Group 7- Halogens / Halides

Question 1

A student investigated the chemistry of halogens and the halide ions
TEST OBSERVATION
1) add chlorine water to colourless to brown
potassium iodide
2) add silver nitrate to colourless to white
potassium chloride ppt forms

i) Using Test 1, identify species responsible for the brown colour, and state the type of reaction occurring?
Write the simplest ionic equation for reaction? [3]

ii) Using Test 2, identify species responsible for white ppt that forms, and state observation made when dilute ammonia is added to the precipitate? [2]
Write the simplest ionic equation for the reactions? [1]

Answer 1

i) species = I2 ( only ) 1
displacement reaction 1
Cl2 + 2I- ———-> 2Cl- + I2 1

ii) species = Cl - 1
white ppt dissolves / forms colourless solution 1
Ag+ + Cl- ———-> AgCl 1

Question 2

In a test, a student made the following observations
TEST OBSERVATION
1) add conc. sulfuric acid to formed misty white fumes
potassium chloride turned litmus paper blue

Identity species responsible for misty white fumes? [1]
Write simplest ionic equation for reaction? [1]

Answer 2

species = HCl 1

H2SO4 + 2Cl- ———–> 2HCl + SO4 [2-] 1

Question 3

A student added sulfuric acid to potassium iodide to produce the following equation
8H+ + 8I- + H2SO4 ———> 4I2 + H2S + 4H2O

Deduce the two half equations for this reaction? [2]
State the role of the sulfuric acid in this reaction? [1]

Identify the yellow solid formed in reaction? [1]

Answer 3

2I- ——-> I2 + 2e- 1
H2SO4 + 8H+ + 8e- ———> H2S + 4H2O 1

acts as a proton acceptor 1
yellow solid = sulfur 1

Question 4

When added to water bromine can be used to kill microorganisms in swimming pools.
Describe why bromine can be used for this purpose despite bromine being toxic? [2]

Answer 4

health benefits outweigh the risks 1

only used in low doses 1

Question 5

Describe and explain the trend in reducing power of halides down Group 7? [3]

Answer 5

Increases 1
(size of ion) increases, e- further from nucleus 1
increased shielding ( more inner shells ) 1

Question 6

When NaCl is added to sulfuric acid no redox reaction occurs, explain why? [2]
Write an equation for this reaction? [1]

Answer 6

(not strong enough) reducing agent 1
electrons to close to nucleus (hard to lose outer e-) 1

NaCl + H2SO4 ——-> NaHSO4 + HCl 1

Question 7

Write an equation for the reaction that occurs when NaBr is added to sulfuric acid? [1]
Deduce the change in oxidation state of sulfur? [1]
State the observations of the reduced product in this reaction? [1]

Answer 7

NaBr + H2SO4 ——–> SO2 + Br2 + 2H2O 1
6 + to 4+ (SO2) 1
colourless choking gas 1

Question 8

When chlorine reacts with water a mixture of both chloride and chlorate ions are formed.
Write an equation for this reaction? [1]

In sunlight a different reaction occurs. Write an equation for the reaction that occurs in sunlight? [1]

Answer 8

Cl2 + H2O ——–> ClO- + Cl- + 2H+ 1

2Cl2 + 2H2O ——–> O2 + 4Cl- + 4H+ 1
sunlight

Question 9

When chlorine reacts with sodium hydroxide two products with different oxidation states are formed
Give the oxidation state of chlorine in the following? [2]
i) NaClO
ii) NaCl

Give a use for the NaClO produced in the reaction? [1]

Answer 9

i) +1 ( the chlorate ion ClO is 1- overall) 1
ii) -1 1

bleach 1

Question 10

Halogens are described as being ‘electronegative’.
Define the term electronegativity? [2]

Describe and explain the trend in electronegativity down Group 7? [3]

Answer 10

ability of an atom to attract a pair of electrons 1
in a covalent bond 1

Decreases 1
size of atom increases 1
more shielding of nucleus so less attraction 1

Question 11

A student added bromine water to potassium iodide solution, and observed a brown solution
Write the simplest ionic equation for the reaction? [1]
Explain why this displacement reaction occurs? [1]

Answer 11

Br2 + 2I- ——–> 2Br- + I2 1

the Br2 will displace I- ,as the halide is less reactive than the halogen 1

NB: [reactivity decreases down Group 7, as they want to GAIN one electron ]