3.1.2 (b,c) Relative Reactivities of Group 2 elements

Syllabus

(b) Relative Reactivities of the Group 2 elements Mg → Ba shown by their redox reactions with:

(i) Oxygen

(ii) Water

(iii) Dilute acids

{Reactions with acids will be limited to those producing a salt and Hydrogen.}

(c) the trend in reactivity in terms of the first and second ionisation energies of Group 2 elements down the group (see also 3.1.1 c)

{Definition of second ionisation energy is not required, but learners should be able to write an equation for the change involved.}

What does this mean?

Reactivity trend

All Group 2 elements need to lose two electrons to react.

The easier it is to lose the electrons the faster the element will be able to react.

It's easier to lose electrons that are only weakly held by the nucleus.

As usual there are three factors to consider:

Increasing nuclear charge down the group - more protons to attract electrons
Increasing atomic radius down the group - decreases the attractive force from the nucleus
Increasing shielding down the group - more inner shells to screen the outer electrons from the attractive force of the nucleus.

The second and third factors outweigh the first - say something like this in your answers.

So, as we go down the group the electrons are more weakly held and more easily lost and so the reactivity increases.

We can put numbers to this by looking a first and second Ionisation Energies.

The lower the combined Ionisation Energies the easier it is to lose electrons.

You can be asked to define the first ionisation energy but not any of the others.

You can be asked to write an equation for any ionisation energy - so you need to recall that you are always losing one electron, and that everything is gaseous.

Here we see the first and second ionisation energy equations for Neon

Reactions with Oxygen

Generally,

Group 2 Metal + Oxygen → White Ionic Metal Oxide

M_(s) + O_2(g) → MO_(s)

The metal is Oxidised from Oxidation No. 0 to +2

The Oxygen is Reduced from Oxidation No. 0 to -2

You should recall from GCSE that you get a brick-red flame when Calcium burns.

You don't really need to know that Barium gives a green flame and Strontium a Crimson one.

It would be difficult to observe much difference in the speed of burning but we can assume that the lower in the group the faster all reactions are for the reasons set out above.

Forget Beryllium - its atoms are so small and losing 2 electrons forms a 2+ ion so polarising that it draws some electron density back from the Oxide ion to form an intermediate ionic-covalent bond that isn't typical of other Grp 2 Oxides.

So no one is likely to ask you much about an element that doesn't fit with the trend.

Reaction with Water

All Group 2 metals (except Be) react with water up to a point - Magnesium only reacts with very hot water when its has been freshly ground into a very fine powder.

As you go down the group they react faster, as expected.

Generally, Group 2 metal + Water → Metal Hydroxide + Hydrogen

M_(s) + H₂O_(l) → M(OH)_2(aq) + H_2(g)

In each case the Metal is again Oxidised from Oxidation No. 0 to +2

This time some of the Hydrogen in water is Reduced from Oxidation No. +1 to 0 in the Hydrogen gas

In the case of Magnesium the Mg(OH)₂ that would be very slowly made dissolves poorly so a precipitate would be formed.

Examiners used to like to ask you about observations when Calcium reacts with water.

You should say:

Calcium slowly dissolves, gets smaller or even disappears
A gas/ bubbles/ fizzing is seen
Steam may be seen
The grains of Calcium rises and fall in the test-tube
A white solid/precipitate is seen.

You may even have to explain the tising and falling of the Calcium - it rises when Hydrogen bubbles form round it and sinks again (due to its density) when they escape at the surface.

The white solid will be Calcium Hydroxide - its not very soluble, which is why there are always bits in bottles of Limewater (Calcium Hydroxide solution)

You should not say:

Hydrogen is seen - you can't see that it is Hydrogen. only that it is a gas
Calcium Hydroxide is made - again you can't see what it is even if you know.

This time the trend is observable from (almost) no reaction for Mg, to quite a vigorous reaction with Ca to very vigorous reactions at the bottom of the group.

You should probably also know how to make Magnesium react with water in the form of steam.

Heating the Magnesium until it almost burns them passing steam over it causes a jet of flammable Hydrogen to be made.

The Magnesium reacts fiercely enough to melt into the glass.

But the white solid made is not Magnesium Hydroxide, it is Magnesium Oxide.

Mg_(s) + H₂O_(g) → MgO_(aq) + H_2(g)

Reaction with dilute acid

Dropping reactive Grp 2 metals in dilute acids is something you did from Year 7 onwards

They react vigorously.

You'll have done this with Magnesium and Calcium - you can assume that the reactions with Ba and Sr will be more vigorous.

They all follow the pattern Metal + Acid → Salt + Hydrogen

Which you dealt with from Year 9 onwards.

Welcome to the Sixth Form

Reactions with Halogens

Hopefully you'd expect reactive metals to react with reactive non-metals by now.

It's not mentioned on the syllabus but you know how examiners are:

Metal + Halogen → Metal Halide

M_(s) + X_2(g) → MX_2(s)

_{And these Metal Halides would be dull looking white solids.}

_{I mean, I'm telling you that this is a picture of Magnesium Chloride but it could be anything, couldn't it. How mysterious.}

Exam-style Questions

1. When Calcium and Barium are added separately to water, similar reactions occur.

(i) Describe two observations that can be made when Calcium reacts with water.

1st observation .....................................................................................................................................................................

.......................................................................................................................................................................

2nd observation ....................................................................................................................................................................

.......................................................................................................................................................................

(2)

(ii) For the two observations you have given in (a)(i), describe the differences that are found when Barium reacts with water.

Difference for 1st observation .....................................................................................................................................................................

.......................................................................................................................................................................

Difference for 2nd observation ....................................................................................................................................................................

.......................................................................................................................................................................

(2)

(iii) Write an equation for the reaction of Calcium with water.

.......................................................................................................................................................................

(1)

(Total 5 marks)

2. (a) Define the term electronegativity.

.......................................................................................................................................................................

(2)

(b) State and explain the trend in electronegativity down Group II.

Trend ....................................................................................................................................................................

Explanation ..........................................................................................................................................................

.......................................................................................................................................................................

(3)

(c) Write an equation for the reaction of Strontium with water, and suggest an approximate value for the pH of the resulting solution.

Equation ................................................................................................................................................................

pH ....................................................................................................................................................................

(2)

(d) Describe what is seen when an aqueous solution of Barium Chloride is added to dilute Sulphuric acid. Write an equation for the reaction which occurs.

Observation ...........................................................................................................................................................

Equation ............................................................................................................................................................(2)

(Total 9 marks)

Answers

1. (i) any two of::

effervescence / fizzing / bubbling / white precipitate or sediment /

solution gets warm / metal dissolves or disappears (2)

(ii) comparisons needed for the two observations given in (a) (i)

more (rapid) effervescence / more vigorous reaction /

less or no (white) precipitate / solution gets hot i.e. hotter than in (i) /

metal dissolves more quickly (2)

(iii) Ca + 2H₂O → Ca(OH)₂ + H₂(1)

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2. (a) ability of an atom to attract electron density (1)

from a covalent bond (1)

(b) Trend decreases (1)

Explanation outer electrons further from nucleus (1)

more shielding from the nucleus (1)

(or more shells or increasing radius)

pH 12 – 14 (1)

(d) Observation white precipitate (1)

Equation Ba2+ + SO₄2– --> BaSO₄ (1)

(or BaCl₂ + H₂SO₄ --> BaSO₄ + 2HCl)

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