Energy Changes Revision Structured Questions

1.    Carbon monoxide is used to make phosgene, COCl₂, which is an important reactant in industries to make polymers. dyes and pharmaceuticals.

Phosgene was first made in 1812 by using a photochemical reaction in which a mixture of carbon monoxide and chlorine was exposed to bright sunlight. This reaction is exothermic and equation is given below.

CO + Cl₂ ⇌ COCl₂
(a) (i) Explain what a photochemical reaction is.

(ii) Explain, in terms of bond breaking and bond making, why the formation of phosgene is an exothermic reaction.

(iii) Draw an energy profile diagram for the decomposition of phosgene. On your diagram, label the activation energy for the reaction.

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2.     A student investigated the reaction between aqueous copper(II) sulfate and two different metals P and Q. A 25.0 cm³ sample of aqueous copper(II) sulfate was added to a Styrofoam cup from a measuring cylinder. The temperature of the solution was taken every 30 seconds. At 1.5 minutes, excess of powdered metal P was added to the cup and the mixture was stirred with a thermometer. The temperature of the solution was taken every 30 seconds. The experiment was repeated using excess of powdered metal Q. The results of the two experiments were plotted on a graph.

(a) is the reaction between aqueous copper(II) sulfate and metal P exothermic or
endothermic? Support your answer with evidence from the graph.

(b) Sketch a labelled energy profile diagram for the reaction between aqueous copper(II) sulfate and metal P.

(c) Deduce from the information provided which reaction would have a larger enthalpy change. the reaction between aqueous copper(II) sulfate and metal P and metal Q. Explain your answer.

(d) Describe two changes you would observe in both reactions.

(e) At the end of the experiments, it was found that 0.6609 of pure copper residue was formed. Calculate the initial concentration, in mol/dm₃. of aqueous copper(II) sulfate.

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Answer:

Energy Changes Revision MCQ Questions

1.    Which statements are true about the diagram below?

l    the value of AH is negative
ll   dissolving ammonium nitrate is an endothermic process
Ill  during the reaction, heat is absorbed from the surroundings

A    land ll only

B    II and III only

C    land lll only

D    I, II and III

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2.     The equations for three reactions are given below.
Reaction 1      H₂ –> 2H
Reaction 2      HCl –> H + Cl
Reaction 3      2H + O –> H₂O

Which reaction/s is/are exothermic?

A    Reaction 3 only.

B    Reaction 1 and 2 only.

C    Reaction 1 and 3 only.

D    Reaction 2 and 3 only.

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3.     When ammonia gas is made from nitrogen and hydrogen, the equation for the reaction is: N₂(g) + 3 H₂(g) —> 2 NH₃(g)
The bonds that are broken and the bonds that are formed are ____________

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4.    Hydrogen reacts with chlorine as follows.The H-H bond energy is 436 kJ/mol and the Cl-Cl bond energy is 242 kJ/mol.
What is the H-Cl bond energy?

A    247 kJ

B    431 kJ

C    494 kJ

D    862 kJ

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5.     Which energy profile diagram best represents the thermal decomposition of
copper(II) carbonate?

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6.    The diagram shows the energy profile of a reversible reaction that occurs with and without a catalyst.

What can be deduced from the diagram?

I    The enthalpy change for the fon/vard catalysed reaction —20 kJ/mol.
II   The catalysed backward reaction is endothermic.
III  The enthalpy change is decreased by using catalyst.
IV  The activation energy for the backward uncatalysed reaction is 80 kJ/mol

A    III only

B    l and ll only

C    I, II and IV only

D    I, II, III and IV

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7.    The relative enthalpies, on arbitrary scale, of the reactants and products of a chemical reaction, are represented on the following diagram.

Which statements are true for the above energy profile diagram of this reaction?

I    The reaction is endothermic.
II   The energy absorbed to break the bonds is greater than energy released during bond formation.
III  The numerical value of the activation energy of the reaction is 50.
IV  The numerical value of enthalpy change of the reverse reaction is 100.

A    III and IV

B    II, III and IV

C    I, II and Ill

D    I, II, III and IV

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Answer:

1.    B

2.    A

3.    C

4.    B

5.    C

6.    C

7.    A

Electrolysis III Revision Structured Questions

The set-up shown was used to compare the reactivity of four metals, P, Q, R and S. Metal strips P, Q. R and S and a copper plate were first cleaned with sandpaper. Each metal strip was then clipped onto the wet filter paper and the voltmeter reading was recorded.A summary of the voltmeter readings obtained is shown in the table below.(a) Give a reason why the metal strips and copper plate must first be cleaned
with sandpaper.

(b) From the results in the table,
(i) arrange the metals. P, Q, R, S in order of increasing reactivity.
(ii) state how the positions of the metals in the reactivity series affect the magnitude of voltage.

(c)(i) Given that R is an element in Group II of the Periodic Table, what is
observed if it is placed in copper(ll) sulfate solution?

(ii) Write an ionic equation for the reaction.
(d) State and explain any difference in the voltmeter readings if the experiment is repeated using filter paper soaked with organic solvent methylbenzene.

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Answer:

Electrolysis III Revision MCQ Questions

1.    Which statement best explains the generation of electrical energy in a simple cell?

A    Free moving ions in the electrolyte.

B    Free moving electrons in the electrolyte.

C    Transfer of electrons from a more reactive metal to a less reactive metal.

D    Transfer of electrons from a less reactive metal to a more reactive metal.

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2.     In the set-up shown below. metal X is more reactive than metal Y.

Which statement about metal X is correct?

A    Metal X decreases in size.

B    Metal X is reduced.

C    Metal X is the cathode.

D    Metal X gains electrons from metal Y.

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3.     In which of the following circuits is the bulb likely to shine most brightly?

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4.    Consider the simple cell shown below. Which of the following changes will occur?

A    Oxygen gas is produced at the copper electrode.

B    The blue colour of the copper(II) sulfate solutions turn darker.

C    Hydrogen gas is produced at the magnesium electrode.

D    The copper electrode is coated with a layer of brown solid.

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5.     Ferroxyl indicator can be used to show if rusting has occurred. It turns blue in the presence of Fe²⁺ (aq). An experiment was set up as shown.

After a few minutes. no blue colour was seen in the solution. What could possibly be the identity of metal X?

A    Copper

B    Lead

C    Magnesium

D    lron

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Answer:

1.    C

2.    A

3.    B

4.    D

5.    C

Electrolysis II Revision Structured Questions

(a) A student was given two different types of electrodes (carbon and copper electrodes) to carry out electrolysis of copper(II) sulfate solution.
First, he carried out the experiment with carbon electrodes. At regular intervals, the cathode and anode were removed, dried and weighed. The results were plotted on the graph below.

(i) Give the formulae of the ions present in the electrolyte.
(ii) Explain why the cathode shows an increase in mass initially.

(iii) Describe what is observed at the anode with an appropriate equation.

(b) He carried out the same experiment with copper electrodes. Similarly, at regular intervals, the cathode and anode were removed, dried and weighed.

(i) Predict and draw on a separate graph how the mass of the anode and cathode will change as electrolysis occurs.

(ii) Explain how the graph for the anode is derived with a relevant equation.

(iii) The student noticed that the colour of the electrolyte remained the same as
electrolysis occurred. Explain what had taken place.

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Answer:

Electrolysis II Revision MCQ Questions

1.    A spoon is to be silver plated. Which one of the following gives suitable materials for the cathode, anode and electrolyte?

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2.    Aqueous copper(II) chloride was electrolysed using copper electrodes. Which of the following graphs best represents the change in mass of the anode with time?

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3.     An experiment on electrolysis is set up as shown.
Which of the following statements is incorrect?

A    Both cathodes gain mass.

B    pH of the electrodes increase after some time.

C    The copper(II) sulfate solution remains blue.

D    The ratio of silver deposited to copper deposited is 2 to 1 mole respectively.

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4.     In an electrolysis experiment, the same amount of charge deposited 16 g of copper and 6 g of titanium. What was the charge on the titanium ion?

A    1⁺

B    2⁺

C    3⁺

D    4⁺

5.     Two electrolytic cells, A and B. were set up as shown below. The electrolyte in both cells is green nickel(II) sulfate solution of the same concentration. Cell A has two nickel electrodes, while Cell 8 has a nickel cathode and a platinum anode.

A steady current was passed through both cells for some time, and the results were summarized in the table below.Which statement about the reactions is true?

A    Equal masses of nickel are formed on the two cathodes.

B    pH of the electrodes increase after sometime.

C    The green nickel solution in both cells decolorise after sometime.

D    The remaining solutions left behind in both cells are acidic.

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Answer:

1.    B

2.    A

3.    B

4.   D

5.   A

Electrolysis I Revision Structured Questions

1.    Complete the following table for the electrolysis of substances using inert electrodes.

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2.    Sodium metal is extracted from molten sodium chloride by electrolysis. The diagram below shows how the process works.

(a) (i) Write an ionic half equation. with state symbols, to show the reaction that happens at the anode.
(ii) Describe a simple test and its result that would identify the gas given off at the
anode.
(b) Calcium chloride is added to the sodium chloride to lower the melting point of the mixture.
(i) Explain why lowering the melting point makes the process cheaper to run.
(ii) The molten sodium contains metallic impurities. Name the main metal impurity you would expect to find and explain how it forms.

 

 

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Answer:

Electrolysis I Revision MCQ Questions

1.    During the electrolysis of concentrated sodium chloride using platinum electrodes, which of the following would be the correct observations at each electrode?

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2. Which of the following equations represents the reaction that takes place at the cathode during the electrolysis of aqueous silver nitrate with carbon electrodes?

A    Ag⁺ (aq) + e^– –> Ag (s)

B    2H⁺ (aq) + 2e^– –> H₂(Q)

C    2N^3- (aq) –>  N₂(aq) + 6e^–

D    4OH^– (aq) –> O₂ (g) + 2H_2O (I) + 4e^–

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3. Electrolysis of the following electrolytes (using inert electrodes) give the same product at the cathode except

A    dilute sulfuric acid

B    aqueous sodium chloride

C    aqueous copper (II) sulphate

D    concentrated potassium hydroxide

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4. Element X is extracted by the electrolysis of a molten compound of elements X and Y. The electrode reactions are as shown.
At the cathode: X²⁺ (I) + 2e^– –> x (I)
At the anode: 2Y^2- (I) –> Y₂ (g) + 4e^–

A    Aluminium oxide

B    Calcium chloride

C    Magnesium oxide

D    Potassium chloride

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5. During the electrolysis of molten silver bromide using graphite electrodes, what will be the mass of the products collected at each electrode when 0.5 mol of electrons was passed through the electrolytic cell?

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6. A solid deposit of element R is formed at the cathode when an aqueous solution containing ions of R is electrolysed. Which statement about element R is correct?

A    Element R is below hydrogen in the reactivity series.

B    R gains electrons to form ions at the cathode.

C    Element R forms negatively charged ions.

D    Ions of R loses electrons at the cathode.

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Answer:

1.    B

2.    A

3.    C

4.    C

5.    B

6.    A

O-Level Chemistry Key Terms and Definitions

Periodic Table