Reactivity trends

Question 1

What is the trend in reactivity down group 2?

Answer 1

Ionisation energy decreases down the group, so reactivity increases.

Question 2

How do group 2 metals react with water?

Answer 2

Produce a metal hydroxide and hydrogen.
The metal hydroxide dissolves in water to produce hydroxide ions, making solutions strongly alkaline. pH is 12-13.
React more readily down group, due to increased atomic radii and shielding.

Question 3

How do group 2 metals react with oxygen?

Answer 3

Solid white oxides produced.
2M(s) + O2(g) –> 2MO(s)
The oxides then react readily with water to form hydroxides.
Hydroxides more soluble down the group, so more alkaline.

Question 4

How do group 2 react with dilute acids?

Answer 4

M(s) + 2HCl(aq) –> MCl2 (aq) + H2(g)

Question 5

What are group 2 compounds used for?

Answer 5

For neutralising acids:
Calcium hydroxide Ca(OH)2 is used in agriculture to neutralise acid soils, in moderation.
Magnesium hydroxide and Calcium Carbonate (CaCO3) are used in indigestion tablets like antacids to neutralise excess stomach acid.
H+ + OH- –> H2O

Question 6

What is the trend in boiling points down group 7?

Answer 6

BP increases down group.
Increasing strength of induced dipole dipole forces as electron number increases.
Volatility decreases down group.

Question 7

What is the trend in reactivity down group 7?

Answer 7

Atomic radius and shielding increases, so harder for large atoms to attract the electron needed to form an ion, so reactivity decreases.
‘Become less oxidising’.

Question 8

What is the ionic equation for the displacement by chlorine?

Answer 8

Cl2 + 2Br- –> 2Cl- + Br2
Cl2 + 2I- –> 2Cl- + I2
Chlorine displaces bromide and iodide.

Question 9

What is the ionic equation for the displacement by bromine?

Answer 9

Br2 + 2I- –> 2Br- + I2
Bromine displaces iodide.

Question 10

What is the displacement by iodine?

Answer 10

No reaction with F-, Cl-, Br-

Question 11

What are the colours of halogens when dissolved in organic solvent?

Answer 11

Very pale yellow/green - chlorine.
Orange/red - bromine.
Violet/pink - iodine.
Shaking it makes the colour changes easier to see.

Question 12

What is the test for halide ions?

Answer 12

Add some dilute nitric acid (HNO3) to each tube.
Add aqueous silver nitrate (AgNO3) - if Cl, I, or Br present a precipitate forms.

Question 13

What are the positive results for halide tests?

Answer 13

Chloride ions - white ppt
Bromide ions - cream ppt
Iodide ions - yellow ppt

Question 14

What is the further test for halide ions?

Answer 14

Add dilute ammonia (NH3) to each ppt.
If ppts haven’t dissolved, add concentrated ammonia.

Question 15

What are the positive results for the further tests of halide ions?

Answer 15

Chloride - ppt dissolves in dilute NH3.
Bromide - ppt doesn’t dissolve in dilute NH3, but does in concentrated.
Iodide - ppt is insoluble in dilute and concentrated NH3.

Question 16

What are disproportionation reactions of halogens?

Answer 16

When they react with cold dilute alkali solutions. (NaOH or KOH).
X2 + 2NaOH –> NaXO + NaX + H2O.
X2 + 2OH- –> XO- + X- + H2O

Question 17

What is the production of bleach?

Answer 17

2NaOH(aq) + Cl2 (g) –> NaClO (aq) + NaCl (aq) + H2O (l).
Sodium chlorate (I) is bleach.
It’s used for water treatment, bleach paper and textiles and for cleaning.

Question 18

What is the reaction of chlorine and water?

Answer 18

Cl2(g) + H2O(l) <–> HCl(aq) + HClO (aq).
Chloric acid is formed.

Question 19

What is water treatment?

Answer 19

Adding chlorate ions to water makes it safe to drink or swim.
It kills disease-causing microorganisms, prevents the growth of algae, eliminates bad tastes and smells, and removes discolouration by organic compounds.

Question 20

What are the limitations of water treatment?

Answer 20

Chlorine gas is toxic.
Liquid Cl causes severe chemical burns.
Chlorine reacts with organic compounds to form chlorinated hydrocarbons, and many are carcinogenic.
But these risks are small compared to untreated water risks - cholera epidemic.

Question 21

What are alternatives of water treatment?

Answer 21

Ozone is a strong oxidising agent that kills microorganism, but it’s expensive and a short half-life.
Ultraviolet light kills microorganisms by damaging DNA, but is ineffective in cloudy water, and won’t stop contamination later on.

Question 22

What are false positives?

Answer 22

If there are ions present that interfere with tests.
Order of tests:
Carbonates -> sulfates -> halides.

Question 23

What is the test for carbonates?

Answer 23

Add dilute strong acid (nitric or hydrochloric).
If carbonates are present, carbon dioxide released.
Observes fizzing and the solid dissolves.
CO3^2- + 2H+ -> CO2 + H2O.

Question 24

What is the limewater test for carbonates?

Answer 24

A delivery tube takes any gas released into a test tube of limewater.
If the gas is carbon dioxide, the limewater turns cloudy.

Question 25

What is the test for sulfates?

Answer 25

Add some barium nitrate solution Ba(NO3)2 or BaCl2 to the sample.
A white ppt shows its barium sulfate.
SO4^2- + Ba2+ -> BaSO4
(this shows sulfates present, because most sulfates are soluble in water, but BaSO4 isn’t)

Question 26

How do you avoid false positives in sulfate tests?

Answer 26

In carbonates, the acid reacts with any carbonates or sulfites so they won’t interfere here.
So if its not done, add dilute acid.

Question 27

What is the test for ammonium ions?

Answer 27

Add some sodium hydroxide to the solution and warm the mixture.
Hold damp litmus paper over the top, it turns blue if ammonia is given off.
NH4+ + OH- -> NH3 + H2O