1. 1
   A technician cools 15 g of molten naphthalene from 95 °C to 60 °C, recording the temperature every minute. The temperature falls steadily until it reaches 80 °C (naphthalene's freezing point), remains at 80 °C for several minutes while the naphthalene solidifies, then falls steadily again.

   (a)

   Which best describes what is happening to the particles of naphthalene during the time the temperature remains constant at 80 °C?

   [1 mark]

   • AThe particles lose potential energy as they form a more ordered solid lattice; kinetic energy stays constant because energy released as attractive forces form balances the energy lost to the surroundings
   • BThe particles stop moving completely as all their kinetic energy is transferred to the surroundings
   • CThe particles gain kinetic energy as new chemical bonds form between them
   • DThe particles' average speed increases because heating continues throughout the process

   (b)

   80 °C is the freezing point of naphthalene. What is this temperature in Kelvin?

   [1 mark]

   • A193 K
   • B353 K
   • C273 K
   • D80 K

   (c)

   Explain, in terms of kinetic and potential energy, why the temperature falls steadily before and after the plateau, but remains constant while the temperature is at 80 °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Silicon, used in computer chips, occurs naturally as three isotopes: silicon-28 (relative abundance 92.2%), silicon-29 (4.7%) and silicon-30 (3.1%). A sample of semiconductor-grade silicon is analysed to check its isotopic composition.

   (a)

   How many neutrons are present in an atom of silicon-29 (atomic number Z = 14)?

   [1 mark]

   • A14
   • B29
   • C15
   • D13

   (b)

   Silicon-28 and silicon-30 have the same chemical properties because they have the same number of:

   [1 mark]

   • ANeutrons
   • BNucleons
   • CMass number
   • DProtons (and therefore electrons)

   (c)

   Calculate the relative atomic mass of silicon from the data, giving your answer to 3 significant figures.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Copper, Z = 29, is unusual because its electron configuration does not follow the pattern predicted by strictly filling the 4s sublevel before the 3d sublevel.

   (a)

   Write the full electron configuration of a copper atom (Z = 29), and explain why it differs from the configuration predicted by strictly filling 4s before 3d.

   [3 marks]

   (b)

   Deduce the full electron configuration of the Cu⁺ ion formed when a copper atom loses one electron. Explain, in terms of orbital energies, why the electron is lost from the 4s sublevel rather than from 3d, even though 4s filled before 3d when the atom was built up.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student reacts 2.70 g of aluminium (M = 27.0 g mol⁻¹) with an excess of dilute hydrochloric acid: 2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g). The hydrogen gas produced is collected at 25 °C (298 K) and a pressure of 100 kPa. (R = 8.31 J K⁻¹ mol⁻¹)

   (a)

   Calculate the amount, in moles, of hydrogen gas produced, and hence calculate the volume of hydrogen gas collected at 25 °C and 100 kPa. Give your answer in dm³.

   [6 marks]

   (b)

   The student assumes the hydrogen gas behaves as an ideal gas. Evaluate this assumption for hydrogen at 25 °C and 100 kPa, and suggest how the conditions could be changed to make the ideal gas approximation less accurate.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Solid carbon dioxide ('dry ice') is left in an open, insulated box at atmospheric pressure. At room temperature the solid steadily disappears, releasing a visible white vapour cloud, without ever forming a visible liquid.

   (a)

   Which change of state is occurring as the solid carbon dioxide disappears without ever forming a visible liquid?

   [1 mark]

   • ASublimation
   • BEvaporation
   • CCondensation
   • DMelting

   (b)

   Which statement correctly compares the particles in solid and gaseous carbon dioxide?

   [1 mark]

   • AThe particles in the solid have more kinetic energy than those in the gas
   • BIn the solid, particles vibrate about fixed positions held by strong attractive forces; in the gas, particles move rapidly and independently with negligible attractive forces between them
   • CThe particles in both the solid and the gas move freely throughout the container
   • DThe particles in the solid and the gas have the same average speed and spacing

   (c)

   Explain, in terms of particle energy and intermolecular forces, why solid carbon dioxide sublimes directly to a gas at atmospheric pressure rather than first melting to a liquid.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Chlorine occurs naturally as a mixture of two isotopes: chlorine-35 (relative abundance 75.8%) and chlorine-37 (relative abundance 24.2%).

   (a)

   How many protons and neutrons are present in an atom of chlorine-37 (atomic number Z = 17)?

   [1 mark]

   • A17 protons, 17 neutrons
   • B20 protons, 17 neutrons
   • C17 protons, 20 neutrons
   • D17 protons, 37 neutrons

   (b)

   Chlorine-35 and chlorine-37 are described as isotopes of the same element because they have:

   [1 mark]

   • ADifferent numbers of protons
   • BThe same mass number
   • CDifferent numbers of electrons only
   • DThe same number of protons but different numbers of neutrons

   (c)

   Calculate the relative atomic mass of chlorine to 3 significant figures using the data given.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   The emission spectrum of atomic sodium contains a series of lines in the ultraviolet region that converge to a series limit at a wavelength of 241 nm. This series corresponds to electron transitions ending at the ground state (n = 1) of the single outer electron. (c = 3.00 × 10⁸ m s⁻¹, h = 6.63 × 10⁻³⁴ J s, NA = 6.02 × 10²³ mol⁻¹)

   (a)

   Write the full electron configuration of a sodium atom (Z = 11), and identify the sublevel occupied by the single electron responsible for this emission series. State what the convergence of the series at higher frequency corresponds to.

   [3 marks]

   (b)

   Calculate the first ionisation energy of sodium, in kJ mol⁻¹, using the convergence wavelength given.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   0.440 g of propane, C₃H₈ (M = 44.0 g mol⁻¹), is burned completely in excess oxygen: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g). All the carbon dioxide produced is collected and cooled to 127 °C (400 K), where it exerts a pressure of 200 kPa. (R = 8.31 J K⁻¹ mol⁻¹)

   (a)

   Calculate the amount, in moles, of carbon dioxide produced, and hence calculate the volume it occupies at 127 °C and 200 kPa. Give your answer in dm³.

   [6 marks]

   (b)

   Evaluate whether carbon dioxide or helium would be expected to deviate more from ideal gas behaviour under the same conditions (127 °C, 200 kPa), explaining your reasoning in terms of the assumptions of the ideal gas model.

   [6 marks]

   Total for question 8: 12 marks

End of questions