Answer all the questions.

1 The element magnesium is an important Group 2 metal. Its presence in distant stars has been

shown using atomic emission spectra.

 (a) (i) The atomic emission spectrum of an element shows a series of coloured lines on a

black background.

 Describe how the appearance of the absorption spectrum of the element is similar to

and different from its emission spectrum.

Similar ...............................................................................................................................

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Different .............................................................................................................................

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 [2]

 (ii) What evidence for the structure of atoms is provided by atomic spectra?

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 (b) Ionisation enthalpies have also been used to develop theories about atomic structure.

 (i) Write an equation for the reaction that represents the first ionisation enthalpy of

magnesium.

 Include state symbols.

 [2]

 (ii) The first ionisation enthalpies of the elements of Period 3 show a general increase

across the period.

 Explain this increase.

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 (c) The mass spectrum of magnesium shows that it has three stable isotopes as shown below.

Isotope Abundance / %

24Mg 78.60

25Mg 10.11

26Mg 11.29

 Calculate a value for the relative atomic mass of magnesium based on these data.

 Give your answer to two decimal places.

relative atomic mass = .......................................................... [2]

 (d) Magnesium-24 is formed in some stars by nuclear fusion of two identical carbon nuclei.

 Complete the nuclear equation for the formation of this isotope.

2

24

12 Mg [1]

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 (e) A student is asked to prepare a sample of hydrated magnesium chloride crystals (containing

water of crystallisation) starting from solid magnesium oxide.

 The student adds magnesium oxide to hot hydrochloric acid until the oxide is in excess.

 The student then evaporates the mixture until just a solid is left.

 Explain why this procedure would not produce hydrated magnesium chloride crystals and

give a correct method.

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 (f) Calcium and barium are two other Group 2 elements.

 A student places a small piece of calcium into 100 cm3 of cold water in a beaker. A steady

fizzing occurs, the calcium disappears and a white, cloudy mixture of pH 11 is left. The

temperature increases by 26 °C.

 The student then repeats the experiment with an equal amount of barium.

 Describe two differences that the student would observe when comparing the reaction of

barium with that of calcium.

1 ................................................................................................................................................

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2 ................................................................................................................................................

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[2]

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 (g) Another student is provided with samples of magnesium carbonate and strontium carbonate

and asked to identify which is which. The student heats equal amounts of each carbonate in

separate test tubes using the same Bunsen flame. The student measures the time taken for

the gas evolved to turn limewater cloudy.

 The student says that the time taken will be shorter when strontium carbonate is heated

because strontium is more reactive than magnesium.

 Comment on the student’s statement, giving the correct chemistry where necessary.

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 (h) Complete the electronic configuration for the magnesium ion, Mg2+.

1s2 ...................................................................................................................................... [1]

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2 Heterogeneous catalysts are used on a large scale for catalytic cracking in industry.

 (a) A student sets up the apparatus shown in Fig. 2.1 to investigate the cracking of ‘liquid

paraffin’.

delivery tube gas collected

clamp

catalyst

strong

heat

warm

mineral wool soaked

in liquid paraffin

iced water bath

liquid

collected

 Fig. 2.1

 (i) Explain why the catalyst is described as heterogeneous.

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 ...................................................................................................................................... [1]

 (ii) The catalyst gets coated with carbon over time and becomes less effective.

 Give the general name of a substance that reduces the function of a catalyst in this way.

 ...................................................................................................................................... [1]

 (iii) The compounds below might be found in the apparatus in Fig. 2.1 when it is in use.

 Match the appropriate formula with the places from Fig. 2.1:

 C2H4 C6H14 C12H26

 Liquid paraffin ........................

 Liquid collected ........................

 Gas collected ........................ [1]

 (iv) The gas collected is found to turn bromine water from orange/brown to colourless.

 What can the student deduce from this?