Practical Techniques

Question 1

What is the purpose of a bunsen valve?

Answer 1

To prevent suck back of cold water back up the delivery tube when the heat is removed, which can cause the glass the explode.

Question 2

Describe how to remove an air bubble from the tip of a pipette

Answer 2

Expel some solution (to remove the air bubble and suck up again with the tip of the pipette in the solution
Or fill pipette above the line and expel some solution (or can gently tap pipette to move air bubble to the top and out of solution).

Question 3

How can you mix a solid with a solution in a conical flask without loss of gas from system?

Seen in core practical 1

Answer 3

Place solid in vial and place in the conical flask. Place bung (attached to delivery tube etc) in conical flask. To mix the two together, shake the conical flask to tip the vial and so the reaction starts.

Question 4

Describe how to make up a standard solution (10g in 250 cm^3) in a 250 cm3 volumetric flask given the pre-weighed solid in a beaker.

Seen in CP2

Answer 4

1. Dissolve solid in distilled water using glass rod
2. Pour solution into a volumetric flask using a funnel
3. Rinse the beaker, glass rod, and funnel with distilled water and transfer the washings to the conical flask
4. Make up to the mark (250cm3)
5. Shake the volumetric flask

Question 5

Describe an experiment to find the concentration of a solution of hydrochloric acid using known conc of sodium hydroxide solution (of approximately equal concentrations)

Seen in CP3

Answer 5

1. Rinse burette and pipette with appropriate solution (one going to be in each) after rinsing with distilled water twice.
2. remove a fixed amount of HCl in conical flask and transfer using a glass pipette, fill burette with the NaOH solution. [ensure no bubbles in burette and tap filled with solution]
3. Add named indicator + colour change (i.e. phenolphthalein going from colourless to pale permanent pink)
4. Add solution from burette to flask until indicator changes colour
5. Add slowly towards end point, with white tile under conical flask whilst stirring.
6. Repeat titrations until concordant results

Question 6

Devise an experiment, giving outline details only, that would enable the relative rates of hydrolysis of halogenoalkanes to be compared.

CP4

Answer 6

1. Use of ethanol as a solvent
2. Use of silver nitrate solution
3. Equal amount of each halogenoalkane used
4. Measure the time taken for the precipitate to form (could be made less subjective by doing time taken for cross to disappear)
5. Use a water bath to control a raised temperature

Question 7

If 10g of your product dissolves in 100cm^3 of hot solvent and the solubility in cold solvent is 1g in 100 cm^3, how much product will crystallise?
What happens if you add excess hot solvent so that there is 200cm^3?

Answer 7

9g

Only 8g if you use 200cm3 as 2g of product stays dissolved

Question 8

Why don’t you use a hot oven (you should use a warm oven or dessicator) to dry your crystals?

Answer 8

Hot oven risks vapourising product or decomposing product so reduces yield.

Question 9

Devise an experimental procedure to determine the enthalpy change of solution for hydrated magnesium sulfate

Answer 9

1. 50 cm3 of water (measure using measuring cylinder) in polystyrene cup.
2. Polystyrene cup should have lid with thermometer placed through hole in lid. (can place in beaker to make more stable)
3. Measure 2g of magnesium sulfate using a balance.
4. Measure temp of solution before adding solid.
5. Add magnesium sulfate to polystyrene cup and stir with thermometer.
6. Measure temperature every 5 seconds. Plot temperature against time on graph and determine the maximum temperature reached during the reaction (change in temp is ΔT)
7. Q = mcΔT and then ΔH = -Q/(1000xn)

Determining max temp change requires extrapolation of the cooling curve as theoretically there is NO loss of thermal energy

Question 10

What assumptions make determining enthalpy change of solution slightly wrong?

Answer 10

• Solution =/= water so specific heat capacity and density not the same
• heat loss to surroundings
• heat absorbed by metal and thermometer
• specific heat capacity of the calorimeter not taken into account

couldnt find the onenote with dqs list of these so if you remember another assumption pls lmk

Question 11

Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?

Answer 11

Heat energy must be supplied for the decomposition so the temperature change measured is not solely due to the decomposition

Question 12

Explain why you would use a polystyrene cup rather than a glass beaker when performing calorimetry experiments with the purpose of calculating the enthalpy change for the thermal decomposition of potassium hydrogencarbonate?

Answer 12

Polystyrene is a better insulator than glass. Therefore, less heat energy is lost to/gained from the surroundings, so temperature changes are more accurate.

Question 13

Suggest 3 reasons why the calculated enthalpy change of combustion of ethanol is different from the data book value

CP8b

Answer 13

Sources of error include:
* heat loss to the surroundings from the spirit-burner, wick, and calorimeter
* evaporation of water
* evaporation of alcohol
* incomplete combustion of the alcohol
* reaction unlikely to occur under standard conditions, especially temperature

Question 14

Suggest ways to improve CP8b (enthalpy change of combustion of ethanol)

Answer 14

• Improving draught exclusion
• minimising the distance between the flame and the calorimeter
• placing a cap on the calorimeter
• insulating the spirit burner

Question 15

Why might you use a copper cup instead of a glass beaker when measuring the enthalpy change of combustion of ethanol

CP8b

Answer 15

Copper is a better thermal conductor than glass and so more of the heat energy is transferred from the container to the solution.

Question 16

Why can the enthalpy change of hydration of a salt not be measured directly by adding water to the anhydrous salt in the correct molar ratio?

Answer 16

1. Water in the hydrated salt is in the form of water of crystallisation (1)
2. which means that there is an ordered arrangement of the water molecules in the crystal structure (2).
3. This cannot be reproduced by adding the measured amount of water to the anhydrous salt as it will simply dissolve a small amount of the anhydrous salt. (3)

Question 17

Give a reason why using a measuring cylinder instead of a burette to measure a reagent would not affect the accuracy of the experiment

This does NOT apply to every situation a burette/measuring cylinder is used

Answer 17

If the reagent is in excess so it doesn’t matter if the volume added isn’t that precise as long as it is still in excess :/

Question 18

When oxidising an alcohol, why is it important to add the ethanol slowly to the hot oxidising agent?

Answer 18

The reaction is very exothermic so you want to avoid a violent reaction occuring (which could potentially, as well as being dangerous, cause some loss of product)

Question 19

Why would you allow a mixture to cool down to room temperature rather than submerging it in an ice bath to cool it faster?

Answer 19

Preventing the glassware exploding due to heat shock (due to rapid decrease in temperature)

Question 20

After chlorinating a tertiary alcohol using hydrochloric acid, why would you add anhydrous calcium chloride to the flask?

Answer 20

Ensures that any unreacted alcohol is in the lower aqueous layer. This is because the chloride ions shift the position of equilibrium of the chloroalkane
R-Cl <=> R+ + Cl-
to the left so that the unreacted alcohol molecules are more likely to form hydrogen bonds with the water molecules in the aqueous layer than the R+ molecules.

Question 21

When using a separating funnel, why must you ensure that the bung is removed before discarding the bottom layer?

Answer 21

So that you don’t create a system of reduced pressure.

Question 22

What is the benefit of using a separating funnel as opposed to a burette?

Answer 22

Allows more space (and the ability to shake in order to mix) when washing.

Question 23

Why would you add a weak base instead of a strong base like sodium hydroxide when neutralising any leftover acid after the chlorination of a tertiary alcohol with HCl?

Answer 23

The weak base, for example sodium hydrogencarbonate, neutralises the leftover acid but doesn’t release enough OH- ions into solution which would reform the alcohol.

Question 24

Why would you not use the tap to remove the upper layer (after discarding the bottom layer) from a separating funnel?

Answer 24

Because the tap still has some of the bottom layer in it. So you should tip it out the top of the separating funnel instead

Question 25

Why should you keep the filter (when removing insoluble impurities from the hot solution) hot during solid product recrystallisation?

Answer 25

Hot filtration prevents product crystallising in the funnel.

Question 26

What is the benefit of using fluted filter paper?

Answer 26

Faster filtration

Question 27

How should you heat a solution which has ethanol in it?

Answer 27

With a mantle or a water bath - DO NOT USE FIRE as ethanol is flammable.

Question 28

In CP16 (synthesis of 2-hydroxybenzoic acid), why do you initially perform the reaction in a cold water bath and then later warm the reaction mixture?

Answer 28

Cold water bath slows the reaction which is important because it is very exothermic - prevents boiling/ spitting which could lead to a loss of product. Later, there is a low concentration of reactants so thermal energy is supplied to increase the rate of reaction.

Question 29

What is dangerous about when you remove the heat during the experiment where you compare the thermal decomposition of group 2 carbonates?

Answer 29

You are bubbling the gas produced by the thermal decomposition through limewater. Removing the heat could lead to suck back of limewater through the delivery tube into HOT boiling tube. This can lead to explosion. How to fix this? remove the delivery tube from the limewater BEFORE removing the heat