CHEM0011 2021 May Exam (Open-Book) PDF

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UNIVERSITY COLLEGE LONDONEXAMINATION FOR INTERNAL STUDENTSMODULE CODE : CHEMASSESSMENT : CHEM0011A4UCPATTERNMODULE NAME : CHEM0011 - Chemistry for BiologistsLEVEL: : UndergraduateDATE : 04/05/TIME : 10:This paper is suitable for candidates who attended classes for thismodule in the following academi...

Description

UNIVERSITY COLLEGE LONDON

EXAMINATION FOR INTERNAL STUDENTS

MODULE CODE

:

CHEM0011

ASSESSMENT PATTERN

:

CHEM0011A4UC

MODULE NAME

:

CHEM0011 - Chemistry for Biologists

LEVEL:

:

Undergraduate

DATE

:

04/05/2021

TIME

:

10:00

This paper is suitable for candidates who attended classes for this module in the following academic year(s): Year 2020/21 Additional material

Periodic table.pdf and Physical constants.pdf

Special instructions

N/A

Exam paper word count

N/A

CHEM0011 page 2 of 8 Candidates should attempt ALL questions. Each question is marked out of 25 and the numbers in square brackets in the right-hand margin indicate the provisional allocation of marks to the subsections of a question. 1.

Answer ALL parts. (a)

Toluene (methylbenzene, C6H5CH3) is a useful hydrocarbon solvent. Sketch the molecular structure and indicate the hybridization of each carbon atom. Draw the orbital overlap(s) between an adjacent pair of ring carbons.

(b)

The infrared spectrum of toluene is shown below. What vibrations give rise to the signals around 3000 cm–1?

(c)

[3]

[2]

How would the infrared spectrum of C6H5CD3 differ from that of C6H5CH3? Predict the wavenumber (to two significant figures) for the peaks associated with the methyl-d3 group.

[4]

(d)

Define the term enthalpy.

[1]

(e)

Define the enthalpy of vaporization for a liquid.

[1]

[Question 1 continued overleaf

CHEM0011 page 3 of 8 Question 1 continued] (f)

The vapour pressure pvap (in Pa) of a liquid at temperature T is related to its enthalpy of vaporization vapH by the following equation, where c is a constant.

pvap

æ exp ç è

vap H ö

c÷ ø

RT

(i)

What is the physical basis for why the equation has this form?

[3]

(ii)

Convert the expression to a linear form.

[2]

(iii)

The table below lists the vapour pressure of toluene as a function of temperature. Using graph paper, plot a graph to determine the enthalpy of vaporization vapH of toluene. Include an estimate of the error in your final value.

(iv)

[6]

T/K

pvap / Pa

298

3800

313

7320

333

22600

343

29500

Rationalize the enthalpies of vaporization of the compounds listed below by comparison with the value you obtain for toluene.

[3]

Compound

Formula

vapH / kJ mol–1

benzene

C6H5–H

33.9

phenol

C6H5–OH

58.8

isopropylbenzene

C6H5–CH(CH3)2

44.0

CHEM0011 page 4 of 8 2.

Answer ALL parts. (a)

Define the rate of a reaction in terms of both reactant and product concentrations. [3]

(b)

What are the units of reaction rate?

[1]

(c)

Explain what is meant by the term first-order kinetics including a mathematical expression that describes such behaviour.

[3]

(d)

What are the units of a first-order rate constant?

[1]

(e)

Write the integrated rate equation for a first-order process, showing how reactant concentration changes with time. Indicate how you can plot concentration data to obtain the rate constant for such a process.

(f)

[3]

The analgesic drug paracetamol A is metabolized to N-acetyl p-benzoquinone imine B.

(i)

Suggest reasons why A is soluble in water.

(ii)

Use a thermodynamic argument to explain why the solubility of A in water increases with temperature.

(iii)

[3]

Suggest a spectroscopic technique that could be used to determine the concentration of B in aqueous solution. Justify your choice of technique.

(iv)

[2]

[3]

The concentration of B in blood plasma changes over time as it is metabolised and excreted, as shown below. time / h

[B]plasma / ng cm–3

0

5590

2

4590

3

4150

7

2820

Using graph paper, plot a graph to show that the decrease in concentration of B is a first-order process and determine its rate constant.

[6]

CHEM0011 page 5 of 8 3.

Answer ALL parts. (a)

In aqueous solution, the -amino acid alanine C3H7NO2 exists in different forms associated with proton transfer reactions. Sketch the structural forms associated with these equilibria.

(b)

How would the position of the equilibria in (a) differ if alanine were dissolved in liquid ammonia (NH3)?

(c)

[4]

Not all of the expected proton environments of alanine are observed in the 1H NMR spectrum above. Explain why this is.

(f)

[2]

The 1H NMR spectrum of alanine recorded in D 2O is shown below. Assign the spectrum, accounting for the observed splittings of the peaks in the spectrum.

(e)

[2]

Draw the likely hydrogen bonding interactions between alanine and liquid ammonia solvent.

(d)

[2]

[3]

Amino acids polymerize to form proteins. Write a general equation for the condensation reaction between two amino acids. Describe the nature of the peptide links, using hybridization models to describe the bonding between the two atoms involved.

(g)

The IR spectra of proteins display bands near 1500 cm–1 and 1650 cm–1. With the aid of a sketch, explain the origin of these bands.

(h)

[3]

Describe two examples of non-covalent interactions other than hydrogen bonds that contribute to the stability of the tertiary structure of a protein.

(i)

[4]

[3]

Rationalize the observation that when a protein folds in aqueous solution, the entropy change S > 0.

[2]

CHEM0011 page 6 of 8 4.

Answer ALL parts. (a)

In the following scheme, identify the products A to D and the reagent X, depicting clearly the absolute stereochemistry of compounds C and D. The 1H NMR spectrum of D is shown below.

(b)

(c) (d)

Identify the mechanism by which benzyl bromide reacts with water to give A, giving reasons for your answer.

[3]

Assign the stereochemistry of compound C as R or S. Show your working.

[2]

1

Assign the H NMR spectrum of compound D (below). The integrated ratios are shown above each peak.

(e)

[10]

[7]

A 0.5 M solution of the hydrochloride salt of C (25 cm3) was titrated using 0.25 M NaOH. Provide a sketch of the change in pH as a function of added base, indicating clearly the equivalence point, and explain how you would determine the pKa of any dissociable protons.

[3]

CHEM0011 page 7 of 8 5.

Iron plays a key role in many biological processes, which is attributed to its chemistry in aqueous environments. (a)

When a salt containing Fe2+ is dissolved in water, the complex [Fe(OH2)6]2+ is formed.

(b)

(i)

Sketch the structure of this complex and state its shape.

[2]

(ii)

What is the d-electron count of the iron in [Fe(OH2)6]2+?

[1]

The pH of a 9.49 × 10–4 mol dm–3 solution of [Fe(OH2)6]2+ was measured to be 6.26. (i)

Write a reaction equation to explain why the solution is acidic, clearly identifying the acid and the conjugate base.

[3]

(ii)

Define the pKa of an acid.

[1]

(iii)

Calculate the pKa of [Fe(OH2)6]2+.

[3]

(iv)

The iron(III) complex [Fe(OH2)6]3+ has a pKa of 2.20. Using your answer to part (iii), rationalize the difference in the two pKa values.

(c)

[3]

A 10.0 cm3 aliquot of the [Fe(OH2)6]2+ solution in part (b) was placed in a 100 cm3 volumetric flask. 10.0 cm3 each of 1,10-phenanthroline (structure below), sodium ethanoate and hydroxylammonium chloride solutions were added to the flask, after which it was made up to the line with deionized water. The resulting solution was placed in a 1 cm pathlength cuvette and analysed using UV/vis spectroscopy to give the spectrum below. The absorbance measured at a wavelength of 508 nm was 1.053.

(i)

Suggest, with reasoning, the roles of 1,10-phenanthroline, sodium ethanoate and hydroxylammonium chloride in the procedure.

[3]

(ii)

Why was the absorbance measured at a wavelength of 508 nm?

[1]

(iii)

Predict the colour of the solution, giving reasons for your answer.

[2]

(iv)

Determine the concentration of iron in the cuvette.

[1]

(v)

Calculate the molar absorptivity ε of the solution at 508 nm. Rationalize the number of significant figures to which you report your answer.

[5]

CHEM0011 page 8 of 8 6.

Answer ALL parts. (a)

(i)

Define the term refraction.

[1]

(ii)

Define the term refractive index in terms of the speed of light.

[1]

(iii)

Sketch and describe an experimental method to estimate the refractive index of a liquid.

(b)

[5]

A student conducts an experiment to determine the enthalpy of vaporization of water using a kettle rated between 2.4 and 2.8 kW and a digital balance that reads to the nearest gram. The following data are obtained. Mass of water before boiling

1413 g

Mass of water after boiling

1341 g

Time water is boiled

60 s

(i)

Calculate vapH in kJ mol–1.

[3]

(ii)

What is the estimated error in the measurement?

[4]

(iii)

Suggest two ways to improve the values obtained from the experiment, explaining which of the two is likely to have the more significant effect.

(iv)

Do these improvements affect the accuracy or the precision of the measurement? Explain your answer.

(c)

[4]

[3]

Design an experiment to test the relative sweetness of a new sweetener compared to that of sucrose.

[4]

END OF PAPER...