Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
15 questions
Unit 18 · Carbon and its Compounds
2. Differentiate between
Graphite and diamond
Answer
| Graphite | Diamond |
|---|---|
| Each carbon atom is bonded to three others, in flat layers of hexagons. | Each carbon atom is bonded to four others, in a rigid 3-D network. |
| Each atom has one free electron, so it is a good conductor of electricity. | No free electrons, so it is a bad conductor of electricity. |
| Soft and slippery: its layers slide over each other. | The hardest natural substance. |
| Black-gray and opaque, with a dull shine. | Colorless and transparent, and sparkles when cut. |
| Density about 2.2 g/cm³. | Density about 3.5 g/cm³. |
| Used in pencils, dry cells and as a lubricant. | Used in jewelry and for cutting glass and drilling rock. |
Organic compounds and inorganic compounds
Answer
| Organic compounds | Inorganic compounds |
|---|---|
| Contain carbon, almost always bonded to hydrogen. | May or may not contain carbon. |
| Atoms joined by covalent bonds. | Mostly ionic (electrovalent) bonds. |
| Mostly don’t dissolve in water, but dissolve in organic solvents. | Mostly dissolve in water, but not in organic solvents. |
| Low melting and boiling points. | High melting and boiling points. |
| Most burn easily. | Most don’t burn. |
| Examples: methane, sugar, urea, ghee. | Examples: common salt, water, limestone, carbon dioxide. |
3. Give reasons
In winter, we should not burn coal or firewood in a room with the windows and doors closed.
Answer
In a closed room, the fire soon uses up the oxygen. With too little oxygen, carbon burns incompletely and forms carbon monoxide (2C + O₂ → 2CO). Carbon monoxide has no color or smell. It combines with the hemoglobin of the blood far more strongly than oxygen does, so the blood can’t carry oxygen, and the people in the room can become unconscious and die in their sleep. (The fire also fills the room with carbon dioxide and smoke.) So the room must have fresh air coming in.
Graphite powder can be used as a dry lubricant.
Answer
In graphite, the carbon atoms are arranged in flat layers. The layers are held to each other only by weak forces, so they slide over one another easily. Graphite powder therefore makes surfaces slippery and reduces friction. Unlike oil, it is dry, so it doesn’t collect dust, and it works even at high temperatures.
Graphite and diamond are both allotropes of carbon, yet graphite conducts electricity and diamond doesn’t.
Answer
In graphite, each carbon atom forms covalent bonds with only three other carbon atoms, so one of its four outer electrons is free. These free electrons can move through the layers and carry an electric current. In diamond, each carbon atom uses all four outer electrons to bond with four other carbon atoms, so there are no free electrons, and no current can flow.
The melting and boiling points of inorganic compounds are higher than those of organic compounds.
Answer
Most inorganic compounds are ionic: their positive and negative ions are held in a crystal by strong electrostatic attractions, which need a lot of heat energy to overcome. Organic compounds are covalent and are made of separate molecules, which are held to one another only by weak forces. Little energy is needed to separate them, so organic compounds melt and boil at low temperatures.
When firewood burns in a stove, it sometimes gives black smoke, and sometimes burns well with little smoke.
Answer
Firewood contains carbon compounds. When the fire gets enough air (dry wood, not packed too tightly, with a good draught), the carbon burns completely to colorless carbon dioxide, and there is little smoke. When the air is not enough (wet wood, wood packed tightly, a choked stove), the carbon doesn’t burn completely: some of it escapes as tiny particles of unburnt carbon, which we see as black smoke (soot), and carbon monoxide forms too.
4. Answer the following questions
Define catenation and allotropy of carbon, with examples.
Answer
Catenation is the property of carbon atoms of joining with one another by covalent bonds to form long chains, branched chains and rings. Examples: butane (a chain of 4 carbon atoms), pentane (5), polythene and PVC (thousands).
Allotropy is the existence of an element in two or more forms with different physical properties but similar chemical properties; the forms are called allotropes. Their atoms are arranged differently. Examples: diamond, graphite and fullerene (crystalline allotropes of carbon), and coal, charcoal and soot (amorphous allotropes).
When sugar is heated for a long time, it gives a black mass. What can we conclude from this?
Answer
The black mass is sugar charcoal, which is carbon. So we conclude that sugar contains carbon. On heating, the hydrogen and oxygen in sugar (C₁₂H₂₂O₁₁) are driven off as water vapor, and the carbon is left behind.
Give two examples each of crystalline and amorphous allotropes of carbon.
Answer
Crystalline: diamond, graphite (also fullerene and graphene).
Amorphous: charcoal, soot (lamp black) (also coal and coke).
Under what conditions does burning carbon form carbon monoxide, and when carbon dioxide? Write the chemical equations.
Answer
When carbon burns in plenty of oxygen (complete combustion), it forms carbon dioxide:
C + O₂ → CO₂ + heat
When carbon burns in a limited supply of oxygen (incomplete combustion), it forms carbon monoxide:
2C + O₂ → 2CO + heat
What is water gas?
Answer
Water gas is a mixture of carbon monoxide and hydrogen gas. It is made by passing steam over red-hot carbon (coke) at about 1,000 °C: C + H₂O → CO + H₂. It is used as an industrial fuel and as a source of hydrogen.
What happens when carbon reacts with steam? Write the chemical equation.
Answer
When steam is passed over carbon heated to about 1,000 °C, the carbon takes the oxygen from the water, forming carbon monoxide, and hydrogen gas is set free. The mixture is called water gas.
C + H₂O → CO + H₂
Rohan couldn’t clean a grease stain from his clothes with soap and water. His mother advised him to rub a little petrol on the stain. Will this solve Rohan’s problem? Explain the cause of his problem and the solution, using scientific facts.
Answer
Yes, it will (mostly) solve it.
The cause: grease is an organic compound. Like most organic compounds, it doesn’t dissolve in water, because its molecules can’t form hydrogen bonds with water. That is why water alone couldn’t lift it.
The solution: organic compounds dissolve in organic solvents, and petrol is one. The petrol dissolves the grease, which can then be washed away. (Dry cleaners use organic solvents in the same way.)
But be careful: petrol catches fire very easily and its vapor is harmful. Use only a little, outdoors, far from any flame, and wash the cloth well afterwards. Kerosene works too, and so does rubbing a little detergent into the stain: detergent molecules have one end that holds on to grease and another that holds on to water.
Write any four examples of organic compounds you use in daily life.
Answer
- Sugar (sucrose) and starch (in rice), in food.
- LPG (butane and propane), as cooking fuel.
- Cotton (cellulose), in clothes.
- Paracetamol, a medicine.
(Others: petrol, kerosene, ghee and oil, soap, plastics, paper, vinegar.)