A-LevelChemistryGroup 29701/22 · May/June 2022 · Q211 Marks

Radium, Ra, is an element found in Group 2 of the Periodic Table. It is a crystalline solid at room temperature and conducts electricity. Radium chloride, RaCl2, has a melting point of 900 °C and is soluble in water.

(a)Predict the lattice structure of RaCl₂(s) based on the properties described.[1]
(b)Draw a dot-and-cross diagram to show the arrangement of outer electrons in RaCl2.[1]
(c)(i)Solid Ra and Ca show similar reactions with H₂O, but the reactions occur at different rates. Separate samples, each containing a single piece of solid Ra or Ca, are added to equal volumes of cold water. Each sample contains equal numbers of moles of solid and the H₂O is in excess. Construct an equation for the reaction of Ra with H₂O.[1]
(c)(ii)Identify which element, Ra or Ca, reacts with H₂O at a faster rate. Suggest how the observations of each reaction would differ.[1]
(c)(iii)Suggest why these reactions occur at different rates.[2]
(c)(iv)One of the solutions is cloudy when the reaction has finished. At the end of each reaction, universal indicator is added to each reaction mixture. Suggest pH values of the solutions made in both reactions. Explain your answer.[2]
(d)A sample of aqueous calcium halide, CaX2(aq), contains either chloride, bromide or iodide ions. Complete Table 2.1 to describe a two-step process that could be used to identify the halide ion present. Table 2.1 step method observation with CaCl₂ step 1 step 2[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] giant ionic
[1] Ra2+ and 2 x Cl- 0 electrons surrounding 2Ra AND 8 electrons surrounding Cl
[1] Ra + 2H2O → Ra(OH)2 + H2
[1] Ra – more bubbles per unit time OR With Ra solid disappears more quickly OR Ra is the first to stop fizzing ora
[2] option 1: suggest why these reactions occur at different rates (collision theory) M1 difference in activation energy / ionisation energy M2 affects the frequency of effective collisions (between particles / molecules) OR affects the proportion of particles with energy greater than activation energy OR option 2: suggest why the elements have different reactivity in terms of atomic structure (which results in a different rate) M1 ionisation energies are different M2 a decrease in nuclear attraction due to EITHER increase in shielding OR increase in distance of outer electron from nucleus OR increase in number of shells of electrons ora
[2] M1 pH value (or values) for each solution must be greater > 7 AND pH value (or range values) identified for solution made from Ra must be greater than pH values (or range of values) stated for Ca M2 any one from: solubility of group 2 hydroxides increases down the group OR Ra(OH)2 is more soluble OR greater concentration of OH-(aq) in the solution involving Ra OR more OH (aq) in the solution involving Ra
[3] M1 step 1 AgNO3(aq) / silver nitrate solution AND step 2 NH3(aq) / ammonia solution / dilute NH3 / concentrated NH3 M2 ALL solid / precipitate AND correct colours described in row 1 M3 correct observations on addition of ammonia