Module 3 - Qualitatitve analysis

Question 1

you are supplied with a sample of ammonum bromde
describe simple tests that woud idenify the cation and anion present

Answer 1

test for bromide ion: silver nitrate results in cream precipitate
Ag+ + Br- = AgBr
test for ammonium ion: heat with NaOH damp red litmus paper turns blue
NH4+ + OH- = NH3 + H2o

Question 2

compare the electrical conductivities of solid and a BaCl2

Answer 2

BaCl2 oly conducts when aq not as solid as it has mobile ions rather than fixed

Question 3

Describe the use of aq bacl2 in qualitative analysis

Answer 3

test for sulfate ions to form a wite precipitate

Question 4

state the reagent needed to identify halide ions
how would the appearance of precipitates allow u to distinguish betwee the halides

Answer 4

silver nitrate
cl- white
br- cream
i- yellow

Question 5

test for carbonate ions

Answer 5

react with acid
forms carbon dioxide
bubble gas through limewate
if cloudy, co2 is presnt therefore so is carbonate ion

Question 6

test for sulfate ions

Answer 6

add hcl
add bacl2
whte precipitate forms

Question 7

test for ammonium ions

Answer 7

heat with NaOH
damp red litmus paper turns blue

Question 8

test for halides

Answer 8

dissolve sample in water
add AgNO3
record colour change
white- cl-
cream- br-
yellow- i-
if difficult to record, add aq NH3
record solubility
cl- dilute nh3
br- conc nh3
i- conc nh3 (dissolves)

Question 9

order for carrying out tests

Answer 9

carbonate
sulfate
halide

Question 10

A student investigates the trend in reactivity of the halogens Cl 2, Br2 and I2.
The student is supplied with:
• solutions of Cl 2, Br2 and I2 in cyclohexane (an organic solvent)
• aqueous solutions of the halides: NaCl , NaBr and NaI.
Cl-pale green
Br-orange
I-violet
Plan an experiment on a test tube scale that would show the trend in the reactivity of the
halogens Cl 2, Br2 and I2.
Include all the expected observations and an ionic equation for one of the reactions.

Answer 10

Plan
Mix (solution of) halogen and (solution of) halide
Observation with chlorine
bromide → orange/yellow
Observation with bromine
iodide → violet/purple/pink
Observation with iodine
No colour change/no reaction
Equation
Cl2 + 2Br– → Br2 + 2Cl–
OR
Cl2 + 2I– → I2 + 2Cl–
OR
Br2 + 2I– → I2 + 2Br–
Reactivity trend
Cl2 > Br2 > I2 /decreases down the group