A-LevelChemistryGroup 29701/21 · May/June 2022 · Q117 Marks

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

(a)Identify the highest energy orbital which contains electrons in a calcium atom. Sketch the shape of this orbital. identity of highest energy orbital in Ca ........................................................................................... shape[1]
(b)(i)Write the equation for the thermal decomposition of calcium nitrate.[1]
(b)(ii)Suggest which of the Group 2 nitrates, calcium, magnesium or radium, requires the highest temperature to decompose. Explain your answer.[1]
(c)Predict what you would observe when aqueous radium chloride is added to aqueous sodium sulfate. Do not refer to temperature changes in your answer.[1]
(d)(i)²⁵Mg is an isotope of magnesium. Determine the number of protons and neutrons in an atom of ²⁵Mg. number of protons ........................................................................................................................ number of neutrons ........................................................................................................................[1]
(d)(ii)State the full electronic configuration of an atom of ²⁵Mg.[1]
(e)(i)The relative atomic mass, Aᵣ, is calculated by comparing the average mass of the isotopes of an element to the unified atomic mass unit. Define the unified atomic mass unit.[1]
(e)(ii)Calculate the mass of X. Use data from Table 1.1 and Aᵣ (magnesium) = 24.31 in your calculation. Show your working. mass of X =[2]
(e)(iii)State one similarity and one difference in the properties of these isotopes of magnesium. Explain your answer.[2]
(f)(i)State one observation when magnesium burns in oxygen. Do not refer to temperature changes in your answer.[1]
(f)(ii)On Fig. 1.1: • sketch a reaction pathway diagram for the reaction that occurs when Mg burns in O₂ • label the diagram to show the enthalpy change, ∆H, and the activation energy, Eₐ, for the reaction.[3]
(g)Cold water reacts slowly with a piece of Mg to produce bubbles of H₂(g). Cold water reacts rapidly with burning Mg to produce H₂(g) in an explosive mixture. Mg + 2H₂O → Mg(OH)₂ + H₂ Explain why the rate of reaction of cold water with burning magnesium is greater.[2]

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] Identify and draw the shape of highest energy orbital of Ca 4s AND s-orbital shape drawn
[1] Ca(NO3)2 → CaO + 2NO2 + ½O2
[1] radium (nitrate) as thermal stability increases down group / has the greatest thermal stability
[1] white precipitate / solid (of radium sulfate)
[1] number of protons: 12
[1] number of neutrons: 13
[1] 1s22s22p63s2
[1] 1/12 (one twelfth) the mass of a carbon-12 / ¹²C atom
[2] M1 correct expression relating Ar to the mass / % abundance of the three isotopes 24.31 = x x 0.7899 + 24.99 × 0.1000 + 25.98 × 0.1101
[2] M2 correct answer to 4 sig figs atomic mass of X = 23.99
[2] M1 (magnesium isotopes have) identical chemical properties AND same electron(ic) arrangement / configuration
[2] M2 different physical properties AND different number of neutrons
[1] white flame / light OR white solid / smoke
[3] M1 sketch shows exothermic reaction with a 'hump' AND labelled reactants (Mg + O2) and products (MgO)
[3] M2 arrow from reactants / Mg + O2 to products / MgO shown as ΔΗ
[3] M3 arrow showing activation energy / Ea / (+)148
[2] M1 (heat/energy released from burning Mg) provides more particles with energy > Ea
[2] M2 frequency of successful / effective collisions is greater