Sample/practice exam April 2019, questions PDF

Preview

Summary

CHEM001x page 1 of 6UNIVERSITY COLLEGE LONDONB. DEGREE 20xy M. DEGREE 20xy CHEM0010: Chemistry for Biology Students CHEM0011: Chemistry for Biologists Unit value: ½ or 1 Time allowed: 2 or 3 hoursCandidates should attempt any THREE/FOUR questions. Each question is marked out of 25 and the numbers in...

Description

CHEM001x page 1 of 6 Candidates should attempt any THREE/FOUR 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. Mephedrone (shown below), is a synthetic stimulant referred to as “bath salts” or “meow meow”. Not only is it illegal in the UK but it is also a solid at room temperature.

UNIVERSITY COLLEGE LONDON B.SC. DEGREE 20xy M.SCI. DEGREE 20xy CHEM0010: Chemistry for Biology Students CHEM0011: Chemistry for Biologists Unit value: ½ or 1 Time allowed: 2 or 3 hours

(a)

Discuss the carbonyl bond in terms of the hybridization of both the C and O atoms, illustrating your answer with clear diagrams of the overlaps and orbitals involved.

(c)

Explain what is meant by a hydrogen bond and describe the key features

(d)

Sketch the possible hydrogen bonding interactions occurring between a pair of

associated with these important interactions.

[3]

[3]

mephedrone molecules in the solid state and indicate how these interactions might affect the relative orientation of the molecules. (e)

[2]

The infrared spectrum of mephedrone is shown below. Indicate the most likely assignments for

(f)

(i)

the strong, broad bands occurring in the 3100-3500 cm–1 region;

[1]

(ii)

the bands around 3000 cm–1;

[1]

(iii)

the strong feature at 1700 cm–1.

[1]

Explain why the peak at 1700 cm–1 is so much stronger than many other peaks in the spectrum.

[2]

(g)

If the spectrum of the

(i)

O isotopically labeled mephedrone were recorded, which

Sketch the structure of the amino acid in solution at pH 1.5. Indicate clearly which are the titratable protons in the molecule.

answer. Calculate the new wavenumber(s) for peaks you would expect to show a

(i)

16

(h)

18

of the peaks listed in (e) would expect to shift? Provide reasoning to justify your O/18O isotopic shift.

[3]

Predict the 1H NMR spectrum of mephedrone. Provide reasons for assigning

(ii)

[2]

Indicate the equilibria and majority species present in the solution as the titration progresses.

[3]

Determine the pKa value for each of the titratable protons.

[3]

chemical shifts and for any coupling patterns that you include in your predicted spectrum. (i)

bonding picture for the aromatic group. 2.

[6]

3.

Account for the chemical shift of the aromatic hydrogens based on a suitable [3]

Answer ALL parts.

Answer ALL parts. (a)

State the First Law of Thermodynamics

[2]

(b)

Write an equation for the First Law, carefully defining all terms.

[2]

(c)

When CuCl2.2H2O is dissolved in acid solution, the temperature decreases

(a)

Explain what is meant by a Brønsted acid.

[1]

slightly. Explain whether the process is endothermic or exothermic and relate

(b)

Define the term pH.

[2]

your answer to the signs of the terms in the equation in part (b) of your answer.

[2]

(c)

Write a chemical equation for the dissociation of a weak acid in water showing

(d)

Rationalise the observation that CuCl2.2H2O is highly soluble in water.

[2]

(e)

An electrochemical cell was constructed as follows

clearly what happens to the hydrogen species involved.

[3]

(d)

Write the equilibrium expression for a weak acid, defining the term pKa

[3]

(e)

Calculate the precise pH for a 0.16 M aqueous solution of 2,6-xylenol, C6H3(CH3)2OH (pKa =10.59).

(f)

Ag(s) | Ag+(aq)|| Cu2+(aq) | Cu(s) Write the half-cell reactions and the balanced cell reaction.

(ii)

Calculate the standard cell potential difference given the standard half-cell

[2]

potentials of Ag|Ag+ and Cu|Cu2+ are +0.798 V and +0.340 V respectively. [2]

Considering bonding issues, suggest why the pKa of xylenol is smaller than that of ethanol (pKa = 15.50)

(g)

[4]

(i)

[3]

The amino acid aspartic acid has the formula HO2CCH(CH2CO2H)NH2. A pH

(iii)

Calculate the standard free energy for the reaction at 25 °C.

(iv)

Calculate the equilibrium constant for the reaction at the same temperature.

titration of a 10 cm3 aliquot of a 0.1 M solution aspartic is conducted using 0.1 M (v)

strong base. The results are shown below:

[3]

[3]

A solution is made up by taking 0.68 g of CuCl2.2H2O and dissolving it in 100 cm3 of water. Calculate the concentration (molality) of the solution together with the error in the concentration.

(vi)

fractions of the ions. (vii)

[2]

If the concentrations of Ag+ in the cell is 0.12 mol kg–1calculate the mole [3]

Using the Nernst equation, calculate the value of E assuming the concentrations given in part (v) and (vi).

[3]

(R = 8.31 J K–1 mol–1, F = 96485 C mol–1) 4.

Answer ALL parts. Urea (H2N)2CO is an important biological molecule which shows a strong absorption at 300 nm. (a)

Sketch the molecular structure of urea paying particularly close attention to the geometry around the carbon and nitrogen atoms.

[2]

(b)

Using a bonding model, account for the geometry around the nitrogen atoms.

[4]

(c)

In what part of the electromagnetic spectrum does urea absorb light and what colour would expect a pure sample of the substance to be?

[2]

(d)

Write the Beer-Lambert equation, defining all terms.

[2]

(e)

Under what conditions might you expect to observe deviations from ideal Beer-

OH I

O

+

A

Lambert behaviour and why?

HBr / H2O2

Urea was prepared by Friederich Wöhler in 1828 by the reaction of ammonia

Br

NaCN

with the cyanate ion under acidic conditions as shown in the following equation. NH4+ + CNO– ⇌ (H2N)2CO (f)

B

x

techniques that you could use in order to monitor the progress of the reaction. +

[2]

NaOH

F

O

y

H2O

OH

OH

cat. H+

K2Cr2O7 H+/H2O

D C

–

NH4 + CNO ⇌ HNCO + NH3 SOCl2

NH3 + HCNO ⇾ (H2N)2CO Write a mathematical expression to describe the overall rate of the reaction.

(h)

Write the rate equations for the elementary steps in the reaction. For one of these

(i)

What assumptions about the components of the reaction might you make in order

equations, define the order with respect to the components.

to derive the kinetic behavior of this reaction? (g)

Answer ALL parts.

H2N

HN

[3]

E

(a)

Identify each of the products A – F.

[7]

(b)

Identify the reagents x and y.

[2]

[3]

(c)

In what range would you expect to observe characteristic infrared bands for the functional groups of C.

[4]

[4]

(d)

Propose a reaction mechanism for the formation of compound C.

[5]

(e)

Predict the 1H NMR spectrum of compound C, explaining your reasoning for the chemical shift, splitting and integrated area of each peak.

[5]

(f)

Draw resonance structures for the nucleophile involved in the formation of F.

[2]

The predicted rate behavior is found to be second order in [NH4+]. What would you plot to confirm this, if you had a means of monitoring its concentration?

5.

O

[1]

Given the assumption in part (i), write a mathematical expression for the concentration of the HCNO in the reaction.

(h)

OH

If you wanted to study the kinetics of the reaction propose two physical

The reaction is known to proceed as follows:

(g)

O

[2]

END OF PAPER...