Paper 2 exam Qs

Question 1

(diacyl chloride attatched to a benzene ring) Suggest the O=C−Cl bond angle in compound A.
Explain your answer.

Answer 1

The red colour comes from lines of specific frequency in the atomic emission spectrum of
strontium.
Explain how these lines are formed.120 degrees
3 pairs of electrons around the central atom
Repel to maximum seperation

Question 2

The red colour comes from lines of specific frequency in the atomic emission spectrum of
strontium.
Explain how these lines are formed.

Answer 2

electrons raised/excited to higher energy levels (by heat)
drop back down energy levels and release specific frequencies of visible light
Frequency proportional to energy gap as E=hv

Question 3

Strontium oxide reacts with water to form strontium hydroxide, Sr(OH)2.
The solubility of strontium hydroxide in water at room temperature is around 10 g dm−3.
A student is given a saturated solution of strontium hydroxide, normal titration equipment and
a variety of different concentrations of hydrochloric acid.
The student wishes to find an accurate value for the concentration of the solution in mol dm−3.
Describe in full a suitable procedure and indicate how the result would be calculated. [6]

Answer 3

Choice of acid concentration
Calculates conc of Sr(OH)2(aq) = 0.08 mol
dm–3;
* use of reaction stoichiometry 2:1 to
determine appropriate concentration of
acid to be used (approx. 0.15 – 0.2 mol
dm-3)

Practical details
* pipette 20/25 cm3 Sr(OH)2/HCl in a suitable
flask;
* add indicator; (details not required)
* place acid/alkali in burette;
* titrate until colour change (details not
required)
* repeat until concordant titres obtained
Relevant fine detail
* Rinses pipette with solution to be delivered
* Rinses burette with solution to be delivered
* Performs a rough titration
* Add dropwise near to end point

Final calculation
* Calculates average volume used
* Use of equation or mole ratio
* Gives example of suitable relationship to
calculate actual concentration
eg use of c = n/v

Question 4

Explain how the solubility of strontium hydroxide in aqueous NaOH at 0 °C compares
with 3.4 × 10−2 mol dm−3.
Use the idea of solubility product in your answer.

Answer 4

Greater concentration of OH- ions
Concentration of Sr2+ decreases in order to keep Ksp constant AND solubility decreases

Question 5

The melting point of strontium is higher than the melting point of rubidium.
Explain the difference in melting point

Answer 5

Rb loses 1 electron while Sr loses 2 electrons
More delocalised electrons in Sr
Sr2+ attracts more delocalised electrons more strongly in metallic structure

Question 6

From data in Table 3.1, the student knows that chlorine reacts with iodide ions in aqueous
solution.(iii) What will be seen when the reaction in (d)(i) occurs

Answer 6

Brown colour solution

Question 7

2Cl O3
− + 4H+ + 2Cl − 2Cl O2 + 2H2O + Cl 2

Suggest why the forward reaction in equation 3.1 does occur in the presence of
concentrated hydrochloric acid.

Answer 7

Increased concentration H+ or Cl-
Eqb shifts to RHS

Question 8

2Cl O3
− + 4H+ + 2Cl − 2Cl O2 + 2H2O + Cl 2
The forward reaction in equation 3.1 does not occur under standard conditions.
Use data from Table 3.1 to explain why.

Answer 8

ClO3–/ClO2 is less positive/ more negative than Cl2/Cl–
so ClO2 is oxidised AND Cl2 is reduced / electrons flow from
ClO3–/ ClO2 (ora) / half equations are reversed