Transcript Group 2 Elements: Reactivity

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.
Group 2 elements
Monday, 13 April 2015
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, reacitvity.
Specification links
Today’s lesson:
Redox reactions of Group 2 metals
• (a) describe the redox reactions of the Group 2 elements Mg→Ba:
– (i) with oxygen,
– (ii) with water;
• (b) explain the trend in reactivity of Group 2 elements down the group
due to the increasing ease of forming cations, in terms of atomic size,
shielding and nuclear attraction;
Next lesson:
Reactions of Group 2 compounds
• (c) describe the action of water on oxides of elements in Group 2 and
state the approximate pH of any resulting solution;
• (d) describe the thermal decomposition of the carbonates of elements
in Group 2 and the trend in their ease of decomposition;
–
(No explanation of thermal decomposition required.)
• (e) interpret and make predictions from the chemical and physical
properties of Group 2 elements and compounds;
• (f) explain the use of Ca(OH)2 in agriculture to neutralise acid soils;
the use of Mg(OH)2 in some indigestion tablets as an antacid.
•
•
•
•
•
Group 2 reactions with water
Add magnesium to water in a test tube
What happens?
Collect the gas produced by putting another tube over the
top and test it with a lit spill (keep the test tube upside
down)
Test the solution that is left with universal indicator.
What colour does it go?
• Do the same with calcium.
• Which is more reactive?
• How do you know.
• What products are made?
Group 2 Elements: Physical
Properties
• High melting point
• Light metals with low densities
• They form colourless compounds
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)
Group 2 Elements: Reactivity
• Reactive metals that are strong reducing agents.
• They are oxidised in reactions – loose 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
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
+2
-2
Oxidation Numbers –
Ca – Oxidised
O – Reduced
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 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
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
From page 89 of text book
Now look at the past exam questions
Complete the summary table below for
group 2
Bonding
Structure
Ions
Redox character
Reactivity
Complete the summary table below for group 2
Bonding
Metallic
Structure
Giant metallic structure;
strong forces between
positive ions and negative
electrons
Ions
M2+
Redox character
Reducing agent reacts by
losing electrons
M  M2+ + 2e-
Reactivity
Increases down the group
Specification links
Today’s lesson:
Redox reactions of Group 2 metals
• (a) describe the redox reactions of the Group 2 elements Mg→Ba:
– (i) with oxygen,
– (ii) with water;
• (b) explain the trend in reactivity of Group 2 elements down the group
due to the increasing ease of forming cations, in terms of atomic size,
shielding and nuclear attraction;
Next lesson:
Reactions of Group 2 compounds
• (c) describe the action of water on oxides of elements in Group 2 and
state the approximate pH of any resulting solution;
• (d) describe the thermal decomposition of the carbonates of elements
in Group 2 and the trend in their ease of decomposition;
–
(No explanation of thermal decomposition required.)
• (e) interpret and make predictions from the chemical and physical
properties of Group 2 elements and compounds;
• (f) explain the use of Ca(OH)2 in agriculture to neutralise acid soils;
the use of Mg(OH)2 in some indigestion tablets as an antacid.