Reactivity Trends and Qualitative Analysis (F6examonly)

Question 1

Describe what happens when group 2 metal oxides react with water:

Answer 1

group 2 metal oxides react with water to form metal hydroxides

Question 2

Write a word, symbol, and ionic equation for the reaction between e.g. calcium oxide and water:

Answer 2

• calcium oxide + water → calcium hydroxide
• CaO + H2O → Ca(OH)2
• CaO(s) + H2O(l) → Ca2+(aq) + 2 OH-(aq)

Question 3

Why are the solutions of group 2 metal hydroxides strongly alkaline?

Answer 3

due to the presence of hydroxide ions (OH-)

Question 4

Are group 2 metal oxides and hydroxides bases? Are the hydroxides soluble? If so, what does this make them?

Answer 4

• yes
• most of them
• the ones that are soluble are alkalis

Question 5

What is an exception to the fact that most group 2 metal oxides react readily with water to form metal hydroxides that are soluble?

Answer 5

magnesium oxide reacts slowly with water and the hydroxide isn’t very soluble

Question 6

What is the approximate pH of the solutions of group 2 metal hydroxides?

Answer 6

pH 12-13

Question 7

Describe the trend of increasing alkalinity in the solutions of group 2 metal hydroxides (formed by the reaction between group 2 metal oxides and water):

Answer 7

the alkalinity of the solutions of group 2 metal hydroxides increases down group 2
(the group 2 metal oxides form more strongly alkaline solutions as you go down group 2)

Question 8

Why does the alkalinity of the solutions of group 2 metal hydroxides increase down group 2?

Answer 8

• the group 2 metal hydroxides become more soluble down group 2
• so more OH- ions are released into the solution
• so the presence of more OH- ions mean that the solution is more alkaline

Question 9

What are some of the uses of group 2 compounds (with equations)?

Answer 9

• group 2 compounds are bases and are used to neutralise acidity
• calcium hydroxide (Ca(OH)2) is used in agriculture to neutralise acidic soils
• magnesium hydroxide (Mg(OH)2) and calcium carbonate (CaCO3) are used in indigestion tablets as antacids which neutralise stomach acid (to treat heart burn)
• overall ionic equation for both: H+ + OH- → H20

Question 10

What is disproportionation?

Answer 10

when the same element is simultaneously oxidised and reduced (the oxidation number both increases and decreases)

Question 11

How is chlorine used to kill bacteria in water?

Answer 11

• Cl2 + H2O ⇌ HCl + HClO
• HClO - chloric (I) acid ionises to form ClO- ions - chlorate (I) ions
• HClO + H2O ⇌ ClO- + H3O+
• the ClO- ions kill bacteria

Question 12

How is disproportionation demonstrated by the reaction of chlorine with water (as used in water purification)?

Answer 12

• Cl2 + H2O → HCl + HClO
• the oxidation number of chlorine decreases from 0 to –1 (in HCl) so Cl is reduced
• the oxidation number of chlorine also increases from 0 to +1 (in HClO) so Cl is oxidised as well

Question 13

How is disproportionation demonstrated by the reaction of chlorine with cold, dilute aqueous sodium hydroxide (as used to form bleach)?

Answer 13

• 2 NaOH + Cl2 → NaClO + NaCl + H2O
• the oxidation number of chlorine increases from 0 to +1 (in NaClO) so Cl is oxidised
• the oxidation number of chlorine also decreases from 0 to –1 (in NaCl), so Cl is reduced as well

Question 14

How is disproportionation demonstrated by the analogous reactions of the halogens?

Answer 14

• e.g. X2 + 2 NaOH → NaXO + NaX + H2O
• the halogen (X) is oxidised (oxidation number increases from 0 to +1 in NaXO)
• the halogen (X) is reduced (its oxidation number decreases from 0 to -1 in NaX)

Question 15

Describe the precipitation reactions (including ionic equations) of the aqueous anions Cl-, Br-, and I- with aqueous silver ions (from aqueous silver nitrate), followed by aqueous ammonia:

Answer 15

• Cl- : white ppt, dissolves in dilute NH3
  Ag+(aq) + Cl-(aq) → AgCl(s)
• Br- : cream ppt, dissolves in concentrated NH3
  Ag+(aq) + Br-(aq) → AgBr(s)
• I- : yellow ppt, insoluble in both dilute or concentrated NH3
  Ag+(aq) + I-(aq) → AgI(s)

Question 16

How are the tests for the halide ions useful?

Answer 16

• the colour of the silver halide precipitate and its solubility in aqueous ammonia will indicate which halide ion is present (Cl-, Br-, or I-)
• the colour of the ppts are often unclear, so the ammonia test will confirm which halide ion is present

Question 17

In what order do you test for ions on a test-tube scale?

Answer 17

the order of tests is:
- test for carbonates, if negative (no co2 produced)
- test for sulfates, if negative (no ppt)
- test for halides

Question 18

When testing for ions on a test-tube scale, what must you remember to do if the unknown substance is a solid?

Answer 18

dissolve it in water

Question 19

What is the test for carbonate ions (including equations)?

Answer 19

• Add dilute acid
• If positive, carbon dioxide will be released
  Carbonate + acid -> carbon dioxide + water
  e.g. CaCO3 + HCl -> CO2 + H2O + CaCl2
  CO3²⁻ + 2H+ -> CO2 + H2O
• To test for co2, bubble the gas through limewater and if positive, limewater turns cloudy

Question 20

What is the test for sulfate ions (including equations)?

Answer 20

• add dilute HCl followed by barium chloride solution (BaCl2)
• if positive, a white ppt will form
  Ba2+ + SO4²⁻ -> BaSO4
  e.g. NaSO4 + BaCl2 -> BaSO4 + 2NaCl

Question 21

What is the test for halide ions (including equations)?

Answer 21

• add dilute nitric acid to remove ions that may interfere with the test
• add aqueous silver nitrate and a precipitate will form (of silver halide)
  Ag+(aq) + X-(aq) -> AgX(s)
  (X is Cl, Br, or I)
• then add aqueous ammonia
• the results:
  Cl – white ppt, dissolves in dilute NH3
  Br – cream ppt, dissolves in concentrated NH3
  I – yellow ppt, insoluble in dilute or concentrated NH3

Question 22

Why do different halides have different solubilities in ammonia solution?

Answer 22

the larger the ion, the more difficult it is to dissolve

Question 23

What are possible false positives to avoid in test-tube tests?

Answer 23

• if carbonate ions or sulfite ions are present during the test for sulfate ions, a ppt will also be produced
• if sulfate ions are present during the test for halide ions, a ppt will also be produced
• using dilute acid will get rid of unwanted anions, but the acid must not interfere with the test (e.g. can’t use HCl before halide test)

Question 24

What is the test for ammonium ions (including equations)?

Answer 24

• add aqueous sodium hydroxide to substance in test tube
• warm the mixture
• if ammonia is given off then there are ammonium ions in the substance
  NH4+ + OH- -> NH3 + H2O
  e.g. NH4Cl + NaOH -> NH3 + H2O + NaCl
• to the test the gas use litmus paper: ammonia gas will turn damp red litmus paper blue (it is alkaline)