8. Reactivity Trends
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In Group 2, what happens to reactivity as you go down the group? Why?
Increases Larger molecules, greater atomic radius, less nuclear attraction, easier to lose an e- and react
Describe the anion test for carbonate ions
add HCl and bubble the gas produced through lime water
Why does the anion test for carbonate ions work?
HCL reacts w CO3- ions to produce water and CO2 CO2 reacts w calcium hydroxide in lime water to make insoluble CaCO3 precipitate
Describe the anion test for sulphate ions
add acid (not H2SO4) to eliminate the possibility of a false positive for carbonate ions add BaCl2 : if there is white precipitate there are SO4 (2-) ions present
Describe the anion test for halide ions
add HNO3 to avoid false +ve add (2cm^3) AgNO3 : white = Cl, cream = Br, yellow = I add dilute NH3, Cl disappears add conc. NH3, Br disappears, I no change
Describe the anion test for nitrate ions
add dilute NaOH to Ca(NO3)2, then add Al powder gently heast on half open flame (anti-bumping) hold damp red litmus => turns blue gas = pungent
Why does the litmus paper turn red to blue in the nitrate ion test if its positive?
NH3 dissolves on damp paper to form NH4+ and OH- OH- indicates alkali and turns paper blue
Describe how to carry out a flame test
do some things
What is the flame colour for lithium?
bright red
What is the flame colour for sodium?
bright yellow
What is the flame colour for potassium?
pale lilac
What is the flame colour for calcium?
orange-red
What is the flame colour for strontium?
red
What is the flame colour for barium?
pale green
How do you see more clearly what colour the flame is?
use cobalt blue glass to filter out bright yellow light from sodium traces
What colour does Ca appear through cobalt blue glass?
green
What colour does Sr appear through cobalt blue glass?
purple
Group 2 element + oxygen = what?
metal oxide : basic
What is the equation for adding chlorine gas to a cold dilute alkaline solution to form bleach?
Cl2 + 2NaOH => NaCl + NaClO + H2O