Group 2 and Group 7 Elements - Unit 2, Section 2

Question 1

what are the group 2 elements?

Answer 1

alkaline earth metals

in the S block

Question 2

atomic radius of Group 2 elements

Answer 2

• gets larger as you go down the group because extra electron shells are added as you go down the group.

Question 3

first ionisation energy of group 2 elements

Answer 3

decreases down the group because each elements down group 2 has an extra electron shell compared to the one above.

the extra inner shells shield the outer electrons from the attraction of the nucleus. this makes it easier to remove outer electrons resulting in a lower first ionisation energy.

Question 4

reactivity of group 2 elements

Answer 4

• become more reactive as you move down the group as it becomes easier to lose electrons due to a lower first ionisation energy

Question 5

melting point of group 2 elements

Answer 5

• generally decreases down the group
• elements have typical metallic structures with positive ions in a crystal structure surrounded by delocalised electrons from the outer electron shells.
• the metal ions get bigger down the group so the ionic radius gets larger and the delocalised electrons are further from the nucleus. This means less energy is needed t break the bonds, which means the melting point decreases.
• Magnesium has a slightly different crystal structure which makes its melting point lower than average.

Question 6

group 2 elements reacting with water

Answer 6

• group 2 elements are oxidised from a state of 0 to +2, forming M 2+ ions.
• form a metal hydroxide and hydrogen
• more reactive down the group due to lower ionisation energies

Question 7

solubility of group 2 compounds

Answer 7

• depends on the anion (negative ion) in the compound
• elements that contain a singly charge negative ion increase in solubility down the group
• elements that contain doubly charged negative ions decrease in solubility down the group
• most sulfates are soluble in water, apart from barium sulfate
• compounds like magnesium hydroxide have very low solubilities so are said to be sparingly soluble

Question 8

barium meals (barium sulfate)

Answer 8

• useful in medicine as it is insoluble
• barium sulfate is opaque to X-rays so it can help to diagnose with the oesophagus, stomach or intestines.
• the patient swallows a ‘barium meal’ which is a suspension of barium sulfate. The sulfate coats the tissues which makes them show up on x-rays.

Question 9

extraction of titanium

Answer 9

the main titanium ore, TiO2, is first converted to titanium chloride, TiCl4, by heating it with carbon in a stream of chlorine gas.

The titanium chloride is then purified by fractional distillation before being reduced by magnesium in a furnace at almost 1000 *C

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

• Magnesium is the reducing agent in this reaction

Question 10

Removal of sulfur dioxide from flue gases

Answer 10

• burning fossil fuels to produce electricity also produces sulfur dioxide which pollutes the atmosphere.
• the acidic sulfur dioxide can be removed from flue gases by reacting with an alkali - this is called wet scrubbing
• Calcium oxide (lime, CaO) and Calcium carbonate (limestone, CaCO3) can both be used for this.
• a slurry is made by mixing the calcium oxide or carbonate with water.
• it is then sprayed on the flue gases. Flue gases are the gases emitted from industrial exhausts and chimneys
• the sulfur dioxide reacts with the alkaline slurry and produces a solid waste product, calcium sulfite.

CaO (s) + 2H2O (l) + SO2 (g) —> CaSO3 (s) + 2H2O (l)

CaCO3 (s) + 2H2O (l) + SO2 (g) —> CaSO3 (s) + 2H2O (l) + CO2 (g)

Question 11

other uses of Group 2 compounds

Answer 11

• common compounds are used for neutralising acids
• calcium hydroxide is used in agriculture to neutralise acidic soils
• Magnesium hydroxide is used in some indigestion tablets as an antacid

Question 12

what are the group 7 elements?

Answer 12

• halogens which are highly reactive non metals

Question 13

boiling points of halogens

Answer 13

• increase down the group
• due to increasing strength of the van der Waals forces as the size and relative mass of the molecules increases
• this trend is shown in the changes of physical state from fluorine (gas) to iodine (solid)

Question 14

electronegativity of halogens

Answer 14

• decreases down the group
• larger atoms attract electrons less than smaller ones because their outer electrons are further from the nucleus and are more shielded as they have more inner electrons

Question 15

displacement reactions

Answer 15

• halogens gain an electron when they react - making them oxidising agents
• they get less reactive down the group because the atoms become larger and the outer shell gets further from the nucleus. - become less oxidising down the group
• the oxidising strengths of the halogens can be seen in their displacement reactions with the halide ions
• a halogen will displace a halide from solution if the halide is below it in the periodic table

KBr (aq) reacts with Cl2 (aq) to form an orange solution of Br2

KI (aq) reacts with Cl2 (aq) to form a brown solution of I2

KI (aq) reacts with Br2 (aq) to form a brown solution of I2

• displacement reactions can be used to help identify which halogen or halide is present in a solution
• halide ions are colourless in solution but when the halogen is displaced it shows a distinctive colour

Question 16

making bleach

Answer 16

if you mix chlorine gas with cold, dilute sodium hydroxide solution at room temperature, you get sodium chlorate (I) solution (NaClO (aq) ) which is common household bleach.

in this reaction, chlorine is both oxidised and reduced which is called disproportionation.

2NaOH (aq) + Cl2 (g) –> NaClO (aq) + NaCl (aq) + H2O (l)

Question 17

Water treatment

Answer 17

• chlorate ions kill bacteria so adding chlorine to water can make it safe to drink or swim in, however chlorine is toxic.

-chlorine is an important part of water treatment. it kills disease causing microorganisms and some chlorine persists in the water to prevent reinfection further down the supply. it also prevents the growth of algae, eliminating bad tastes and smells, and removes discolouration caused by organic compounds.

• chlorine gas is very harmful if it’s breathed in as it irritates the respiratory system
• liquid chlorine can cause severe chemical burns on the skin or eyes
• water contains a variety of organic compounds from the decomposition of plants
• chlorine reacts with these compounds to form chlorinated hydrocarbons and many are carcinogenic. this risk is small compared to the risks of untreated water such as cholera

Question 18

Halide ion formation and oxidation

Answer 18

• halide ions are 1- ions as they gain one electron
• when a halide ion takes part in a redox reaction, it reduces something and is oxidised itself.

Question 19

does the reducing power of halides increase or decrease down the group?

Answer 19

• how easy it is form a halide ion to lose an electron depends on the attraction between the nucleus and outer electrons.
• the attraction gets weaker because the ions get bigger, so the electrons are further away from the positive nucleus.
• there are extra inner electron shells so there is a greater shielding effect
• the reducing power of the halides increases down the group

Question 20

what is produced when a halide reacts with sulfuric acid?

Answer 20

a hydrogen halide

NaX + H2SO4 –> NaHSO4 + HX

Question 21

Half equations for reactions with sulfuric acid when the halide is strong enough to reduce sulfuric acid to water and sulfur dioxide

Answer 21

2X- (g) –> X2 (s) + 2e- = halide is oxidised

H2SO4 +2H+ + 2e- —> SO2 + 2H2O = sulfuric acid is reduced