8 - Reactivity Trends Flashcards
How does reactivity increase down G2
Reactivity increases as atomic rad increases and shielding. This decreases efoa from outer e and easier to remove making it more reactive
How does solubility increase down G2
increases as it is easier to form bonds with water and interactions with water. pH increases and alkalinity increases
Reactions
- oxidation
- with water creating hydroxide and H2
- with acid to form salt and H2
- with steam to form metal oxide and H2
Ionisation energy of G2
As you go down the group, it is easier to ionise due to shielding and atomic radius
Solubility/saturation
When the solution become saturated, any further metal hydroxide ions will form a solid precipitate of M(OH)2 (solid)
Uses of G2 compounds
- used in agriculture to lime acidic soil as it neutralises
- used in medicine as antacids to treat acid indigestion using magnesium and calcium carbonate
G7 physical properties
F - pale yellow gas
Cl - pale green gas
Br - orange brown liquid that vaporises
I - grey black crystalline that sublimes into purple vapour
At - never been seen but probably black solid
Halogen- Halide displacement reactions
Shows that reactivity decreases down the group as a more reactive halogen is able to displace a hydrogen/ metal halide from its compound
Trend in reactivity down G7
Decreases down the group as atomic rad and shielding increases so harder to gain electron as efoa is weaker, decreasing reactivity
Colour changes seen in hydrogen halide displacement reactions
In water:
Cl - pale green
Br - orange
I - brown
What else can be done in hydrogen halide displacement reactions
To tell iodine and bromine apart in water (can look similar due to orange brown), test with cyclohexane which doesn’t react and only colour change occurs
Cl - pale green
Br - orange
I - purple
What is disproportionation
A redox reaction in which the same element is both oxidised and reduced
Two disproportionation reactions
- chlorine with water
- Chlorine with cold dilute, sodium hydroxide
Reaction of chlorine with water
Cl2 + H2O ——> HClO + HCl
Chloric (I) acid kills the bacteria due to the chlorate ion (ClO-) which is also a weak bleach
How to test for bleach
This is a test of chloric (I) acid by adding indicator which first turns red due to acid and then bleach dyes to colourless and disappears
Reaction of Chlorine with NaOH
Cl2 + 2NaOH ——> NaClO + NaCl + H2O
This also forms a household bleach
Why is NaOH reaction used more than H2O
- reaction of chlorine with water is limited because of the low solubility of chlorine in the water
- When using sodium hydroxide much more chlorine dissolves, and another disproportionation reaction takes place
Benefits and Risks
- chlorine is extremely toxic
- respiratory irritant and large concentration can be fatal
- Cl2 reacts with organic hydrocarbon such as methane: formed by decaying vegetation
- chlorinated hydrocarbons cause cancer
- required to have potable water
- diseases like typhoid and cholera can break out
- helpful for natural disaster relief
Test for halide ions
- adding AgNO3 results in AgCl and colour change of white, cream, yellow
- also must add nitric acid to get rid of any carbonate ions
What is qualitative analysis?
This is a test that relies on simple observations, rather than measurements that can be carried out quickly on a test tube scale
Test for carbonate
- adding any acid (sulphuric or nitric) results in carbon dioxide being formed
- When added and see bubbles, compound contains carbonate ions
- To confirm, bubble, the gas produced through limewater (calcium hydroxide) and it turns into white solid precipitate and milky (calcium carbonate) - confirms carbonate ion is present
Test for sulphate
Ba +2 + SO4 - 2 ——> BaSO4 which is a white solid precipitate
Usually add BaCl2 however use BaNO3 if will be testing for halides after
Confirming halide tests
Add equals ammonia to test the solubility of the precipitate, as sometimes the colour differences may be difficult to tell apart:
- chlorine soluble in dilute NH3
- Bromine soluble in conc NH3
- Iodine insoluble in NH3
Sequences for anions
- carbonate
- Sulphate
- Halides
Why is there a sequence for anions?
- Carbonate will be able to bubble in dilute acid whereas others do not. Will confirm if there is carbonate present and proceed to the next test.
- BaCO3 is white and insoluble in water, so if you carry out a sulphate test before testing for carbonate, will also get white precipitate, so must test for carbonate before sulphate
- Silver, carbonate and silver sulphate are both insoluble in water and form precipitates. Therefore, must carry out carbonate and sulphate test first before halide
Mixture of ions testing
- First test for carbonate again, however, use nitric acid. Sulphuric or HCl will have its ions that show up as a precipitate.
- Test for sulphate, however use BaNO3 and not BaCl2 as a chlorine ions, will show up
- Test for halide, using AgNO3. Any sulphate or carbonate will already have been removed. Use NH3 to confirm your halide
Test for ammonium ion NH4+
- when heated together, ammonium ions and hydroxide ions form ammonia gas
NH4+ + OH- ——> NH3 + H2O - NaOH added to NH4+ ion.
- Ammonia gas is produced, however unlikely to see as ammonia is soluble in water
- Heat slightly and gas is produced
- Will be able to be smelt, but test with moist PH indicator paper.
- As it is an alkali, it turns the paper blue
