Redox - exam practice
Redox
Question 1 of 9
Chlorine is an important chemical and has a number of applications including sterilising water and extracting bromine from seawater. Chlorine can be formed by the oxidation of chloride ions.
In terms of electrons, what is meant by the term oxidation?
Write a half equation to show the oxidation of chloride ions to chlorine.
In the extraction of bromine from seawater, chlorine acts as an oxidising agent.
What is meant by the term oxidising agent?
Chlorine can also oxidise water to form oxygen.
Write an equation for this reaction and give the oxidation state of chlorine in the chlorine-containing product that is formed.
Iodine reacts with nitric acid to form nitrogen dioxide, NO2.
Balance the following equation for this reaction.
I2 + 10HNO3 ➔ ......HIO3 + ......NO2 + ......H2O
Give the oxidation state of iodine in I2 and HIO3.
Two half equations involving iodine are shown below. Use these two half equations to write an overall equation for the reaction.
IO3− + 5e− + 6H+ ➔ 3H2O + I2
I− ➔ I2 + e−
Iodide ions can behave as a reducing agent in the reaction with chlorine. State the meaning of the term 'reducing agent'.
Write the simplest ionic equation for the reaction of chlorine and iodide ions.
This question is about redox reactions.
State the meaning of the term redox as applied to a reaction.
SO32− ions can be oxidised by Cr2O72− ions in acidic conditions to form Cr3+ and SO42−. Write the following equations:
- A half equation for the oxidation of SO32− to SO42−
- A half equation for the reduction of Cr2O72− to Cr3+
- An overall equation for the reaction
Salts can be prepared in redox reactions of metals with acids. What is meant by a salt?
The aluminium sulfate salt can be prepared by reacting aluminium with sulfuric acid as shown in the equation below.
2Al(s) + 3H2SO4(aq) ➔ Al2(SO4)3(aq) + 3H2(g)
Use oxidation numbers to show which element has been oxidised and which element has been reduced.
This question is about the redox reactions of chlorine.
Chlorine and water react in a redox reaction according to the equation below.
Cl2 + H2O ⇌ HClO + HCl
Write two half equations to show the redox processes happening in this reaction.
Hydrochloric acid can be oxidised with sodium chlorate (I) (NaClO) solution. Write the following equations:
- A half equation for the oxidation of chloride ions to chlorine
- A half equation for the reduction of chlorate ions (ClO−) to chlorine in acidic conditions
- An overall equation for the reaction
ClO− ions are found in bleach and their concentration can be found by the reaction with iodide ions.
ClO− + 2I− + 2H+ ➔ I2 + Cl− + H2O
10 cm3 of bleach, containing ClO− ions, was reacted with 69.5 mg of potassium iodide.
Find the concentration of ClO− ions in the bleach solution, giving your answer to 3 significant figures.
Copper can be extracted from CuS by reaction with nitric acid to produce a solution of copper sulfate, as shown in the reaction below.
3CuS(s) + 8HNO3(aq) ➔ 3CuSO4(aq) + 8NO(g) + 4H2O(l)
Give the oxidation states of nitrogen in HNO3 and NO.
Deduce a half equation for the reduction of the nitrate ion in acidic conditions to form NO and H2O.
Deduce a half equation for the oxidation of the sulfide ion to form the sulfate ion and H+ ions in solution.
NO can also be formed when sodium nitrite (NaNO2) is reacted with iodide ions.
Give the oxidation state of nitrogen in NO2− and NO, and write a half equation for the conversion of NO2− to NO.
During this reaction, iodide ions are converted to iodine.
Write an overall equation for the reaction, and state the role of NO2− in this reaction.
Iodine can be used on a small scale to purify water. When iodine reacts with water, the following equilibrium is set up.
I2 + H2O ⇌ HI + HIO
Using oxidation numbers, explain why this equilibrium is an example of disproportionation.
Chlorine is also used to purify water but on a much larger scale. Chlorine can be manufactured by the reaction below.
MnO2(s) + 4HCl(aq) ➔ MnCl2(aq) + 2H2O(l) + Cl2(g)
Use oxidation numbersto show which substance has been oxidised and which has been reduced.
Nitrite ions, NO2−, react with chlorate (V) ions, ClO3−, to produce chlorine as shown in the reaction below.
2ClO3− + 5NO2− + 2H+ ➔ Cl2 + 5NO3− + H2O
In a titration experiment, 25.0 cm3 of sodium nitrite solution was completely reacted with 26.79 cm3 of 0.0200 mol dm-3 potassium chlorate(V) solution.
Calculate the concentration, in g dm-3, of sodium nitrite to 1 decimal place.
The processes of oxidation and reduction can be defined in terms of the transfer of electrons.
Define the term reduction in terms of electrons
The following reaction between copper and nitric acid can be used to produce NO.
3Cu + 8H+ + 2NO3− ➔ 3Cu2+ + 4H2O + 2NO
If the nitric acid is concentrated and warm, then NO2 can be produced instead of NO.
Give the oxidation states of nitrogen in NO3−, NO and NO2.
Write a half equation for the formation of the NO2 from NO3− and state whether oxidation or reduction is occurring.
Write a half equation for the formation of NO from NO3−, in acidic conditions, and state whether oxidation or reduction is occurring.
Bromine can form the oxyanion BrO3-.
Deduce the oxidation state of bromine in the BrO3- ion and give its systematic name.
BrO3- ions act as oxidising agents in acidic conditions to form bromide ions.
Deduce the half-equation for this reduction reaction.
BrO3- ions oxidise silver metal, Ag, to silver(I) ions, Ag+.
Deduce the equation for this redox reaction.
The dichromate ion, Cr2O72−, can be used to oxidise iodide ions to iodine.
What is the oxidation state of chromium in Cr2O72−?
Write the following equations:
- A half equation for the oxidation of iodide ions to iodine
- A half equation for the reduction of Cr2O72− to Cr3+ in acidic conditions
- An overall redox equation for the reaction
MnO2 is another oxidising agent that can be used to oxidise iodide ions to iodine.
2I− + MnO2 + 4H+ ➔ Mn2+ + 2H2O + I2
Give the oxidation state changes of Mn and I and show that this is a redox reaction.
The percentage purity of a 0.850 g sample of impure iodine can be determined using a titration. The iodine is dissolved in potassium iodide solution and titrated with sodium thiosulfate according to the reaction below.
2S2O32− + I2 ➔ 2I− + S4O82−
The titration requires 28.85 cm3 of 0.200 mol dm−3 sodium thiosulfate.
Calculate the percentage purity of the sample of iodine, giving your answer to 3 significant figures.