Enne Retics

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    1. Which of these diagrams correctly represents an endothermic reaction?

    (Total 1 mark)

    2. This question is about methanol and the energy changes that accompany some of its reactions.

    (a) Complete the diagram (using dots and crosses) to show the bonding in methanol, CH!H.

    "ou should show outer electrons only.

    (2)

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    (b) The Hess cycle below can be used to calculate the standard enthalpy change of

    combustion of methanol, using standard enthalpy changes of formation.

    (i) Complete the cycle by filling in the empty bo#.(2)

    (ii) $efine the term of a compound, ma%ing clear the meaning of standardin this

    conte#t.

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    (iii) &se your cycle and the data below to calculate the standard enthalpy change of

    combustion of methanol, 'Hc

    'Hf

    %* mol+

    C!-(g) +./

    H-!(l) +-0/.0

    CH!H(l) +-.

    (2)

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    (c) 1n e#periment was carried out, using the apparatus below, to estimate the standard

    enthalpy change of combustion of methanol.

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    1fter burning the methanol for a few minutes, the temperature of water in the bea%er had

    risen by -2.3 4C and the mass of methanol burnt was 2.050 g.

    (i) Calculate the amount of energy transferred to the water.

    6nergy transferred (*) 7 mass of water 8 5.0 8 temperature change(1)

    (ii) Calculate the number of moles of methanol, CH!H, burnt during the e#periment.

    (1)

    (iii) &se your answers to (c)(i) and (ii) to calculate the e#perimental 9alue for the

    standard enthalpy change of combustion. :nclude a sign and units in your answer,

    which should be gi9en to threesignificant figures.(1)

    (i9) Compare your answers to (b)(iii) and (c)(iii) and gi9e TW! reasons to e#plain any

    differences.

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    (Total 14 marks)

    3. Which of the following is true for the e#othermic reaction shown below?

    ;g(s) < -HCl(aq) = ;gCl-(aq) < H-(g)

    A 'H positi9e

    B 'Ssurroundings positi9e

    C 'Ssystem negati9e

    D 'Stotal negati9e

    (Total 1 mark)

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    4. (a) Crystals of hydrated cobalt(::) chloride, CoCl-.>H-!, lose water when they are heated,

    forming anhydrous cobalt(::) chloride, CoCl-.

    CoCl-.>H-!(s) = CoCl-(s) < >H-!(l)

    (i) Calculate the entropy change of the system, 'S

    system, at -0 . :nclude a sign and

    units in your answer. "ou will need to refer to your data boo%let.(2)

    (ii) 6#plain whether the sign of your answer to (a)(i) is as e#pected from the equation

    for the reaction.

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    ...........................................................................................................................(1)

    (iii) The standard enthalpy change for the reaction, 'H

    , is

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    (b) 1 student attempted to measure the enthalpy change of solution of anhydrous cobalt(::)

    chloride by adding -.22 g of cobalt(::) chloride to /2.2 cmof water in a [email protected]

    container. 1 temperature rise of ./ 4C was recorded.

    The student used a balance which reads to 2.2g, a /2.2 cm

    pipette, and a thermometerwhich can be read to 2.-/ 4C.

    (i) Which measuring instrument should be changed to gi9e a result which is closer to

    the accepted 9alue? *ustify your answer.

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    (ii) Auggest !B6 otherchange the student could ma%e to gi9e a result which is closer

    to the accepted 9alue. *ustify your suggestion.

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    ...........................................................................................................................(2)

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    (c) The lattice energies of magnesium chloride, ;gCl-, calcium chloride, CaCl-, and

    strontium chloride, ArCl-are shown in the table below.

    Chloride attice energy%* mol+

    ;gCl- +-/->

    CaCl- +--/0

    ArCl- +-/>

    (i) &se data on ionic radii, from your data boo%let, to e#plain the trend in these 9alues.

    6stimate a 9alue for the lattice energy of cobalt(::) chloride, gi9ing !B6 piece of

    data to Dustify your estimate.

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    (ii) 6#plain how lattice energy 9alues, together with other data, can be used to predict

    the solubility of ionic compounds.

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    ...........................................................................................................................(3)

    (d) Cobalt forms another chloride, CoCl, but scientists predict that ;gClcannot be made.

    Auggest a reason for this.

    "ou should consider the enth