Transcript GROUP 2 ELEMENTS File

GROUP 2
SYLLABUS POINTS 3.1.2
SYLLABUS POINTS
Group 2 Elements:
Redox Reactions
Lesson Objectives:
• Describe the redox reactions of the Group 2
elements
Mg → Ba with oxygen and with water.
• Explain the trend in reactivity of Group 2 elements
down the group.
Key Words: periodicity, redox reactions, reactivity.
Recap
1. What do you know about the elements in
group 2?
2. Write the chemical equation for
magnesium with:
a) Water
b) oxygen
3. Assign oxidation numbers to all species
in the above equations.
Group 2 Elements:
Physical Properties
Group 2 elements exhibit the following
physical properties:
• High melting point
• Light metals with low densities
• They form colourless compounds
Group 2 Elements: Melting Points
Group 2 Elements:
Electron Configuration
Element
Electron Configuration
1st I.E. /kJ mol-1
Be
[He]2s2
900
Mg
[Ne]3s2
736
Ca
[Ar]4s2
590
Sr
[Kr]5s2
548
Ba
[Xe]6s2
502
Ra
[Rn]7s2
509
• Highest energy electrons in an s sub-shell
• Each element has two more electrons than the
preceding noble gas
• two electrons in the outer shell. (form 2+ ions)
• All elements in the group undergo similar
chemical reactions.
Group 2 Elements:
Ionisation energy
First energy
& Secondisionsation
energies
of Group
2
The first ionisation
the enthalpy
change
when
one mole of
3000 atoms forms one mole of gaseous ions with a single positive
gaseous
charge.
2500
Ionisation energy (kJ/mol)
1757
1450
It is
an endothermic process, i.e. delta H is positive.
2000
Second
Proton
First ionisation
ionisation
Element
Symbol
A general equation
for
this
enthalpy
change
is:
energy
(kJ/mol)
number
1500
energy (kJ/mol)
X(g) → X+(g) + e1145
1064
965
1000
beryllium
500
magnesium
calcium0
strontium
barium
899
4
Be
12
Mg
20
Ca
Be
Mg
38
Sr
56
Ba
1st ionisation
energy
738
899
738
590
Ca
Sr
549
Group 2
503 energy
2nd ionisation
590
549
1757
1450
1145
Ba
1064
965
503
Explanation of this trend
Going down Group 2:
• there are more filled energy levels between the
nucleus and the outer electron these shield the outer
electron from the attraction of the nucleus ...
• the radius of the atom increases, so the distance
between the nucleus and the outer electron increases
...
• therefore the force of attraction between the nucleus
and outer electron is reduced ...
• so less energy is needed to remove the outer electron.
Explanation of this trend
As the number of protons in the nucleus increases
going down Group 2, you might expect the first
ionisation energy to increase because the nuclear
charge increases.
This does not happen, because the factors described
before have a greater influence on the value of the first
ionisation energy.
Group 2 Elements:
Reactivity
• Reactive metals that are strong reducing agents.
• They are oxidised in reactions – lose 2 electrons
M  M2+ + 2e- (+2 oxidation state)
• Reactivity increases down the group
• Reactivity increases because of the same
factors that cause 1st I.E. to decrease.
Group 2 Elements:
Reaction with Oxygen
• Group 2 metals react vigorously with oxygen
• It is a redox reaction
• Produces an ionic oxide with general formula MO
e.g. 2Ca(s) + O2(g)  2CaO(s)
0
0
Oxidation Numbers –
Ca – Oxidised
O – Reduced
+2 -2
Group 2 Elements:
Reaction with Water
• Group 2 metals react with water to form
hydroxides – M(OH)2
• Hydrogen gas is also formed
e.g. Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g)
0
+1 -2
+2 -2 +1
0
Oxidation Numbers –
Ca – Oxidised
H – Reduced
Only one H atom in each water has been reduced – The other
doesn’t change
Mg reacts slowly with water – on descending the groups
reactions get more vigorous.
Questions -textbook pg89
1. Mg + 2HCl  MgCl2 + H2 is a redox reaction
a) Identify the changes in oxidation number
b) Which species are being oxidised and which are
being reduced?
c)Which species are the oxidising and reducing
agents?
2. a) Write the equations for these reactions;
i) Barium with water
ii) Strontium with oxygen
b) Using oxidations numbers, identify what has
been oxidised and reduced