8) Reactivity trends Flashcards
Describe Group 2 elements
reactive metals
do not occur in their elemental form naturally
reducing agents in redox reactions which form 2+ ions
Group 2 element + oxygen ->
metal oxide (MO)
Describe Magnesium’s reaction with oxygen
burns with a white light to form a white metal oxide
Group 2 element + water ->
alkaline hydroxide (M(OH)2) + hydrogen gas
metal + acid ->
salt + hydrogen
A group 2 element’s reaction with water becomes _ vigorous down the group as reactivity _?
more
increases
A group 2 element’s reactions with oxygen, water and acids are all examples of what type of reaction?
redox
State and explain the reactivity trend down Group 2
reactivity increases
first and second ionisation energies decrease down the group because the attraction between the nucleus and outer e- decreases as a result of increased atomic radius and shielding
Describe the reaction of group 2 oxides with water
release OH- and form alkaline solutions of the metal hydroxide
however, group 2 hydroxides are only slightly soluble in water so when the solution becomes saturated, any further metal and hydroxide ions will form a solid precipitate (M(OH)2 (s))
Solubility of group 2 hydroxides in water _ down the group, so resulting solutions contain _ OH- ions and are _
increases
more
more alkaline
How can the solubility trend of Group 2 hydroxides be shown experimentally?
add a spatula of oxide to a test tube of water
shake
you will see whether or not there is sufficient water to dissolve the hydroxide that forms
pH of solutions can be tested
What are group 2 oxides, hydroxides and carbonates all useful as?
bases
Give two uses of group 2 bases
Ca(OH)2 added to fields as lime to increase pH of acidic soils in agriculture
as antacids for treating acid indigestion in medicine - often magnesium / calcium carbonates or ‘milk of magnesia’ (suspension of white Mg(OH)2 in water) to neutralise HCl(aq) in stomach
Describe halogens
reactive non-metal elements
do not occur in their elemental form in nature (instead as stable halide ions dissolved in sea water / combined with sodium or potassium as solid deposits)
exist as diatomic molecules at RTP
as solids form lattices with simple molecular structures
Boiling point _ down the halogen group because _?
increases
more electrons
stronger London forces
more energy required to break intermolecular forces
What do displacement reactions of halogens show?
reactivity of halogens decreases down the group
Why does reactivity of halogens _ down the group?
decrease
increased atomic radius and shielding
decreased nuclear attraction
Describe the displacement reactions of halogens
a solution of each halogen is added to aqueous solutions of the other halides
if the halogen is more reactive than the halide present:
- halogen displaces halide from solution
- solution changes colour
What colour is Cl2 in water?
pale green
What colour is Cl2 in cyclohexane?
pale green
What colour is Br2 in water?
orange
What colour is Br2 in cyclohexane?
orange
What colour is I2 in water?
brown
What colour is I2 in cyclohexane?
violet
Cl2 + Br- (aq)?
Br2 formation
orange
Cl2 + I- (aq)?
I2 formation
violet
Br2 + Cl- (aq)?
no reaction
Br2 + I- (aq)?
I2 formation
violet
I2 + Cl- (aq)?
no reaction
I2 + Br- (aq)?
no reaction
Give the equation for Cl2 + Br- (aq)?
Cl2 (aq) + 2Br- (aq) -> 2Cl- (aq) + Br2 (aq)
Describe fluorine
pale yellow gas
very reactive
Describe astatine
radioactive + decays rapidly
predicted to be the least reactive halogen
Define disproportionation
a redox reaction in which the same element is both oxidised as reduced
Give two examples of disproportionation with halogens
chlorine + water -> chloric (I) acid, HClO (weak bleach) + HCl
chlorine + cold dilute NaOH (aq) -> sodium chlorate (I) (household bleach) + NaCl (aq) + water
Give 1 benefit and 2 risks of chlorine use
benefit: disinfectant for water purification
risks:
extremely toxic gas (respiratory irritant in small concentrations)
reacts with organic hydrocarbons to form chlorinated hydrocarbons - suspected of causing cancer
What is the test for carbonate ions?
add dilute nitric acid
bubbles / white ppt in lime water turning it cloudy
What is the test for sulfate ions?
add Ba 2+(aq) as barium nitrate
white ppt of BaSO4 (s)
What is the test for halides?
add aqueous silver nitrate (precipitation reaction)
Ag+ (aq) + X- (aq) -> AgX (s) then add aqueous ammonia
Cl- -> white ppt; soluble in dilute NH3(aq)
Br- -> cream ppt; soluble in concentrated NH3(aq)
I- -> yellow ppt; insoluble in concentrated NH3(aq)
Give the correct sequence of tests for anions
carbonate -> sulfate -> halide
What is significant about the carbonate test within the sequence of tests for anions?
always correct conclusion as neither sulfate nor halide ions produce bubbles with dilute acid
What should you ensure with the carbonate test within the sequence of tests for anions?
continue to add dilute nitric acid until bubbling stops to ensure all carbonate ions have reacted / been removed
What should you ensure with the sulfate test within the sequence of tests for anions? (2)
ensure carbonate test is carried out first as BaCO3 is also white and insoluble in water
add an excess of barium nitrate and filter solution after to remove ppt
What should you ensure with the halide test within the sequence of tests for anions?
carried out last as both Ag2CO3 and Ag2SO4 are insoluble in water and will form ppts
What is the test for ammonium ions, NH4 +?
add NaOH(aq) and warm produces ammonia gas which smells and turns moist pH indicator blue as an alkaline
