Unit 17Redox Reactions

Multiple-Choice Qrestions

l. A drop of solution S turns acidified orange potassium dichromate(Vl) to green chromium(Ill)ions. What must solution S contain?A an acid B an oxidising agentC a reducing agent D a substance that can be reduced ( )

2. Which of the following statement is not true about the reaction between copper(Il) oxide andhydrogen gas?

A Copper(Il) oxide is reduced.B Copper(Il) oxide acts as a reducing agent.C The colour of the solid changes from black to pink.D Water is formed. ( )

3. The element astatine (At) lies immediately below iodine in Group VII of the Periodic Table. Itcan be deduced thatA the At- ion is a strong oxidising agentB the Ar ion is both a reducing agent and an oxidising agentC astatine can oxidise the I-to I,D bromine can oxidise the At- ion to At,

4. In the blast furnace, iron is extracted from its ore. The equation for the reaction is shown below

FerO,(s) + 3CO(g)--* 2Fe(s) + 3CO,(g)

Which of the following is correct about this reaction?A FerO, is a reducing agent. B CO is an oxidising agent.C FerO., is neutralised by CO. D CO is oxidised to CO".

5. Which reaction is not an example of a redox reaction?A Ag*+Ct-AgCl B Mg+2H.-Mgr*+H,C Cl,+2I--ZCl-+I, D Cu2*+Fe-Cu+Fe2*

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Structured Qrestions

The equation for the reaction between acidified potassium manganate(Vll) and iron(Il) ions isgiven below.

5Fe2*(aq) + Mnoo-(ag) + 8H.(aq) - 5Fe3*(aq) + Mn2*(aq) + 4H,o(/)

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(b) Outline how acidified potassium manganate(Vll) can be used to test for a reducing agent. [2]

2. Acidified potassium manganate(Vll) reacts with aqueous hydrogen peroxide, HrOr. The equation

for the reaction is shown below.

5HrOr(ag) + 2MnOo (aq) + 6H.(aq)--- 5Or(B) + 2Mn2*(aq) + 8HrO(/)

400 cm3 of aqueous hydrogen peroxide reacts with 0.050 moles of MnOo-(ag).

(a) What is the oxidation state of manganese in the following ions?

(i) MnO; t1l

(ii) Mn2. I1l

(b) State two changes you would expect to observe when acidified potassium manganate(Vll) is

added to aqueous hydrogen peroxide. 12)

(c) What type of chemical change happens to the MnO; in the reaction? Give a reason for your

answer. t2)

(a) What is the reducing agent in this reaction? Explain your answer.

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(d) (i) Calculate the number of moles of HrO, that react with the MnOo tll

(ii) Calculate the concentration of the aqueous hydrogen peroxide in mol dmr t1l

(e) A compound with the formula MnSO., is one of the products of the reaction. Name this

compound. tll

3. Acidified potassium dichromate(Vl) is a common oxidising agent used in the laboratory.

(a) What is the meaning of (VI) in potassium dichromate(Vl)? t1l

(b) What is the colour of potassium dichromate(Vl) in aqueous solution?

(c) The unbalanced ionic equation for the reduction of acidified potassium dichromate(Vl) isgiven below.

CrrOr2-(ag) + Ht(aq) + e + Cr3*(aq) + HrO(/)

(i) Balance the equation.

(ii) what is the change in the oxidation state of chromium in this reaction?

(d) What colour change is observed when sulfur dioxide gas is bubbled through acidifiedpotassium dichromate(Vl)?

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4. Hydrogen peroxide is a colourless liquid at room temperature and pressure. The structure ofhydrogen peroxide is shown below.

H oaa

aa o X

Ha

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Xa

aa aa

(a) When a few drops of acidified potassium manganate(Vll) are added to aqueous hydrogenperoxide, the purple potassium manganate(Vll) is decolourised.

(i) What is the function of the acidified potassium manganate(Vll)? t1l

(ii) What can you deduce from the observation?

(b) When aqueous hydrogen peroxide is added to acidified aqueous iron(Il) sulfate, the palegreen iron(Il) sulfate solution turns brown.

(i) What is the function of the acidified iron(Il) sulfate?

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(ii) What can you deduce from this observation?

(c) Based on the observations in (a) and (b), what can you conclude about the nature ofhydrogen peroxide?

5. Hydrogen iodide dissolves in water and ionises to form an acidic solution.

(a) (i) Write the equation for the ionisation of hydrogen iodide in water.

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(ii) Give the formula of the ion that causes the acidity ttlDanya

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(b) Bromine is added drop by drop to the solution.

(i) What is observed in this reaction? I2l

(ii) Explain the changes observed in (b)(i). tzl

(iii) Write an ionic equation for the reaction.

6. Hydrogen peroxide, HrO, is an oxidising agent in acidic conditions.

ttrOr(ag) + 2H.(aq) + 2e- --- zHro(l)

(a) What is the oxidation state of H and O in hydrogen peroxide?

(b) Which element in hydrogen peroxide is reduced when it acts as an oxidising agent?Explain your answer.

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(c) When aqueous acidified hydrogen peroxide is added to aqueous iron(Il) sulfate, a redoxreaction occurs. Fe2* ions are oxidised to Fe3* ions.

Fe2*(aq) --* Fe3*(aq) + e-

(i) What is observed in the reaction?t1l

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(ii) Write an ionic equation for the reaction.

7. When sulfur dioxide gas is bubbled through aqueous iron(III) chloride, the browniron(III) chloride solution turns pale green. The equation for the reaction is given below.

zFeClr(ai) + ZHrO(l) + SOr(e) --- 2FeClz(aq) + 2HCl(aq) + HrSOo(ag)

(a) Which substance is oxidised? Give your reason.

(b) Which substance is reduced? Give your reason.

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(c) Give a chemical test which can indicate the presence of iron(Il) ions. tzl

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