Chemistry 5070/21 — October/November 2011
Cambridge O-Level · Theory · worked solutions for every part, with the mark scheme
Topics Stoichiometry · Acids, Bases and Salts · Chemical Reactions · Experimental Techniques and Chemical Analysis · Atoms, Elements and Compounds · Chemistry of the Environment · +4 more
Choose from the following list of compounds to answer the questions below.
calcium hydroxide
carbon monoxide
methane
nitrogen dioxide
potassium manganate(VII)
silver nitrate
sulfur dioxide
Each compound can be used once, more than once, or not at all.
Which compound
is used as a bleach in the manufacture of paper,
______
Answer
sulfur dioxide
sulfur dioxide
Walkthrough
The question gives a list of seven compounds and asks which one is used as a bleach in the manufacture of paper. Sulfur dioxide is a bleaching agent and is used to bleach wood pulp during paper making. None of the other listed compounds is used for this purpose, so the answer is sulfur dioxide.
Key Takeaways
- Sulfur dioxide has industrial uses as a bleach and as a preservative.
- A 'use' question is answered by recalling the characteristic application of each substance.
Common Mistakes
- Choosing chlorine instead of sulfur dioxide: chlorine is also a bleach, but it is not in the list.
- Writing the formula instead of the name: the question asks for the compound from the list, so give the name as printed.
Things to Be Careful About
- Read the list carefully; the answer must be one of the seven names given.
- Use the exact name 'sulfur dioxide', not 'sulfur(IV) oxide' unless the mark scheme allows it.
changes from purple to colourless when its acidified solution is used to oxidise ethanol,
______
Answer
potassium manganate(VII)
potassium manganate(VII)
Walkthrough
Acidified potassium manganate(VII) is a strong oxidising agent. Its solution is purple because of the manganate(VII) ion. When it oxidises ethanol, the manganate(VII) is reduced to colourless manganese(II) ions, so the purple colour disappears. Therefore the compound is potassium manganate(VII).
Key Takeaways
- Acidified potassium manganate(VII) is a purple oxidising agent.
- A purple-to-colourless change shows that the manganate(VII) has been reduced while oxidising another substance.
Common Mistakes
- Saying 'manganate' without 'potassium' or without the oxidation state (VII).
- Confusing it with potassium dichromate(VI), which changes from orange to green; that compound is not in the list.
- Forgetting that the solution must be acidified for this oxidation to take place.
Things to Be Careful About
- The answer must be the name as printed in the list: potassium manganate(VII).
- The colour change is purple to colourless, not purple to pink or green.
has an aqueous solution that reacts with aqueous sodium chloride to give a white precipitate,
______
Answer
silver nitrate
silver nitrate
Walkthrough
Aqueous sodium chloride contains chloride ions. Silver nitrate solution provides silver ions. Silver chloride is insoluble, so a white precipitate of silver chloride forms. This is the standard test for chloride ions. Therefore the compound in the list is silver nitrate.
Key Takeaways
- Silver ions react with chloride ions to form insoluble silver chloride, a white precipitate.
- Silver nitrate solution is used to test for halide ions such as chloride.
Common Mistakes
- Writing 'silver chloride' as the answer: the question asks for the compound from the list that reacts, which is silver nitrate.
- Choosing a compound that does not react with sodium chloride to give a white precipitate.
Things to Be Careful About
- The precipitate is white because silver chloride is insoluble.
- The answer must be one of the seven names given in the list.
can be formed by the action of lightning on gases in the atmosphere,
______
Answer
nitrogen dioxide
nitrogen dioxide
Walkthrough
During thunderstorms, lightning provides enough energy for nitrogen and oxygen in the air to react. Nitrogen monoxide is formed first and is then further oxidised to nitrogen dioxide. Nitrogen dioxide is an oxide of nitrogen, so the compound in the list is nitrogen dioxide.
Key Takeaways
- Lightning fixes nitrogen by providing energy for nitrogen and oxygen to react.
- Nitrogen dioxide is an atmospheric pollutant formed naturally by lightning and also by car engines.
Common Mistakes
- Choosing carbon monoxide, which is formed by incomplete combustion of fuels, not by lightning.
- Choosing sulfur dioxide, which comes from burning fossil fuels containing sulfur impurities.
Things to Be Careful About
- The phrase 'action of lightning on gases in the atmosphere' points to oxides of nitrogen.
- Give the name 'nitrogen dioxide' exactly as it appears in the list.
is formed by the decay of vegetable matter?
______
Answer
methane
methane
Walkthrough
When vegetable matter decays without oxygen, a process called anaerobic decay, methane is produced. Methane is the simplest alkane and the main component of natural gas. In the list, the compound formed by the decay of vegetable matter is methane.
Key Takeaways
- Methane is produced by the anaerobic decay of vegetable matter.
- Methane is a greenhouse gas and the simplest member of the alkane homologous series.
Common Mistakes
- Choosing carbon dioxide: carbon dioxide is produced by complete combustion or aerobic decay, not by the anaerobic decay of vegetable matter.
- Choosing carbon monoxide: this is formed by incomplete combustion, not by decay.
Things to Be Careful About
- The answer is 'methane', not 'natural gas' or a formula, because the question asks for the compound from the list.
- Remember that anaerobic decay means decay in the absence of oxygen.
The rest of this paper
8 more questions- Q2Stoichiometry · Atoms, Elements and Compounds · Metals · The Periodic Table9M
- Q3Chemical Reactions · Atoms, Elements and Compounds8M
- Q4Experimental Techniques and Chemical Analysis · Chemistry of the Environment · Stoichiometry · Electrochemistry · Organic Chemistry14M
- Q5Atoms, Elements and Compounds · Stoichiometry9M
- Q6Acids, Bases and Salts · Experimental Techniques and Chemical Analysis · Stoichiometry · Chemistry of the Environment10M
- Q7Organic Chemistry · Acids, Bases and Salts · Chemical Reactions10M
- Q8Acids, Bases and Salts · Stoichiometry · Experimental Techniques and Chemical Analysis · Electrochemistry · Metals10M
- Q9Stoichiometry · Atoms, Elements and Compounds · Acids, Bases and Salts · Chemical Reactions10M