Senior Secondary (HKDSE) · Chemistry

Oxidation numbers, redox reactions: Practice Questions

5 multiple-choice questions marked as you go, and 5 written questions with worked solutions. All on Oxidation numbers, redox reactions.

10 questions24 marksFree, no account

Question 1

1 mark

Arsenic(III) oxide ( $\text{As}_2\text{O}_3$ ) reacts with acidified potassium dichromate ( $\text{Cr}_2\text{O}_7^{2-}$ ) to produce arsenic acid ( $\text{H}_3\text{AsO}_4$ ) and chromium(III) ions ( $\text{Cr}^{3+}$ ). The unbalanced ionic equation for the reaction is:

$\text{As}_2\text{O}_3(\text{s}) + \text{Cr}_2\text{O}_7^{2-}(\text{aq}) \rightarrow \text{H}_3\text{AsO}_4(\text{aq}) + \text{Cr}^{3+}(\text{aq})$

When this equation is balanced using the smallest whole-number coefficients in acidic medium, what is the stoichiometric coefficient of water ( $\text{H}_2\text{O}$ )?

Question 2

1 mark

Consider the reaction:
$ \text{H}_2\text{S}(\text{g}) + \text{I}_2(\text{aq}) \rightarrow 2\text{HI}(\text{aq}) + \text{S}(\text{s}) $

Which species acts as the oxidizing agent?

Question 3

1 mark

Chlorine gas ($Cl_2$) is bubbled through hot concentrated aqueous potassium hydroxide ($KOH$) to produce chloride ($Cl^-$) and chlorate(V) ($ClO_3^-$) ions. After balancing the equation with the smallest whole number coefficients, what is the sum of the stoichiometric coefficients of the reactants ($Cl_2$ and $OH^-$)?

Question 4

1 mark

What is the oxidation number of chlorine in the chlorate ion, $ \text{ClO}_3^- $?

Question 5

1 mark

In the reaction:
$ 3\text{Cu}(\text{s}) + 8\text{HNO}_3(\text{aq}) \rightarrow 3\text{Cu}(\text{NO}_3)_2(\text{aq}) + 2\text{NO}(\text{g}) + 4\text{H}_2\text{O}(\text{l}) $

What is the change in the oxidation number of nitrogen?

Question 6

2 marks

Calculate the oxidation number of chlorine in the chlorite ion ($ \text{ClO}_2^{-} $).

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Question 7

4 marks

Consider the following reaction: $\text{Sn}^{2+}\text{(aq)} + 2\text{Fe}^{3+}\text{(aq)} \rightarrow \text{Sn}^{4+}\text{(aq)} + 2\text{Fe}^{2+}\text{(aq)}$. Determine the change in oxidation number for tin and iron. Identify the oxidising agent and the reducing agent in this reaction.

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Question 8

5 marks

When urea ($ \text{CO(NH}_2)_2 $) reacts with nitrous acid ($ \text{HNO}_2 $), the products are carbon dioxide, nitrogen gas, and water. Determine the oxidation number of nitrogen in both urea and nitrous acid, and find the mole ratio of $ \text{HNO}_2 $ to $ \text{CO(NH}_2)_2 $ in the balanced chemical equation.

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Question 9

3 marks

Consider the reaction between aqueous sulfur dioxide and an acidified potassium dichromate(VI) solution.
(a) Determine the oxidation number of sulfur in $ \text{SO}_2 $.
(b) State the color change of the solution observed during the reaction.
(c) Identify the species that acts as the oxidizing agent in this reaction.

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Question 10

5 marks

Consider the unbalanced reaction for the catalytic oxidation of ammonia:

$$\text{NH}_3(\text{g}) + \text{O}_2(\text{g}) \rightarrow \text{NO}(\text{g}) + \text{H}_2\text{O}(\text{l})$$

(a) Determine the oxidation number of nitrogen in $$\text{NH}_3$$ and $$\text{NO}$$.

(b) Determine the oxidation number of oxygen in $$\text{O}_2$$ and $$\text{H}_2\text{O}$$.

Write your answer out first, then check it against the worked solution.

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