Group 2&7 and tests Flashcards
solubility of group 2 hydroxides
solubility of the hydroxides in water increases down the group so solution has more OH- ions and is more alkaline
ionisation energies down the group
decrease down, attraction between the nucleus and outer electrons decreases as increasing atomic radius and increased shielding
so reactivity increases down the group and stronger reducing agents down the group
why is ca(oh)2 added to fields
increase the pH of acidic soils
group 2 bases used as medicines?
used as anti acids to treat acid indigestion e.g. milk of magnesia Mg(OH)2
colour of Ca flame
red/orange
colour of Sr flame
red
colour of barium flame
pale green
colour of Mg flame
white
state of bromine at RTP
orange-brown liquid
state of iodine at RTP
shiny grey black solid
state of chlorine at RTP
pale green gas
Usual reaction of halogens
usually redox where halogen is reduced to increase outer electrons from 7 to 8
Boiling point of halogen down the group
decreases as: - more electrons - stronger London forces - more energy needed to break IMF - boiling point increases
colour of chlorine in solution in water
pale green
colour of bromine in solution in water
orange
colour of iodine in solution in water
brown
why is cyclohexene added to water solutions
iodine and bromine colours are similar in water, cyclohexene added to tell them apart as iodine turns from brown to violet whereas bromine stays orange
colour of bromine in cyclohexene
orange
colour of iodine in cyclohexene
violet
trend in reactivity down the gorup
- atomic radius increases - more inner shells so shielding increases - less nuclear attraction to capture electron from another species - reactivity decreases
Halogens are….. agents
halogens - oxidising agents
flourine is the strongest oxidising agent
Purpose of chlorine in water purification
Kills pathogen to avoid disease
Chlorine reacts with water to form acidic solution that kills pathogens
Chlorine and water reaction
Forms
hydrochloric acid
Chloric acid (HOCl)
Why is Cl2 reaction with H2O disproportionation reaction
Chlorine is both oxidised and reduced
Oxidised on HOCl
Reduced in HCl
Risks of using chlorine to purify water
Gas is toxic
Chlorinated hydrocarbon may form which are carcinogenic
Benefits of Chlorine purifying water
Kills many harmful microorganisms
Reduces rates of waterborne disease; cholera and typhoid
How is bleach created
Cl2 and cold dilute NaOH
Forms NaOCl and NaCl
NaOCl is the bleach
qualitative tests can test for:
- carbonates
- sulfates
- halides
carbonate test
carbonates react with acids to form CO2
- add dilute HNO3 to test tube with compound - bubble gas through limewater Ca(OH)2to see its CO2 , cloudy
CO2 + Ca(OH)2 –> CaCO3 +H20
caco3 turns water cloudy
sulfate test
- Barium ions are added to unknown compound BaNO3
- Forms BaSO4 which is insoluble and is therefore a white ppt
Equations for Sulfate test. (2)
eg BaNO3 + XSO4 = BaSO4 + XNO3
Ionic: Ba2+ + SO4 2- = BaSO4 (s)
Describe the test for halide ions.
Including ionic equation.
- Add AgNO3 to unknown solution
- Precipitation reaction will form a Silver halide
- X- + Ag+ = AgX
- Colour of the ppt indicates the type of halide ion;
White- Chloride
Cream- Bromide
Yellow- Iodide
The further distinguish, NH3 is added;
Dilute NH3 dissolves Cl- (more reactive) but not Br-
Concentrated NH3 dissolves Br-
NH3 dissolves iodine
Describe the test for ammonium ions. (3)
- Add NaOH (aq) to the unknown solution.
- NH3 gas should be produced but since it dissolves in water, it has to be heated to release the gas.
- Test the gas released with pH indicator paper which should turn purple as ammonia is alkali.
Ionic equation for ammonium test.
NH4+ + OH- = NH3(g) + H20(l)
Explain the sequence of tests for all the qualitative analysis. (3)
- ) Carbonate Test: other tests do not produce bubbles with dilute acid
- ) Sulfate test: BaCO3 is also white ppt so CO3- test is before to ensure it is not that.
- ) Halide test: Silver carbonate and sulfate are also insoluble ppt so the previous tests go first to eliminate those ions.
