Unit 4 - Inorganic Chemistry and the Periodic Table
Instelling
Pearson (PEARSON)
A comprehensive in-depth, specification led, revision booklet of the 2015 spec chemistry AS level course. Written during AS studies by an AS student, meaning it is written in an easily comprehensible stance for any student wishing to find extra resources to revise from.
Unit 4 - Inorganic Chemistry and the Periodic Table
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Voorbeeld van de inhoud
A Cook 2015
Chemistry Led Specification Revision:
Topic 4 - Inorganic Chemistry and the Periodic Table
Topic 4a: The elements of Groups 1 and 2
Group 2 Elements Background:
• Group 2 elements consist of the second column of the Periodic Table
• They are all shiny metals, but when exposed to air they oxide and subsequently
become matted
(a) Understand reasons for the trend in ionisation energy down Group 2
Ionisation Energy Background:
1st and 2nd Ionisation Energies:
• 1st IE = The energy required to remove 1 electron from every atom in 1 mole of
gaseous atoms to form 1 mole of gaseous singly charged (1+) gaseous ions.
M(g) —> M+(g) + e-
Group 2 elements typically lose 2 electrons from each atom, so we consider the
2nd IE too. (hence the first + second IE in table)
• 2nd IE = The energy required to remove 1 electron from every atom in 1 mole of
singly charged (1+) gaseous ions, to produce 1 mole of 2+ charged gaseous
atoms.
M+(g) —> M2+(g) + e-
Table below shows the metallic radius (Atomic Radii of metallic bonded atoms), the 1st IE,
2nd IE and 1+2 IE to allow comparison of factors effecting ionisation energy
Element Metallic Radius / 1st IE /kJ mol-1 2nd IE /kJ mol-1 First + Second /
nm kJ mol-1
Beryllium 0.112 900 1757 2657
Magnesium 0.160 738 1451 2189
Calcium 0.197 590 1145 1735
Strontium 0.215 550 1064 1614
Barium 0.224 503 965 1468
First Ionisation Energy DECREASES down group 2:
The energy needed for ionisation is to overcome the electrostatic attraction between the electron
being removed snd the protons in the nucleus
Factors affecting Ionisation Energy:
1) Nuclear Charge (number of protons in nucleus)
2) The orbital in which the electron exists
, A Cook 2015
3) The shielding effect - the repulsion between filled inner shells and the electron being removed
Why IE decreases:
Nuclear Charge:
• As the nuclear charge increases, the force of attraction between the nucleus and electrons
increases, meaning the ionisation energy down the group increases, due to the reduced energy
of the removed electron.
Quantum Shell:
• As each quantum shell is added, energy of the outermost electron increases
Shielding:
• As the number of filled inner shells increases, their force of repulsion on the electron being
removed increases - meaning a decrease in ionisation energy down the group
Nuclear charge causes an increase in ionisation energy, while the quantum shell and shielding
effect both cause a decrease in ionisation energy.
(b) Understand reasons for the trend in reactivity of the Group 2 elements down the group
Increase in reactivity down Group 2:
• There is a general increase in reactivity down the group
Explanation for increase down:
• Explained by the decrease in energy needed to remove the two electrons from each atom of the
element
• Making the elements less stable as it now takes less energy to remove the valence electrons
(c) Know the reactions of elements Mg to Ba in Group 2 with oxygen, chlorine and water
• When group 2 elements react they form ions with a charge of 2+, this is because Group 2
elements contain 2 electrons in their outer shell.
• They lose both of these electrons when they react.
Reactions with oxygen:
In air O2 levels:
• Burning Magnesium in air creates a bright UV light and the formation of a MgO
• Similar flame colour creating reactions occur with other group 2 elements, but they are different.
• When calcium, Strontium and Barium are burned in air they are more vigorous.
In oxygen rich environment:
• If the burning metal is placed in a jar of oxygen, then the same reaction occurs although more
vigorously
Reaction Initiation:
• For all elements, they need to be heated to initiate full reaction
• However without heating there is a slow reaction between oxygen and the group 2 element
regardless. Oxidation occurs creating a thin layer of oxide over the top of the element to stop
further reactions taking place
Barium, being the most reactive, is needed to be kept under oil to stop it reacting with oxygen and
water vapour in the air.
The general equation for group 2 metal reactions with oxygen is:
, A Cook 2015
2M(s) + O2(g) —> 2MO(s)
The products formed consist of Mg2+ ions and O2- ions
Reactions with Chlorine:
Group 2 elements combine with chlorine when heated in a gas
Reactions become more vigorous going down group 2, like with oxygen
General equation:
M(s) + Cl2(g) —> MCl2(s)
Products form contain M2+ and Cl2- Ions
Reactions with Water:
• Water and magnesium is a very slow reaction and does not complete
• Calcium, strontium and various react with increasing vigour, which can be seen in the
effervescence
General equation of group 2 with water is:
M(s) + 2H2O(l) —> M(OH)2(aq) + H2(g)
The products are M2+ and OH- ions
Calcium with water:
Ca(s) + 2H2O(l) —> Ca(OH)2(s) + H2(g)
(Ca(OH)2 is a SOLID because it is insoluble in water)
Barium and water:
Ba(s) + 2H2O(l) —> Ba(OH)2 (aq) + H2(g)
(Ba(OH)2 is an aqueous solution because it is SOLUBLE in water)
Magnesium and Steam
• Magnesium reacts differently when heated in steam
• It rapidly forms Magnesium Oxide and hydrogen gas in a vigorous reaction:
The equation is:
Mg(s) + H2O(g) —> MgO(s) + H2(g)
The hydrogen should be burned as it leaves so that it does not fill the lab with highly flammable
gasses
Reactions with Beryllium and Radium:
• Beryllium will be less reactive than magnesium as the trend of reactivity goes down the group
• Radium is more reactive than Barium as the trend is downwards flowing
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