Periodicity

Question 1

What is Periodicity?

Answer 1

A regular periodic variation of properties of elements with atomic number and position in the periodic table

Question 2

What is the trend for atomic radius across a period?

Answer 2

It decreases because as the number of protons increases, the positive charge of the nucleus increases. This means that the electrons are pulled closer to the nucleus, making the atomic radius smaller.

Question 3

What is the general trend for melting point across Period 3

Answer 3

The melting point increases from sodium to silicon, but then generally decrease from silicon to argon.

Question 4

What is the First ionisation energy?

Answer 4

The energy needed to remove one electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Question 5

What is the trend for first ionisation energy for Period 3?

Answer 5

There is a general increase as you go across, because of the increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing.

Question 6

What is the trend in atomic radius for Group 2?

Answer 6

Down the group the atomic radii increases, because an increasing number of shells means that the atoms get bigger

Question 7

What is the general trend in ionisation energies for Group 2?

Answer 7

They decrease down the group because:
- Atoms get bigger so the outermost electrons are further from the nucleus
- More electron shells mean shielding increases
So overall attraction to the outer electron decreases.

Question 8

What is the general trend in melting point for Group 2?

Answer 8

It decreases down the group:
- Atoms get bigger so the delocalised electrons are further from the nucleus
- More electron shells meaning shielding increases
Overall attraction to the delocalised electrons decreases, which means the metallic bonding is weaker and requires less energy to break it

Question 9

What is the trend in reactivity for Group 2?

Answer 9

Reactivity increases down the group:

• Atomic radii increases down group - increased number of shells means atoms get bigger and there is less attraction between the outermost electron and nuclei meaning they are more reactive.
• They react by losing 2 electrons, forming positive ions - easier to lose electrons = more reactive
• Shielding increases
• Lower nuclear attraction to outermost electron

Question 10

For Group two melting point, why is there a ‘blip’ for magnesium?

Answer 10

Its crystal structure (the arrangement of metallic ions) changes.

Question 11

What is formed from a reaction between water and a metal?

Answer 11

A hydroxide is formed.

Question 12

What is the equation for the reaction between a metal and water?

Answer 12

M = metal

- M + 2H2O (l) —-> M(OH)2 (aq) + H2 (g)

Question 13

What is the equation for the reaction between Magnesium and water vapour?

Answer 13

• Mg (s) + H2O (g) —> MgO (s) + H2 (g)

During this reaction a bright white flame will be observed.

Question 14

What is the trend with the reaction of water down Group 2?

Answer 14

• The reaction gets more vigorous further down the group.

Question 15

What observations are made from the reaction between a metal and water?

Answer 15

• Fizzing
• The metal will dissolve
• Exothermic - the reaction will feel hot
• The resulting solution will be alkaline
• Some precipitate, amount depends on solubility of the hydroxide produced

Question 16

What is the trend in solubility for the hydroxides?

Answer 16

The solubility increases down the group.
Ba(OH)2 is most soluble
Mg(OH)2 is least soluble

Question 17

What is Calcium Hydroxide used for?

Answer 17

• In agriculture to neutralise acidic soil
• Can be used to test for CO2:
  Ca(OH)2 (aq) + CO2 (g) —> CaCO3 (s) + H2O (l)

Question 18

What is Magnesium Hydroxide used for?

Answer 18

• Used in medicine to treat indigestion - it does this by neutralising stomach acid.

Question 19

What is the trend in solubility for sulfates?

Answer 19

• Solubility decreases down the group.
  MgSO4 most soluble
  BaSO4 least soluble

Question 20

What is Barium sulfate used for?

Answer 20

• It is used in medicine as a barium med - used for x-rays to observe soft tissues like internal organs
• It is very toxic, but can be used as it is completely insoluble

Question 21

How do we test for sulfate ions?

Answer 21

• Acidified BaCl2 is added to the unknown solution
  (Acidified in HCl as this removes impurities such as carbonate ions - solution will fizz)
• If sulfate ions are present a white precipitate will form: Ba2+ (aq) + SO42- (aq) –> BaSO4 (s)

Question 22

Why is titanium useful?

Answer 22

• It is abundant
• It is low density
• It resists corrosion
  However it must be extracted from its ore.

Question 23

Extraction of Titanium

Answer 23

• Mg is used to extract Ti from its ore (Mg is more reactive than Ti)
• Main titanium ore TiO2 is converted to TiCl4 by heating to 900 degrees C with carbon in stream of Cl2 (g)
• TiCl4 is then purified by fractional distillation in an Ar atmosphere, then reduced by Mg in a furnace at 500 degrees C, again under Ar

Question 24

What is the equation for the conversion of TiO2 to TiCl4?

Answer 24

TiO2 + 2Cl2 + 2C —> TiCl4 + 2CO

Question 25

What is the equation for the reduction of TiCl4 by Mg?

Answer 25

TiCl4 (g) + 2Mg (l) —> Ti (s) + 2MgCl2 (l)

Question 26

Why is Titanium expensive?

Answer 26

• Both purification steps require high temperatures
• It is a batch process
• Magnesium is expensive
• Argon is expensive (maintaining argon atmosphere)