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The liquids trichloromethane and water separate into two immiscible layers when shaken together and allowed to stand. Ammonia can dissolve in both of these layers. The distribution of ammonia between these two solvents is called partition, where the concentration of ammonia in each solvent will be different. The partition coefficient represents the ratio of the distribution.
You are to plan an experiment, using a titration with sulfuric acid, to determine the value of the partition coefficient of ammonia between water and trichloromethane at room temperature.
The following information gives some of the hazards associated with trichloromethane and ammonia.
You are provided with the following.
• trichloromethane
• aqueous ammonia of concentration 5.00 mol dm$^{-3}$
• sulfuric acid, of concentration 0.500 mol dm$^{-3}$
• distilled water for dilution of aqueous ammonia
(a) Explain why ammonia is likely to be more soluble in water than in trichloromethane.
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(b) Define the partition coefficient, $K_{partition}$, for ammonia between water and trichloromethane. State whether the partition coefficient you have defined will be greater or less than 1.
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(c) In the experiment, explain whether it is important that the volumes of water and trichloromethane are the same.
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(d) Write an equation for the reaction of aqueous ammonia and sulfuric acid.
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(e) (i) The partition coefficient for ammonia distributed between water and trichloromethane is approximately 25. Calculate the concentration of aqueous ammonia that should be used so that a 25.0 cm$^3$ sample of the aqueous ammonia layer would require approximately 24.0–26.0 cm$^3$ of 0.500 mol dm$^{-3}$ sulfuric acid for complete neutralisation. Then state the factor by which the 5.00 mol dm$^{-3}$ aqueous ammonia should be diluted to give that concentration.
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(ii) Describe, in detail, how you would dilute 5.00 mol dm$^{-3}$ aqueous ammonia to make 250 cm$^3$ of aqueous ammonia ready for use in the experiment.
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(f) Other than the use of eye protection and gloves, state one safety precaution you would take while setting up the experiment.
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(g) State a suitable indicator for use in the titration of a sample taken from the experiment. Explain your answer.
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(h) It is unnecessary to titrate both layers of the partition. Explain why it would be better to titrate a sample of the aqueous layer rather than the trichloromethane layer.
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(i) Once a mean titre has been calculated, outline the steps you would take to calculate the partition coefficient.
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536
565
514
Nitrogen(II) oxide, NO, can be oxidised by ozone, O_3, in the atmosphere to form nitrogen(IV) oxide, NO_2.
It is possible to simulate this process in the laboratory to measure the rate at which this reaction takes place.
In this experiment, nitrogen(II) oxide is reacted with ozone. Since the concentration of nitrogen(II) oxide, [NO], in the air is very low, specialist equipment is required and [NO] oxide is measured as the number of molecules present in a volume of 1 cm^3.
The results obtained from the experiment are shown below.
[Table_1]
(a) Use the results obtained to plot a graph to show the relationship between [NO] and time.
(b) On your graph circle the single result that you consider to be the most anomalous. Suggest a reason why anomalous results may occur during an experiment to measure the rate of a reaction.
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(c) Use construction lines at concentrations of nitrogen(II) oxide equal to 13.5 \times 10^8 molecules cm^{-3} and 6.75 \times 10^8 molecules cm^{-3} to determine the order of reaction with respect to nitrogen(II) oxide.
Show the construction lines on your graph and your working.
[3]
(d) (i) Use your graph to calculate the initial rate of the reaction.
[2]
(ii) In the reaction between ozone and nitrogen(II) oxide the order of reaction with respect to ozone is 1.
In the experiment the initial concentration of ozone used was 4 \times 10^{11} molecules cm^{-3}.
Calculate the value of the rate constant for the reaction and give its units.
[2]
(e) 1 mole contains 6.02 \times 10^{23} particles.
Convert the initial rate in (d)(i) to a value with units of mol dm^{-3}s^{-1}.
(If you have no answer to (d)(i) you may use 3.0 \times 10^5 as the value of the initial rate.)
[2]
(f) The concentration of ozone used in the experiment is considerably greater than the concentration of nitrogen(II) oxide.
Explain why this is necessary for the experiment, in order to determine the order with respect to nitrogen(II) oxide.
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517
535
590
