Topic 4b - Group 7

Question 1

What is the trend in melting/ boiling points as you go down group 7

Answer 1

They increase because the number of electrons in the molecule and the surface area of contact between molecules increases therefore the intermolecular forces (London forces) increase

Question 2

What is the trend in X-X bond strength going down group 7

Answer 2

It decreases as the size of X increases and the bond length increases therefore the orbital overlap is less effective (EXCEPTION F-F BOND IS WEAK AS IT IS SHORT)

Question 3

What is the trend in the H-X and C-X bonds going down group 7

Answer 3

It decreases as the size of X increases and bond length increases less effective orbital overlap

Question 4

Equation and conditions of reaction of chlorine and hydrogen

Answer 4

Cl2 + H2 -> 2HCl

In sunlight

Question 5

Equation and conditions of reaction of bromine and hydrogen

Answer 5

Br2 + H2 -> 2HBr

300C & Pt catalyst

Question 6

Equation and conditions of reaction of iodine and hydrogen

Answer 6

I2 + H2 -> 2HI
300 C & Pt catalyst
(Slow and reversible)

Question 7

How do distinguish between I2 and Br2

Answer 7

Add chlorine water to a solution of X- ions

• if Br- is present solution goes orange
• if I- is present solution goes brown & black precipitate may form

Question 8

Test for halide ions

Answer 8

CHLORINE turns damp red litmus paper white and blue litmus paper red then white
BROMINE turns damp red litmus paper white slowly and will turn starch-iodide paper blue-black
IODINE turns blue-black with starch

Question 9

Equation for reaction of chlorine and water

Answer 9

Cl2 + H2O -> HCl + HOCl

HOCl + H2O -> H3O + OCl-

Has to be stored in brown bottle away from sunlight or else:
2HOCl -> 2HCl + O2

Question 10

Equation of reaction of chlorine and cold alkali (15c)

Answer 10

Cl2 + 2OH- -> Cl- + ClO- + H2O

Question 11

Equation for reaction of chlorine with cold alkali heated

Answer 11

3ClO- -> ClO3- + 2Cl-

Question 12

Equation for reaction of chlorine with hot alkali (60 degrees)

Answer 12

3Cl2 + 6OH- -> 5Cl- + ClO3- + 3H2O

Question 13

How do you test for halide ions with aqueous silver nitrate

Answer 13

Add dilute aqueous nitric acid (HNO3) to destroy any OH- (brown ppt) or CO3– (cream ppt) ions which may interfere. Then add AgNO3

Question 14

What are the result when you test a halide ion with aqueous silver nitrate AgNO3

Answer 14

• AgF no ppt (soluble in dilute & conc ammonia)
• AgCl white ppt (soluble in conc ammonia not dilute)
• AgBr cream ppt (not soluble in ammonia)
• AgI primrose yellow ppt (not soluble in ammonia)
• AgAs dark yellow ppt (not soluble in ammonia)

Question 15

Reaction of chlorine with concentrated sulfuric acid

Answer 15

KCl + H2SO4 -> KHSO4 + HCl(g)

Stops at first step
Steamy fumes of HCl test with stopper from bottle of ammonia solution gives white smoke

Question 16

Reaction of bromine with concentrated sulfuric acid

Answer 16

KBr + H2SO4 -> KHSO4 + HBr

2HBr + H2SO4 -> Br2 + SO2 + 2H2O

Steamy fumes of HBr & orange liquid Br2

Question 17

Reaction of iodine with concentrated sulfuric acid

Answer 17

KI + H2SO4 -> KHSO4 + HI

6HI + H2SO4 -> 3I2 + S + 4H2O

8HI + H2SO4 -> 4I2 + H2S + 4H2O

Yellow solid - S
Rotten egg smell - H2S
Steamy fumes - HI
Black solid - I2

Question 18

Reaction of hydrogen halide with water

Answer 18

HX + H2O -> H3O + X-

All HX form strong acids in water except for HF as the H-F bond is small and strong

2HF + H2O -> H3O+ + HF-