KERBOODLE PRACTICE QUESTIONS: ES

Question 1

Look at the two reactions of chlorine with ethene:
Reaction 1: C₂H₄ + Cl₂ –> C₂H₄Cl₂
Reaction 2: C₂H₄ + Cl₂ –> C₂H₃Cl + HCl
Which of the following answers is correct about the atom economies of these reactions in terms of the organic product

A) R1 - 80%
R2 - 50%

B) R1 - 100%
R2 - 37%

C) R1 - 100%
R2 - 63%

D) R1 - 40%
R2 - 100%

Answer 1

C) R1 - 100%
R2 - 63%

Question 2

Which is the correct answer for the electrolysis of aqueous potassium iodide?

A) Cathode: Potassium
Anode: Iodine

B) Cathode: Hydrogen
Anode: Oxygen

C) Cathode: Iodine
Anode: Potassium

D) Cathode: Hydrogen
Anode: Iodine

Answer 2

D) Cathode: Hydrogen
Anode: Iodine

K salts give H₂ at cathode; iodides give I₂ at anode

Question 3

What are the correct half equations for the electrolysis of molten sodium chloride?

A) Cathode: Na⁺ + e^- –> Na
Anode: Cl^- –> Cl + e^-

B) Cathode: 2H⁺ + 2e^- –> H₂
Anode: Cl^- –> ¹/₂Cl₂ + e^-

C) Cathode: Na⁺ –> Na + e^-
Anode: Cl₂ –> 2Cl^- + 2e^-

D) Cathode: Na⁺ + e^- –> Na
Anode: 2Cl^- –> Cl₂ + 2e^-

Answer 3

D) Cathode: Na⁺ + e^- –> Na
Anode: 2Cl^- –> Cl₂ + 2e^-

Products are sodium and chlorine

Question 4

Which answer correctly describes the halogen elements at room temperature?

A) Chlorine: green gas
Bromine: brown gas
Iodine: purple gas

B) Chlorine: colourless solution
Bromine: brown solution
Iodine: brown solution

C) Chlorine: green gas
Bromine: red liquid
Iodine: grey solid

D) Chlorine: yellow-green gas
Bromine: brown liquid
Iodine: purple solid

Answer 4

C) Chlorine: green gas
Bromine: red liquid
Iodine: grey solid

Question 5

Sodium bromide is reacted with silver nitrate solution.
The result is…

A) a white precipitate that is soluble in dilute ammonia solution

B) a cream precipitate that is soluble in concentrated ammonia solution

C) a yellow precipitate that is insoluble in ammonia solution

D) a white precipitate that is soluble in concentrated ammonia solution

Answer 5

B) a cream precipitate that is soluble in concentrated ammonia solution

Question 6

Which of the following will react with sodium iodide to produce the purest sample of hydrogen iodide?

A) concentrated sulfuric acid

B) dilute sulfuric acid

C) dilute hydrochloric acid

D) phosphoric acid

Answer 6

D) phosphoric acid

Question 7

An aqueous solution of chlorine is added to an aqueous solution of sodium iodide. Some cyclohexane is added, forming the upper layer. Which of the following is the correct observation?

A) There is no reaction

B) The upper layer goes purple and the aqueous layer goes brown.

C) Both layers go brown.

D) The lower layer is brown and the upper layer is yellow.

Answer 7

B) The upper layer goes purple and the aqueous layer goes brown.

Question 8

When sulfuric acid reacts with a bromide, which of the following are correct?

• *Statement 1:** hydrogen bromide is produced
• *Statement 2:** sulfur dioxide is produced
• *Statement 3:** bromine is produced

A) 1,2 and 3 are correct

B) 1 and 2 are correct

C) 2 and 3 are correct

D) Only 1 is correct

Answer 8

A) 1,2 and 3 are correct

Question 9

Which of the following are true about hydrogen chloride, hydrogen bromide and hydrogen iodide?

• *Statement 1:** they all react with ammonia
• *Statement 2:** they are all acidic
• *Statement 3:** they all reduce sulfuric acid

A) 1,2 and 3 are correct

B) 1 and 2 are correct

C) 2 and 3 are correct

D) Only 1 is correct

Answer 9

B) 1 and 2 are correct

HCl does not reduce sulfuric acid (H₂SO₄)

Question 10

A solution of copper sulfate is electrolysed with copper electrodes. Which of the following is true?

• *Statement 1:** Copper is transferred from the anode to the cathode.
• *Statement 2:** Copper is plated on the anode.
• *Statement 3:** Sulfur dioxide is produced at the anode.

A) 1,2 and 3 are correct

B) 1 and 2 are correct

C) 2 and 3 are correct

D) Only 1 is correct

Answer 10

D) Only 1 is correct

Copper is plated on the cathode, no SO₂ is produced at all

Question 11

In the manufacture of bromine from sea water:
Step 1: Chlorine is bubbled through sea water containing a very dilute bromide solution to release bromine.
Step 2: Air is blown through to produce bromine vapour
Step 3: The vapour is mixed with sulfur dioxide and passed into water:
Br₂ + SO₂ + ………. –> 2HBr + ……….
Step 4: Steam and chlorine are blown through to release bromine from the hydrogen bromide
Step 5: The bromine is dried using concentrated sulfuric acid.

Write an ionic equation for the reaction that occurs in both steps 1 and 4. Which property of the halogens does this reaction illustrate?

Answer 11

Cl₂ + 2Br^- –> 2Cl^- + Br₂
Ignore state symbols

Chlorine/Chloride is a better oxidising agent than bromine/bromide

Question 12

In the manufacture of bromine from sea water:
Step 1: Chlorine is bubbled through sea water containing a very dilute bromide solution to release bromine.
Step 2: Air is blown through to produce bromine vapour
Step 3: The vapour is mixed with sulfur dioxide and passed into water:
Br₂ + SO₂ + ………. –> 2HBr + ……….
Step 4: Steam and chlorine are blown through to release bromine from the hydrogen bromide
Step 5: The bromine is dried using concentrated sulfuric acid.

Suggest why is it necessary to produce bromine in Step 1 and then again in Step 4.

Answer 12

Because the bromine in Step 1 is too dilute.

Question 13

In the manufacture of bromine from sea water:
Step 1: Chlorine is bubbled through sea water containing a very dilute bromide solution to release bromine.
Step 2: Air is blown through to produce bromine vapour
Step 3: The vapour is mixed with sulfur dioxide and passed into water:
Br₂ + SO₂ + ………. –> 2HBr + ……….
Step 4: Steam and chlorine are blown through to release bromine from the hydrogen bromide
Step 5: The bromine is dried using concentrated sulfuric acid.

On what property of bromine does Step 2 depend?

Answer 13

volatile or low boiling point

Question 14

In the manufacture of bromine from sea water:
Step 1: Chlorine is bubbled through sea water containing a very dilute bromide solution to release bromine.
Step 2: Air is blown through to produce bromine vapour
Step 3: The vapour is mixed with sulfur dioxide and passed into water:
Br₂ + SO₂ + ………. –> 2HBr + ……….
Step 4: Steam and chlorine are blown through to release bromine from the hydrogen bromide
Step 5: The bromine is dried using concentrated sulfuric acid.

Suggest the appearance of the gas stream after Step 2.

Answer 14

brown or red (brown-red)

Question 15

In the manufacture of bromine from sea water:
Step 1: Chlorine is bubbled through sea water containing a very dilute bromide solution to release bromine.
Step 2: Air is blown through to produce bromine vapour
Step 3: The vapour is mixed with sulfur dioxide and passed into water:
Br₂ + SO₂ + ………. –> 2HBr + ……….
Step 4: Steam and chlorine are blown through to release bromine from the hydrogen bromide
Step 5: The bromine is dried using concentrated sulfuric acid.

Use oxidation states to complete and balance the equation in step 3 and explain your reasoning.

Answer 15

Br₂ + SO₂ + 2H₂O → 2HBr + H₂SO₄.

Two bromine atoms go from zero to -1.
so S must be oxidised from +4 to +6.

Question 16

Chlorine is made by electrolysing an aqueous solution of sodium chloride.
Give the half equations for the reactions at the positive and negative electrodes during this electrolysis.

Answer 16

negative electrode (cathode): 2H<sup>+</sup> + 2e<sup>-</sup> --\> H<sub>2</sub>
positive electrode (anode): 2Cl<sup>-</sup> --\> Cl<sub>2</sub> + 2e<sup>-</sup>

Question 17

Chlorine reacts with cold aqueous sodium hydroxide as follows:
Cl₂ + H₂O ⇌ HCl + HClO

Write the oxidation states underneath the chlorine atoms in Cl₂, HCl, and HClO. What is being reduced and what is being oxidised in this equation?

Answer 17

oxidation state of Cl in Cl₂ = 0
oxidation state of Cl in HCl = -1
oxidation state of Cl in HClO = +1

Chlorine is both reduced and oxidised

Question 18

Give the systematic name of HClO

Answer 18

chloric(I) acid

Question 19

Hydrogen chloride is made industrially by the reaction:
H_2(g) + Cl_2(g) ⇌ 2HCl_(g)

The reaction can reach dynamic equilibrium. Explain the meaning of the term dynamic equilibrium.

Answer 19

when the rate of the forward reaction = the rate of the backward reaction

in a closed system
concentrations of products and reactants are constant

Question 20

Hydrogen chloride is made industrially by the reaction:
H_2(g) + Cl_2(g) ⇌ 2HCl_(g)

Write the equation for the equilibrium constant K_c of the reaction

Answer 20

[HCl]² /[H₂] [Cl₂]

Question 21

Hydrogen chloride is made industrially by the reaction:
H_2(g) + Cl_2(g) ⇌ 2HCl_(g)

K_c= 2 x 10³³ for this reaction at 298K. What conclusion can be drawn about the composition of an equilibrium mixture at 298K.

Answer 21

nearly all product (HCl)

Question 22

Hydrogen chloride is made industrially by the reaction:
H_2(g) + Cl_2(g) ⇌ 2HCl_(g)

The reaction is exothermic. Discuss the effect on an equilibrium mixture of, separately, changing the hydrogen concentration, varying the temperature and varying the pressure.

Answer 22

pressure - no effect
equal moles on each side
increased temperature - moves position of equilibrium to the left (reverse for lowered temperature)
forward reaction - exothermic
more H₂ - position of equilibrium moves to the right (reverse for less H₂)

reference to opposing change or Le Chatelier for temp
use of K_c for more H₂
increased temperature - more reactants/less product (HCl)
adding hydrogen H₂ and HCl greater, Cl₂ less

Question 23

The chemist Max Bodenstein investigated the equilibrium shown below:
H_2(g) + I_2(g) ⇌ 2HI_(g)
He allowed mixtures of known masses of hydrogen and iodine to react in sealed tubes at high temperatures until equilibrium had been established. Then he rapidly cooled the tubes and analysed the iodine present with sodium thiosulfate.

Suggest why the flasks are rapidly cooled.

Answer 23

so the position of equilibrium does not change

Question 24

The chemist Max Bodenstein investigated the equilibrium shown below:
H_2(g) + I_2(g) ⇌ 2HI_(g)
He allowed mixtures of known masses of hydrogen and iodine to react in sealed tubes at high temperatures until equilibrium had been established. Then he rapidly cooled the tubes and analysed the iodine present with sodium thiosulfate.

Describe how a sealed tube could be investigated to measure the mass of iodine it contains

Answer 24

Dissolve contents of tube in water
Titrate with sodium thiosulfate … of known concentration
mass I₂ = (0.5 x moles thiosulfate x 253.8)g

Question 25

The chemist Max Bodenstein investigated the equilibrium shown below:
H_2(g) + I_2(g) ⇌ 2HI_(g)
He allowed mixtures of known masses of hydrogen and iodine to react in sealed tubes at high temperatures until equilibrium had been established. Then he rapidly cooled the tubes and analysed the iodine present with sodium thiosulfate.

From such an experiment at 500K , the masses of substances in a 100cm³ tube were found to be:
H₂ = 0.20g
I₂ = 25.38g
HI = 161.15g

Calculate a value for K_c for the reaction at 500K.

Answer 25

[HI] = 161.15/127.9 (= 1.26)

[H<sub>2</sub>] = 0.2/2 (= 0.1) AND
[I<sub>2</sub>] = 25.38/253.8 (= 0.1)

Use of [HI]² /[H₂] [I₂]

answer 159 (±2)

Question 26

Give the systematic name of KIO₃

Answer 26

potassium iodate(V)

Question 27

20.0cm³ of a solution of 0.002mol dm^-3 KIO₃ is reacted with excess iodide ions in the presence of acid.
KIO₃ + 5I^- + 6H⁺ –> 3I₂ + 3H₂O

Calculate the volume of 0.50mol dm^-3 Na₂S₂O₃ that would react with the iodine formed. Give your answer to a suitable number of significant figures.

Answer 27

n(KIO3) = 20 × 0.002/1000
= 4 x 10–4

n (Na2S2O3) = 6 × (4 x 10–4)

vol Na2S2O3 = (6 × (4 x 10–4)) x 1000/0.5
= 4.8

to 3 sf with units 4.80cm³

Question 28

20.0cm³ of a solution of 0.002mol dm^-3 KIO₃ is reacted with excess iodide ions in the presence of acid.
KIO₃ + 5I^- + 6H⁺ –> 3I₂ + 3H₂O

A teacher tells a student that this is not a very satisfactory titration result. Suggest why the teacher says this and suggest what the student could do to improve the titration result without changing the apparatus used.

Answer 28

titre can be measured to ±0.1
larger percentage uncertainty for smaller titre
use more conc KIO₃ or more dilute thiosulfate