Complaining about the face masks that we have to wear in the sweltering heat of Singapore? It was much worse in the 1900s, when masks were uncomfortable, unwieldy, and unsightly. Yet, they were essential. Miners and chemical workers depended on breathing masks for a fresh supply of air in their hostile working environments.
The Danger of Oxygen
It was the early 1900s. During the time when electric lighting was a novelty in Singapore, there was no safe way to transport oxygen to far-flung mines and ulu factories. Shipping companies refused to convey compressed oxygen gas.
Their caution was not unwarranted. Compared to oxygen in the atmosphere, compressed oxygen gas has an elevated pressure. This could cause fire to start and spread more quickly.
The Trouble of Oxygen
To reduce the risk of a chemical accident, solid sources of oxygen were transported instead. They were then reacted on-site to produce oxygen. A common source of oxygen was potassium chlorate(V), KClO3 with the old-school name of chlorate of potash. Potassium chlorate(V) undergoes thermal decomposition to form oxygen gas and potassium chloride only.
However, the reaction was a trouble to prepare and control. It required retorts, a furnace and purifiers.
The Cake of Oxygen
In 1907, Scientific American announced the invention of a new substance to produce oxygen “with the minimum of trouble”. It was sodium peroxide, Na2O2 marketed as oxylithe. It could produce oxygen at room temperature by reacting with carbon dioxide in the air:
2Na2O2 + 2CO2 ⟶ 2Na2CO3 + O2
It could also produce oxygen indirectly on contact with water:
Na2O2 + 2H2O ⟶ 2NaOH + H2O2
2H2O2 ⟶ 2H2O + O2
The reaction with water produces hydrogen peroxide. The hydrogen peroxide then decomposes spontaneously to liberate oxygen gas.
As the reactions do not require heating, oxylithe could be placed in portable breathing apparatus. It was also easy to transport, as oxylithe was made into small cakes.
The Blessing of Oxygen
It has been more than a hundred years since oxylithe was lauded on Scientific American. Unlike what the article promised, oxylithe is no longer used in modern self-contained breathing apparatus. Instead, we have mastered the safe storage and use of compressed oxygen. It is used by firefighters and SCUBA divers alike.
Data-Based Questions à la Paper 2 Section B
QUESTION 1: Air
In mines, carbon monoxide can be present at elevated concentration. Explain why it is necessary to use breathing masks that provide a fresh supply of air in mines. [2 marks]
Carbon monoxide is a poisonous gas. It combines irreversibly with haemoglobin, which is a protein found in red blood cells that piggybacks oxygen. This forms carboxyhaemoglobin that can no longer transport oxygen. Therefore, body tissues will receive less oxygen, resulting in breathing difficulties, loss of consciousness, and even death.
To prevent carbon monoxide poisoning, breathing masks should be used to supply atmospheric air that contains nearly undetectable, trace amount of carbon monoxide.
QUESTION 2: Speed of Reaction
Compressed oxygen is a fire hazard. It hastens the speed of combustion, whereby oxygen gas reacts with combustible substances like hydrocarbons and even steel.
Explain, in terms of collisions between reacting particles, why compressed oxygen at a higher pressure increases the spread of fire. [3 marks]
At higher pressure, reacting oxygen molecules are closer together. Collisions and the attendant effective collisions become more frequent, leading to an increased speed of combustion.
Marker’s comment: for questions on speed of reaction, always mention:
- Distance between reacting particles
- Frequency of collisions
- Frequency of effective collisions
- Speed of reaction
QUESTION 3: Formulae
Write an equation for the decomposition reaction of potassium chlorate(V) that leads to the formation of oxygen gas. [1 mark]
2KClO3 ⟶ 2KCl + 3O2
Marker’s comment: read the stem of the question carefully, which tells you that “potassium chlorate(V) undergoes thermal decomposition to form oxygen gas and potassium chloride only.”
QUESTION 4: Chemical Bonding
Sodium peroxide present in oxylithe is an ionic compound. Its cation is sodium ion, while the anion is peroxide polyatomic ion.
Explain what it means by ‘polyatomic’ and deduce the chemical formula of the anion. [2 marks]
A polyatomic ion is a charged particle made up of multiple atoms covalently bonded. The chemical formula of the anion is O22-.
Marker’s comment: this is an application question in which you have to infer from the hints given.
- The question tells us that peroxide is a polyatomic anion, much like sulfate and carbonate. Therefore, the anion cannot be oxide, O2-, which is a monoatomic anion.
- Besides sodium, its formula unit of Na2O2 has 2 oxygen atoms, implying that the 2 oxygen atoms are part of the polyatomic anion.
- Given that 2 sodium ions with a combined charge of 2+ are needed to balance the negative charge of 1 peroxide anion, it must have a 2- charge.
QUESTION 5: Chemical Bonding
The peroxide polyatomic ion contains a single covalent bond between its atoms. All the atoms in the ion have a stable octet electron arrangement.
Draw a dot and cross diagram to show the arrangement of outer shell electrons in a peroxide anion. [2 marks]
Marker’s comment: the doubly negative charge suggests that there are 2 extra electrons, which are gained equally by each of the oxygen atom; each oxygen has 7 valence electrons, and hence only need to form a single covalent bond to gain the stable octet electron arrangement.
QUESTION 6: Acids, Bases and Salts
When water is added to oxylithe, the pH of the resulting mixture is tested with Universal Indicator.
Describe and explain the results you would expect. [3 marks]
The Universal Indicator turns blue. Sodium peroxide of oxylithe reacts with water to form sodium hydroxide, which is a strong alkali that dissociates completely in water to produce hydroxide ions. Therefore, the pH is higher than 7.