AS CHEMISTRY PAST PAPERS

Example 10

Calculate ΔfH for propane, C3H8(g), given the following data.

C(s) —> C(g) ΔatH = 715 kJ mol-1

3C (s) + 4H2 (g) —→ C3H8(g),

A student recorded the temperature of aqueous ethanoic acid in a polystyrene cup for

three minutes.

At the fourth minute, the student added sodium hydrogencarbonate.

The student stirred the mixture and carried on recording the temperature every minute

for several minutes.

The student’s measurements are shown in Figure 2.

A best-fit line showing the temperature before mixing has been drawn.

Draw an appropriate best-fit line on Figure 2 and use it to find the temperature

change at the time of mixing.

(17.2 – value read from graph line at 4 minutes) 0.2

( C)

(b) The student considered using a glass beaker on a tripod and gauze instead

of the clamped copper calorimeter.

Suggest two disadvantages of using a glass beaker on a tripod and gauze

Glass is a poorer conductor than copper

M1

M2

M1

Tripod and gauze would reduce heat transfer

Explain why the value given for the O=O bond enthalpy in part (b) is not a

mean value.

Oxygen / O2 is the only substance that has O=O bond

With reference to the structure shown above give one reason why ice is less dense than water.

Water or H2O or molecules (in ice) are held further apart (than in liquid water)/(more) space/gaps/holes in structure/Water or H2O or molecules (in ice) are more spread out

(c) The temperature of the mixture was changed to T2 and the mixture left to

establish a new equilibrium.

In the new equilibrium mixture the concentration of sulfur trioxide was

found to be 0.07 mol dm–3

Deduce which of T1 and T2 is the higher temperature.

Explain your deduction.

Higher temperature _____________

Explanation

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T2 (Not worth a mark alone)

T1, CE=0

Equilibrium has moved / shifted to RHS/forward in endothermic direction

Both RHS / forward and endothermic needed

Equilibrium has opposed the increase in T / Equilibrium moves to decrease

the T

Not just to oppose the change

Q10.

Nitryl chloride reacts with nitrogen monoxide according to the equation:

ClNO2(g) + NO (g) ⟶ NO2(g) + ClNO(g)

The Maxwell–Boltzmann distribution curve in Figure 1 shows the distribution of

molecular energies in 1 mol of this gaseous reaction mixture (sample 1) at 320

K.

The amount of gas present (or number of molecules) has been reduced /

or the pressure has been reduced

1

Rate of reaction decreases (no mark)

Particles are spread further apart

1

Fewer collisions between gas particles so fewer successful collisions

0 1 . 4 When Reaction 2 is carried out under different conditions, a compound with

molecular formula C9H21N is produced.

Draw the skeletal formula of the compound.

Identify the functional group in the compound including its classification.

[2 marks]

Skeletal formula

Functional group including classification

Cl is more electronegative (than C) or

C and Cl have different electronegativities

0 6 . 2 Suggest why tetrachloromethane molecules are non-polar.

idea that dipole moments (or dipoles) cancel out (due to symmetry

Allow polar bonds / polarities cancelling out

0 6 . 3 Explain why tetrachloromethane has a higher boiling point than dichloromethane.

M1 M2 van der Waals' forces between molecules in CCl4 stronger than

(combined van der Waals' and) dipole-dipole forces between

molecules in CH2Cl2

as CCl4 has (many) more electrons than CH2Cl2

M1 must refer to the forces being between

molecules at some point

NOT M1 for any reference to bond breaking

NOT M1 for any reference to incorrect

intermolecular forces

Allow London forces or temporary (induced) dipole-

dipole forces for van der Waals' forces

For M2, allow CCl4 has higher mass or higher Mr or

bigger than CH2Cl2

The distillate is collected in the range 77–82 °C

Explain why the water should enter the condenser at the bottom and not at the top

Marking guidance idea of ensuring condenser fills with water

idea that condenser is cool(er) or

ensuring (more) vapour condenses

Oxidation half equation

2 I– → I2 + 2 e–

Reduction half equation

2 IO3– + 12 H+ + 10 e– → I2 + 6 H2O