Transcript Group 2 Elements

Starter.......
1. If M can be any element what group do
you think MO is the general formula for?
Why?
2. Write equations for M reacting with water
and oxygen.
3. Write any other general formulas for M.
Groups 1 and 2 elements
Monday, 11 April 2016
Groups 1 and 2 elements
Lesson Objective: To know the chemical and physical properties of
the Group 2 elements and to know the flame tests for Groups 1 and
2
Success criteria:
•understand reasons for the trend in ionisation energy down
Group 2
•understand reasons for the trend in reactivity of the Group 2
elements down the group
•know the reactions of the elements Mg to Ba in Group 2 with
oxygen, chlorine and water
•understand the formation of characteristic flame colours by
Group 1 and 2 compounds in terms of electron transitions
•know the flame colours for Groups 1 and 2 compounds
•understand experimental procedures to show flame colours in
compounds of Group 1 and 2 elements
Group 2 Elements (The Alkaline Earth
Metals): Physical Properties
• High melting point
• Light metals with low densities
• They form colourless compounds
Group 2 Elements: Melting Points
Trend in first ionisation energy and atomic radius down a group
• 1st IE’s decrease
• The number of shells
increases, so the
distance between
the nucleus and the
outer electrons –
weaker attractive
force.
• More inner shells, so
the shielding
increases – less
attraction.
• Ionic radius increases
• More shells of electrons
• Increased electron
shielding
• Also less attraction
means electrons are
pulled in less by the
nucleus
For each element the +2 ion is smaller than the atom
because of the loss of the outer electrons.
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)
• What will their oxidation states be?
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.
Reducing agents reduce (add electrons to) another species
Oxidation is the loss of electrons
Explaining the trend in reactivity
The reactivity of the elements
down group 2 from beryllium to
barium increases.
Mg
This is because it is
successively easier to remove
electrons to form the 2+ ion.
Ca
Although increased shielding
cancels the increased nuclear
charge down the group, the
increase in atomic radius results
in a decrease in the attractive
force between the outer
electrons and the nucleus.
Sr
Ba
Group 2 Elements:
Reaction with Oxygen
• Group 2 metals react vigorously with oxygen (apart
from Be)
• It is a redox reaction
• Produces an ionic oxide with general formula MO
What are
the
oxidation
numbers?
e.g. 2Ca(s) + O2(g)  2CaO(s)
0
0
+2
Oxidation Numbers –
Ca – Oxidised
O – Reduced
-2
An oxidation number is a measure of the number of electrons
that an atom uses to bond with atoms of another element.
Group 2 Elements:
Reaction with Water
• Group 2 metals (Mg – Ba) 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)
What are
the
oxidation
numbers?
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.
Redox reaction with chlorine
When group 2 metals react with chlorine, they form the
metal chloride. For example:
Ca(s) + Cl2(g) CaCl2(s)
0
0
+2 -1
What are
the
oxidation
states?
The product is a white metal chloride.
The oxidation state of calcium has increased from 0 in its
elemental form to +2 when it is in calcium chloride. This
means the calcium has been oxidized.
The oxidation state of chlorine has decreased from 0 in its
elemental form to -1 when it is in calcium chloride. This
means the chlorine has been reduced.
Groups 1 and 2 elements
Lesson Objective: To know the chemical and physical properties of
the Group 2 elements and to know the flame tests for Groups 1 and
2
Success criteria:
•understand reasons for the trend in ionisation energy down
Group 2
•understand reasons for the trend in reactivity of the Group 2
elements down the group
•know the reactions of the elements Mg to Ba in Group 2 with
oxygen, chlorine and water
•understand the formation of characteristic flame colours by
Group 1 and 2 compounds in terms of electron transitions
•know the flame colours for Groups 1 and 2 compounds
•understand experimental procedures to show flame colours in
compounds of Group 1 and 2 elements
Flame tests practical
When metals are burned in oxygen they burn with
characteristic flame colours.
We are going to investigate the colours of groups 1
and 2 metals.
1. Mix a small amount of the compound you are
testing with a few drops of concentrated
hydrochloric acid
2. Heat a piece of nichrome wire in a hot Bunsen
flame to clean it.
3. Dip the wire into the compound/acid mixture.
Hold it in a very hot flame and note the colour
produced.
Results
• What results did we get?
• Why do we get the different coloured
flames?
Flame tests
When group 2 metals are burned in oxygen, coloured flames
are produced. This is due to the presence of metal ions. Flame
tests exploit this fact.
The presence of certain metal ions can be identified by noting
the characteristic flame colour that results from burning. The
colours for group 2 metal ions are:
magnesium – burns with a bright white light, not
classed as a flame colour
calcium – brick red strontium – bright red
barium – pale green
Group 1 flame tests
It's possible to test a compound to detect the presence of
an alkali metal ion.
Different metals give different coloured flames.
Explaining flame tests
When heated, some electrons in an atom or ion are excited
to higher energy levels. When they fall back to their initial
levels, energy is emitted; sometimes seen as visible light.
light
heat
energy
Electrons may be excited by
different amounts into
different energy levels and
drop back at different times.
The colour of the flame is a
combination of all these
energy emissions.
Groups 1 and 2 elements
Lesson Objective: To know the chemical and physical properties of
the Group 2 elements and to know the flame tests for Groups 1 and
2
Success criteria:
•understand reasons for the trend in ionisation energy down
Group 2
•understand reasons for the trend in reactivity of the Group 2
elements down the group
•know the reactions of the elements Mg to Ba in Group 2 with
oxygen, chlorine and water
•understand the formation of characteristic flame colours by
Group 1 and 2 compounds in terms of electron transitions
•know the flame colours for Groups 1 and 2 compounds
•understand experimental procedures to show flame colours in
compounds of Group 1 and 2 elements
Practice questions
1) Use the electron configurations of magnesium
an calcium to help explain the difference in
their first ionisation energies. (5 marks)
Mg 1s22s22p63s2
Ca 1s22s22p63s23p64s2
(1 mark)
First ionisation energy of Ca is smaller (1 mark)
because Ca has (one) more electron shell(s) (1
mark). This reduces the attraction between
the nucleus and the outer electrons because it
increases the shielding effect (1 mark) and
because the outer shell of Ca is further from
the nucleus (1 mark)
Practice questions
2) Calcium can be burned in chlorine gas.
a) Write an equation for the reaction (1 mark)
Ca (s) + Cl2 (g)  CaCl2 (s)
b) State the change in oxidation state of
calcium (1 mark)
From 0 to +2
c) Predict the appearance of the product (2
marks)
White (1 mark) solid (1 mark)
d) What type of bonding would the product
have? (1 mark)
Ionic
Practice questions
3) The table shows the atomic radii of three
elements from Group 2.
Element
X
Y
Z
Atomic radius (nm)
0.089
0.198
0.176
a) Predict which element would react most rapidly
with water (1 mark)
Y
b) Explain your answer (2 marks)
Y has the largest radius (1 mark) so it will have
the smallest ionisation energy (1 mark)
Practice questions flame tests
a) When a substance is heated, what changes
occur within the atom that give rise to a
coloured flame? (2 marks)
Energy is absorbed and electrons move
to higher energy levels (1 mark). Energy
is released in the form of coloured light
when the elctrons fall nack to the lower
levels (1 mark)
b) A compound gives a lilac flame in a flame test.
What s block metal ions might this compound
contain? (1 mark)
Potassium
Questions
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
Groups 1 and 2 elements
Lesson Objective: To know the chemical and physical properties of
the Group 2 elements and to know the flame tests for Groups 1 and
2
Success criteria:
•understand reasons for the trend in ionisation energy down
Group 2
•understand reasons for the trend in reactivity of the Group 2
elements down the group
•know the reactions of the elements Mg to Ba in Group 2 with
oxygen, chlorine and water
•understand the formation of characteristic flame colours by
Group 1 and 2 compounds in terms of electron transitions
•know the flame colours for Groups 1 and 2 compounds
•understand experimental procedures to show flame colours in
compounds of Group 1 and 2 elements