Practicals paper 2/3 Flashcards
(15 cards)
Practical 2 (Paper 1, 2 + 3) Topic Energetics
Measurement of an Enthalpy Change
Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous magnesium chloride.
Stage 1 Method
(1a) Measures water with named appropriate apparatus
(1b) Suitable volume/mass / volume/mass in range 10 – 200 cm3/g
(1c) Into insulated container / polystyrene cup (NOT just ‘lid’)
(1d) Add known mass of MgCl2(s)
(1e) Use of ‘before and after’ weighing method. NOT ‘added with washings’
Stage 2 Measurements (could mark from diagram)
(2a) Record initial temperature (min 2 measurements)
(2b) Record T at regular timed intervals for 5+ mins / until trend seen
(2c) Plot T vs time
Stage 3 Use of Results (3a and 3b could come from diagram)
(3a) Extrapolate lines to when solid added (to find initial and final T)
(3b) Tfinal – Tinitial = ΔT / idea of finding ΔT from graph at point of addition
(3c) q = mcΔT
(3d) amount = mass/Mr (0.80/95.3 = 8.39 × 10−3 mol)
(3e) ΔHsoln = –q/8.39 × 10−3 or in words
what can you do to improve burning a fuel ?
Reduce the distance between the flame and the beaker
Put a sleeve around the flame to protect it from drafts.
what can you do to improve adding a solid to a solution or adding 2 solutions together to reduce heat loss ?
Add a lid/Insulate the beaker
Practical 3 (Paper 2 +3) – Topic Kinetics/Rates
Initial rates example: Investigation of how the rate of a reaction changes with temperature.
The initial rate of the reaction between sodium thiosulfate and hydrochloric acid can be monitored by measuring the time taken for a fixed amount of sulfur to be produced.
Na2S2O3(aq) + 2 HCl(aq) → 2 NaCl(aq) + SO2 (g) + S(s) + H2O(l)
Describe an experiment to investigate the effect of temperature on the initial rate of this reaction.
Include
- a brief outline of your method
- how you will measure the time taken for a fixed amount of sulfur to be formed
- how you will present your results in graphical form
- a sketch of the graph that you would expect.
Stage 1 Method
(1a) Idea of using disappearing cross or colorimetry
(1b) Puts acid or thiosulfate into container on/with cross or in colorimeter
(1c) Add second reactant and start timing
Stage 2 Measurements
(2a) Repeat at different temperatures (if number of temperatures stated, must be more than two)
(2b) Record time, t, for cross to disappear / defined reading on colorimeter
(2c) Idea of ensuring other variables (cross, volumes, concentrations) kept constant (apart from T)
Stage 3 Use of Results
(3a) 1/t (or 1000/time, etc) is a measure of rate
(3b) plot of rate (or 1/t etc) (y-axis) against T (x-axis) (can come from labelled axes on sketch) (IGNORE T against rate)
(3c) sketch of plot as shown (Allow 3c if axes not labelled but NOT if incorrectly labelled)
Practical 5 (Paper 2 +3) – Distillation of a product from a reaction.
Propanone can be made by reacting propan-2-ol with an excess of acidified potassium dichromate(VI).
The propanone is removed from the reaction mixture by distillation.
(a) The figure below shows the apparatus set up by a student to make propanone by this method. Suitable clamps are used to hold all the apparatus firmly in place.
There are three problems with the apparatus set up in the figure above.
For each problem:
- identify the problem
- describe the issue it would cause
- suggest how the problem can be solved.
Stage 1
Anti-bumping granules
1a no anti-bumping granules / add anti-bumping granules
1b to create smaller bubbles / to prevent large bubbles / to prevent mixture jumping into condenser
Stage 2
Open system with no thermometer
2a system should be closed (above flask) to prevent gases escaping
2b should be closed with (bung +) thermometer
2c to allow collection of propanone (only) / to prevent distillation of other components / to stay in suitable temperature range
Stage 3
The water direction in the condenser
3a water flows in wrong direction through condenser / change water direction
3b condenser not cool enough / not full of water
3c product may not condense / comes through as gas
Practical 6 (Paper 2+3) Tests for alcohol, aldehyde, alkene and carboxylic acid
COOH
a) NaHCO3 / Na2CO3 (or correct alternative)
b) effervescence /gas turns limewater milky
c) K and /or M but not L and/or N
-OH and -CHO
d) acidified K2Cr2O7
e) solution turns green
f) K and/or L and/or N but not M
-CHO
g) Fehlings OR Tollens
h) red ppt OR silver mirror
i) N only but not K and/or L and/or M
-Br
j) Silver nitrate
k) cream ppt
l) L and/or N but not K and/or M
Practical 7 – (Paper 2+3) Measuring Rates of Reaction
Initial Rates
A(aq) + B(aq) + C(aq) → D(aq) + E(aq)
In aqueous solution, A, B, C and D are all colourless but E is dark blue.
A reagent (X) is available that reacts rapidly with E. This means that, if a small amount of X is included in the initial reaction mixture, it will react with any E produced until all of the X has been used up.
Explain, giving brief experimental details, how you could use a series of experiments to determine the order of this reaction with respect to A. In each experiment you should obtain a measure of the initial rate of reaction.
Stage 1 Preparation
1a Measure (suitable/known volumes of) A, B and C (ignore quoted values for volume)
1b Use of colorimeter
1c into separate container(s) – (allow up to two reagents measured together into one container) – ignore use of X
Stage 2 Procedure
2a Start clock/timer at the point of mixing
2b Take series of colorimeter readings at regular time intervals
2c Use of same concentration of B and C / same total volume / (same volume/amount of X)
2d Same temperature
2e Repeat with different concentrations of A (can be implied through different volumes of A and same total volume)
Stage 3 Use of Results
3a Plot absorbance vs time and measure/calculate gradient at time=0
3b plot of gradient against volumes/concentrations of A
3c description of interpreting order from shape of 1/time vs volume or concentration graph
Example explanation of how to monitor the production of a gas in a rates question:
- Measure 50 cm3 of the 1.0 mol dm–3 hydrochloric acid and add to conical flask.
- Set up the gas syringe in the stand.
- Weigh 0.20 g of magnesium.
- Add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the stopwatch.
- Record the volume of hydrogen gas collected every 15 seconds for 3 minutes.
- Plot a volume of H2 gas produced (y) against time (x) graph
Continuous Monitoring
Potential 6 marker:
Describe how you can determine the order with respect to NaOH in the hydrolysis of methylethanoate ester [6]
CH3COOCH3 + NaOH → CH3COONa + CH3OH
Stage 1: Method
1a- Add 50cm3 of 1 moldm-3 CH3COOCH3 to a 250cm3 beaker + 50cm3 of 1 moldm-3 NaOH, start the timer.
1b – Every 30 seconds, transfer 5cm3 sample from mixture to a clean 250ml conical flask using a 10ml measuring cylinder and teat pipette
1c – Quench the reaction by adding 20cm3 of cold water
1d – Titrate sample with 1moldm-3 HCl in burette to determine the concentration of NaOH
Stage 2: Use of results
2a – Plot the concentration of NaOH /moldm-3 (y axis) vs time/secs (x axis)
2b – Draw tangents t=30 secs & t=60 secs and calculate the gradients
2c – Rate = gradient which will allow you to determine the rate at two different concentrations
Stage 3: Analysis
3a – Compare the change in concentration to change in rate.
3b – If the conc is x ½ and the rate x ½ = 1st order wrt NaOH as the concentration is directly proportional to rate.
3c – If the conc is x ½ and the rate decreases by x ¼ = 2nd order wrt NaOH as concentration2 is directly proportional to rate.
3d – If conc is x ½ and rate does not change, 0 order wrt to NaOH.
Practical 10 (Paper 2+3) Preparation of a pure organic solid and a pure organic liquid
Pure Solid and Purity Test – e.g. for purifying aspirin or solid benzoic acid
- Dissolve the impure compound in a minimum volume of hot solvent to create a saturated solution.
- Filter whilst hot to remove any insoluble impurities and heat will prevent crystals reforming.
- Cool the filtered solution by inserting beaker in ice to increase size of crystals formed.
- Pour filtrate through a Buchner funnel and filter under reduced pressure to separate out crystals.
- Wash the crystals with distilled water/cold solvent to remove soluble impurities.
- Dry the crystals between absorbent paper/in air.
Purifying an Organic Liquid
Example reaction:
Cyclohexanol is converted into cyclohexene by dehydration using distillation with concentrated H2SO4.
Once the initial reaction is complete:
Place your crude liquid sample in a separating funnel. (Often after distillation)
Add an equal volume of saturated NaCl (aq) solution – This ensures all water and polar compounds separate from the sample and dissolve in NaCl solution.
Shake and open separating funnel to release pressure build up.
Let two layers separate out. – There will be an aqueous polar layer and a non-polar organic layer.
Use the tap at the bottom of the separating funnel to run off lower layer.
To complete the purification process add a drying agent to the sample eg anhydrous Na2SO4 to remove any remaining water. Shake and allow to stand. The sample should go from cloudy to clear.
Note: the higher density liquid will form the lower layer.
Practical 12 (Paper 2+3)Separation of a species by thin-layer chromatography
what solvent can be used ?
Conc HCl
State the two developing agents
UV light
Ninhydrin
why is a lid used ? [3]
prevents escape of vapour / evaporation of solvent from beaker
so atmosphere in beaker is saturated with solvent vapour
·to reduce evaporation from the plate
State why the amino acids separate on the TLC plate ?
Difference in the balance between solubility in solvent/mobile phase and attraction to/retention on stationary phase
