sir michelangelo refalo sixth form -...

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SIR MICHELANGELO REFALO

SIXTH FORM

Half-Yearly Exam 2016

Subject: Chemistry ADV 1ST Time: 3 hours

Answer 6 questions. All questions carry equal marks. You are reminded of the importance

of clear presentation in your answers, and the use of good English.

A copy of the Periodic Table of Elements is printed on the last page of this booklet.

Important information: The molar gas constant R = 8.31

Question 1: This question is about energetics.

(a) State Hess’s Law. (1 mark)

(b) Use the following data to calculate the standard enthalpy of

formation, ΔHf°, of butane, C4H10, whose structural formula is:

C4H10 (g) + 7½ O2 (g) → 4CO2 (g) + 5H2O (l) ΔH°combustion = − 2878 kJ mol−1

H2 (g) + ½ O2 (g) → H2O (l) ΔH°combustion = − 286 kJ mol−1

C (graphite) + O2 (g) → CO2 (g) ΔH°combustion = − 394 kJ mol−1

Give a 3-significant figure answer. (3 marks)

(c) A theoretical value for the standard enthalpy of formation, ΔHf°, of butane can be

calculated from bond energy terms for C−C and C−H and appropriate standard enthalpies of

atomisation. Draw a properly labelled Hess cycle that shows how this can be done. (3 marks)

(d) A similar exercise using data as in question (c) to determine the

standard enthalpy change of formation, ΔHf° of methylpropane gives the

same value for ΔHf° as obtained for butane. Explain why this is so and

discuss why the two enthalpy changes should, in fact, be different. The

structural formula of methylpropane is as shown on the right.

(2 marks)

(e) Write a thermochemical equation, including state symbols, for each of the following

enthalpy changes:

(i) The enthalpy of solution of LiCl (1 mark)

(ii) The enthalpy of solvation of Li+ (1 mark)

(iii) The lattice enthalpy of LiCl (1 mark)

(f) Dissolving 1.50g of LiCl in 100cm3 of water results in a temperature change from 18.8°C

to 21.9°C. Calculate the standard enthalpy of solution of LiCl, in kJ mol−1. Assume that the

specific heat capacity of the solution is 4.18 J cm−3 °C−1. (3 marks)

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CCC

C

H

H

H

H

H

H

H

HH H

CCCC H

H

H

H

HH

HH

H

H

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Question 2: This question is about gases.

(a) State the units for all variables in the ideal gas equation, and use them to find the units of

the molar gas constant R. (3 marks)

(b) The combustion of methylhydrazine, CH3NHNH2, in oxygen produces steam, carbon

dioxide and nitrogen gas according to the following equation:

2CH3NHNH2 (g) + 5O2 (g) → 6H2O (g) + 2CO2 (g) + 2N2 (g)

0.500 mol of methylhydrazine are ignited in 2.00 mol of oxygen gas.

(i) Calculate the total volume of the mixture of gases, in m3 obtained when the reaction is

complete. Include the volume of any excess reactant, if any is present, in your final answer.

Assume that the temperature of the mixture is kept at 500 K and that its pressure is 100 000

Pa. (4 marks)

(ii) Calculate the partial pressure that is exerted by the nitrogen gas present in the mixture of

gases, at 500K. (2 marks)

(iii) What will the partial pressure of nitrogen become if the gaseous mixture is allowed to

stand overnight in the presence of some solid KOH pellets? (1 mark)

(c) (i) State the physical conditions under which real gases deviate from ideal behaviour.

(1 mark)

(ii) Give two reasons that explain why gases do not exhibit ideal behaviour under the

conditions listed as your answer for (c) (i). (2 marks)

(iii) Sketch a graph which represents the distribution of molecular speeds at two different

temperatures, T1 and T2, where T2 = T1 + 10 K. (2 marks)

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Question 3: This question is about redox reactions and volumetric analysis.

(a) Write down the oxidation number of the underlined particle:

(i) S8 (ii) Na2S (iii) SiO2 (iv) OF2 (2 marks)

(b) Complete and balance the following redox reactions:

(i) Cr2O72− (aq) + H+ (aq) + NO2

− (aq) → Cr3+ (aq) + NO3− (aq) + H2O (l) (2 marks)

(ii) ClO− (aq) + Mn2+ (aq) + OH− (aq) → MnO2 (s) + Cl− (aq) + H2O (l) (2 marks)

(c) Iron (II) sulfate crystals have the formula FeSO4.nH2O. In an experiment to determine n,

8.1675g of the salt were dissolved and made up to 250 cm3 of solution with distilled water. A

25.0 cm3 portion of the solution was acidified by adding some dilute sulfuric(VI) acid and

titrated against potassium manganate(VII) solution of concentration 0.0300 mol dm−3, of

which 22.5 cm3 was required. The unbalanced equation for the redox reaction occurring

during the titration is as follows:

MnO4− (aq) + 8H+ (aq) + 5Fe2+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)

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(i) Identify the oxidising agent and the reducing agent reacting during the titration. (2 marks)

(ii) Find the number of moles of potassium manganate(VII) used in 22.5 cm3. (1 mark)

(iii) Find the number of moles of iron(II) present in 25.0 cm3 of the salt solution. (1 mark)

(iv) Calculate the number of moles of iron(II) that were present in 250.0 cm3. (1 mark)

(v) Calculate the value of n in the formula FeSO4.nH2O. (2 marks)

(d) Define the term disproportionation. Illustrate your answer by a balanced chemical

equation of a chemical reaction that involves disproportionation. (2 marks)

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Question 4: This question is about entropy and free energy.

(a) State what is meant by the term spontaneous change when applied to chemical reactions,

in terms of the Gibbs free energy. (1 mark)

(b) State, without doing any calculations, whether the following reactions will have a positive

or negative value of ΔSθ:

(i) NH4NO3 (s) → N2O (g) + 2H2O (g) (½ mark)

(ii) 3O2 (g) → 2O3 (g) (½ mark)

(c) Natural gas is converted into hydrogen by reaction with steam in the presence of a nickel

catalyst. The balanced chemical equation for this reaction is as follows:

CH4 (g) + H2O (g) → CO (g) + 3H2 (g)

Substance ΔHfθ/kJ mol−1 Sθ/J K−1 mol−1

CH4 (g) −74.9 186

H2O (g) −242 189

CO (g) −111 198

H2 (g) 131

(i) State and explain the approximate value of ΔHfθ for H2 (g). (1 mark)

(ii) Calculate the entropy change, ΔSθ, for the reaction of steam with methane. (1 mark)

(iii) Calculate the enthalpy change, ΔHθ, for the reaction of steam with methane. (2 marks)

(iv) Use your answers to (ii) and to (iii) to calculate the temperature at which the reaction will

be feasible. (2 marks)

(v) Explain whether the reaction of steam with methane becomes more spontaneous or less

spontaneous if carried out at a temperature which is higher than that calculated in (c) (iv).

(2 marks)

(d) At 0°C, the change H2O (s) ⇌ H2O (l)

is at equilibrium and so ΔGθ is zero. If ΔHθ for the forward process is +6.01 kJ mol−1,

calculate the value of the ΔSθ for the following processes at 0°C:

(i) ice → water (2 marks)

(ii) water → ice (1 mark)

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(e) Icing sugar can be hazardous (it may ignite) when it is being produced by powdering

sucrose in a factory. Explain why sucrose is stable at room temperature, but its powdering

process is hazardous. (2 marks)

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Question 5: This question is about bonding and structure.

(a) Draw a dot-cross diagram, showing the outer electrons only, for SrCl2. State the

predominant type of bonding present in this compound. (2 marks)

(b) Draw a Lewis diagram showing the shape and the type of bonding present in one XeOF4

molecule. Refer to the VSEPR theory to state the shape that this molecule has. (2 marks)

(c) (i) Draw Lewis diagrams showing the shape and bonding in the molecules BF3 and NH3.

(1 mark each)

(ii) Indicate the polarities present, if any, in the molecules BF3 and NH3, by drawing the

symbols δ+ and δ− on the diagrams drawn in (d) (i) wherever appropriate. (2 marks)

(iii) Explain, by referring to intermolecular bonding, why the boiling temperature of BF3 is

much lower than the boiling temperature of NH3. (2 marks)

(d) Account for the following observations:

(i) The ground state electron configuration of carbon shows that 1 carbon atom has 2 unpaired

electrons in its valence shell, yet carbon forms CH4 and not CH2. (3 marks)

(ii) Sodium chloride is a high melting point solid while ammonium chloride sublimes upon

heating. (2 marks)

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Question 6: This question is about nuclear chemistry and mass spectrometry.

(a) The nucleus of an isotope of uranium, A, contains 92 protons and 146 neutrons. The

isotope decays by the emission of a particle B containing 2 protons and producing a nuclide

C. The half-life of the process is 4.5 × 109 years. The nuclide C decays by a process whereby

a neutron changes into a proton with the emission of a particle D and a nuclide E. Particle D

has a very small mass.

(i) Define the terms isotope and half-life. (2 marks)

(ii) Write a balanced nuclear equation for the decay of A into B and C. In your answer

represent all particles by their appropriate chemical symbols. (1 mark)

(iii) State the name of the particle B. (½ mark)

(iv) Write a balanced nuclear equation for the decay of C into D and E. In your answer

represent all particles by the appropriate chemical symbols. (1 mark)

(v) State the name of the particle D. (½ mark)

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(vi) A sample of uranium ore contains 1.28 × 1022 atoms of isotope A. Determine the mass of

uranium atoms of isotope A present in the sample 1.35 × 1010 years ago. (3 marks)

(b) The following are the mass spectrum of ethanal. Ethanal is an organic molecule whose

chemical formula is C2H4O.

(i) What is the name given to the peak at m/z = 29? (1 mark)

(ii) Draw the displayed structural formula of the chemical species that are responsible for the

peaks at m/z =15 and at m/z = 29. (2 marks)

(iii) By referring to your answers to (e) (ii), draw the displayed structural formula of ethanal.

State the m/z value of the peak that supports your answer. (1 mark)

(iv) A small peak appears further to the right of the molecular ion peak in the mass spectra of

many organic compounds. Give an explanation for this common occurrence.

(1 mark)

(c) The relative atomic mass of Cl is 35.5. Calculate the percentage abundance of the 37Cl

isotope in the element chlorine given that this element has the isotopes 35Cl and 37Cl.

(2 marks)

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Question 7: This question is about dynamic equilibria.

(a) Write an expression for Kc of the following equilibrium and give the units for Kc:

Cu2+ (aq) + 4NH3 (aq) ⇌ [Cu(NH3)4]2+ (aq) (1 mark)

(b) Write an expression for Kp of the following equilibrium and give the units for Kp:

P4 (s) + 6Cl2 (g) ⇌ 4PCl3 (g) (1 mark)

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(c) The copper mineral malachite is principally composed of copper(II) carbonate. On

heating in a closed container the following equilibrium is established forming copper(II)

oxide and carbon dioxide, as shown by the following equation:

CuCO3 (s) ⇌ CuO (s) + CO2 (g)

Explain briefly what would happen to the amount of CuCO3 that is decomposed if these

changes are applied separately to the equilibrium mixture:

(i) CO2 is added to the mixture. (1 mark)

(ii) A few drops of concentrated NaOH are added to the mixture. (1 mark)

(iii) The volume of the container is increased. (1 mark)

(d) Nitrogen and oxygen react at high temperatures to form NO.

N2 (g) + O2 (g) ⇌ 2NO (g) ΔHθ = +180 kJ mol−1

The value of Kc at 600°C is 3.3 × 10−10.

(i) State the sign of the enthalpy change of reaction of the reverse process. (1 mark)

(ii) Calculate the fraction of N2 that is converted to NO when an equimolar mixture of N2 and

O2 are heated to 600°C, in closed container. (4 marks)

(iii) For the reaction of N2 and O2, sketch and label on the same set of axes, graphs to show

changes in (I) concentration of the reactants with time and (II) concentration of the product

with time. Indicate on the sketch the point at which equilibrium is reached. (2 marks)

(iv) Explain qualitatively what would happen to Kc when the temperature is increased at

constant volume. (2 marks)

(v) Use the value of Kc given for the formation of NO to calculate Kc of the equilibrium

reaction by which NO decomposes, as shown by the equation below:

2NO (g) ⇌ N2 (g) + O2 (g) (1 mark)

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Question 8: This question is about atomic structure.

(a) Give a value for the following, for the species

(i) atomic number

(ii) mass number

(iii) neutron number

(iv) electron configuration, in terms of orbitals. (2 marks)

(b) Explain what is meant by the following statement: Cl is a p-block element while Fe is a d-

block element. (1 mark)

(c) The successive ionisation energies of an element X are shown in the following table:

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Process I1 I2 I3 I4 I5

Ionisation energy

(kJ mol−1)

787 1577 3232 4356 16091

(i) Write a thermochemical equation, including state symbols and the enthalpy change value,

for the third ionisation energy of element X. (1 mark)

(ii) Explain why the difference between the 4th and the 5th ionisation energies is much greater

than that between any two other successive values. (2 marks)

(iii) The first ionisation energy of the other elements in the group in which X lies in the

periodic table are, in order of increasing value, in kJ mol−1: 709, 716, 762 and 1086.

(I) State and explain the electronic configuration, in terms of orbitals, of the outer shell of

element X by referring to the ionisation energies given. (2 marks)

(II) Use the periodic table provided to suggest a possible identity for element X. (1 mark)

(d) (i) Sketch a graph of the first ionisation energies against atomic numbers of the elements

in period 3. (2 marks)

(ii) Explain the general trend that appears in the graph. (2 marks)

(iii) Explain any particular exceptions to the general trend that appears in the graph. (2 marks)

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