorimetry method for measuring enthalpy change

used to find enthalpy changes for reactions occuring in aqueous solutions such as enthalpy change of neutralisation

Answer 21

 put polystyrene cup in a beaker for insulation and support
 Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
 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

• curve can be extrapolated backwards to obtain a value for the exact temperature rise

Question 22

What are the main errors in the solution calorimetry method ?

Answer 22

• energy transfer from surroundings (usually loss)
  * approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.
• neglecting the specific heat capacity of the calorimeter- **we ignore any energy absorbed by the apparatus. **
• reaction or dissolving may be **incomplete or slow. **
• Density of solution is taken to be the same as water

Question 23

What are the main errors in the combustion calorimetry method?

Answer 23

• energy losses from calorimeter !!!!
• 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 **as H2O is gas, not liquid, in this experiment

Question 24

How to determine accurate temperature change from a temperature-time graph?

Answer 24

1. establish a baseline and a post reaction line
2. extrapolate these lines
3. The difference between the two extrapolated lines of best fit at the start of the reaction gives the accurate temperature change.

Question 25

Describe the method for purification of an organic liquid

Answer 25

1. add impure product to a **separating funnel** 2. WASH with** sodium hydrogencarbonate solution** (shake, and release pressure from CO2 produced) - neutralises any remaining acid 3. Allow the layers to **separate in the funnel,** and then run and discard the aqueous layer. 4. Add **drying agent** (**anhydrous calcium chloride**) to the organic layer 5. **Decant** liquid into a distillation flask and **DISTILL** to collect pure product

Question 26

Describe the technique of recrystallisation

Answer 26

1. dissolve the impure compound in the MINIMUM volume of HOT solvent 2. **Hot filter** through fluted filter paper - removes insoluble impurities - heat stops crystals reforming 3. **Cool** in ice - recrystalisation occurs (slower cooling = bigger crystals) 4.** Filter under reduced pressure** with a Buchner funnel to collect crystals 5. **Wash** crystals with distilled water (removes residual soluble impurities) and allow to **dry** ## Footnote An appropriate solvent is one which will dissolve both compound and impurities when hot and** one in which the compound itself does not dissolve well when cold.**

Question 27

Give three reasons for a **loss of yield **during the technique of recrystallisation

Answer 27

* Crystals lost when filtering or washing * Some product stays in solution after recrystallisation * other side reactions occurring

Question 28

Why are anti-bumping granules used in distillation and reflux?

Answer 28

prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles

Question 29

How do the melting points of pure and impure substances differ?

Answer 29

Pure - higher melting point with a narrow range Impure - lower melting point, wider range - compare experimental melting point with data source to determine purity

Question 30

Method for determing melting point of an organic solid

Answer 30

Two methods - electronic melting point machine - capilliary tube strapped to a thermometer, which is immersed in heating oil - Capilliary tube - seal end with bunsen burner and fill with crystals - heat tube slowly near melting point - record point at which it melts - then heat again but more gradually - more accurate determination of melting point

Question 31

Describe the method for thin layer chromatography

Answer 31

a) draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line. b) Use a **capillary tube** to add a tiny drop of each solution to a different spot and allow the plate to air dry. c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal. e) When the level of the solvent reaches about 1 cm from the top of the plate, **remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.** f) Place the plate** under a UV lamp in order to see the spots**. Draw around them lightly in pencil. g) Calculate the Rf values of the observed spots.

Question 32

What is the purpose of the salt bridge in an electrochemical cell?

Answer 32

connects the circuit - allows movement of IONS (usually made from filter paper soaked in salt solution - salt should be unreactive with electrodes and electrode solutions)

Question 33

What is the continuous monitoring method?

Answer 33

When we follow one experiment over time recording the change in concentration - the gradient represents rate of reaction

Question 34

What is the initial rate?

Answer 34

the rate at the start of the reaction, where it is fastest. It is obtained by taking the gradient of a continuous monitoring conc vs time graph at time = zero.

Question 35

Why are reactants in excess essentially zero order?

Answer 35

concentration stays virtually constant and does not impact rate

Question 36

Describe the half life of a first order reaction

Answer 36

**Half life is independent of concentration and is constant** (if half lives rapidly increase than the reaction is 2nd order) Rate constant (k) = ln(2) / t 1/2****

Question 37

What is the clock reaction? ## Footnote What are the assumptions?

Answer 37

- used to estimate initial rates, avoiding continuous measurements - The** time taken to produce a fixed amount of product is measured **as the initial concentrations of reactants are varied. - faster clock reaction = faster initial rate (INITIAL RATE = INVERSELY PROPORTIONAL TO TIME TAKEN FOR COLOUR CHANGE TO APPEAR) ## Footnote - temperature remains constant - reactant concentrations do not change significantly over the experiment

Question 38

Describe the method for the iodine thiosulfate clock reaction

Answer 38

1. add **sodium thiosulfate and starch indicator** to an EXCESS of **hydrogen peroxide and acidified iodide** 2. thiosulfate rapidly consumes any iodine produced 3. one thiosulfate runs out -** iodine accumulates causing a sudden blue-black colour change** 4. By varying the concentration of iodide or hydrogen peroxide while keeping other reactants constant, the time to reach this endpoint will change, reflecting the effect on reaction rate.

Question 39

Give four examples of continuous monitoring methods

Answer 39

1. measuring volume of gas evolved 2. measuring mass loss as gas escapes 3. measuring pH change 4. using colorimetry to quantify colour changes

Question 40

Describe the colourimetry method

Answer 40

1. callibrate colourimeter using a blank sample (distilled water) to set absorbance to zero 2. withdraw samples from reaction mixture at fixed points and measure absorbance CALLIBRATION CURVE 1. prepare a series of standard solutions of known concentrations 2. measure absorbance of each 3. plot a graph - concentration on X-axis, Absorbance on Y-axis 4. Unknown sample concentrations can then be determined by locating their absorbance on the curve and reading the corresponding concentration from the x-axis.

Question 41

How is a pH meter callibrated?

Answer 41

put the probe in two set buffers (pH 4 and 7) and press the callibration button

Question 42

Describe the method for construction of a pH curve

Answer 42

1. Transfer 25cm3 of acid to a conical flask with a volumetric pipette 2. Measure** initial pH of the acid with a pH meter** 3. Add** alkali in small amounts** (2cm3) noting the volume added 4. **Stir** mixture to equalise the pH 5. **Measure and record the pH to 1 d.p.** 6. Repeat steps 3-5 but when approaching endpoint add in smaller volumes of alkali 7. **Add until alkali in excess**