Which one of the following forms of carbon is thermodynamically most stable?
- A.Coal
- B.Graphite
- C.Diamond
- D.Fullerene
Correct answer
Explanation
The correct answer is B, Graphite. Graphite has the lowest energy of all forms of carbon at ordinary temperature and pressure, so it is the thermodynamically most stable allotrope.
Chemists choose graphite as the standard state of carbon, which is why its standard enthalpy of formation is taken as zero. In graphite each carbon atom is bonded to three others in flat hexagonal layers, and the layers are held by weak forces, which also makes it soft and a good conductor of electricity. Diamond turns into graphite on its own in theory, but the change is so slow that a diamond lasts for ages; it is stable in practice, not in energy terms.
A, coal, is an impure, largely amorphous fuel, not a pure crystalline allotrope.
C, diamond, has slightly higher energy than graphite and forms only under very high pressure.
D, fullerene (such as C60), is a strained cage molecule with more energy than graphite.
Exam tip: graphite is the standard state of carbon (ΔHf = 0); diamond is the hardest, graphite the most stable.
Practice Questions
View allConsider the following actions :
1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
An accelerometer measures acceleration, including the pull of gravity, and each of the three actions depends on reading it. An accelerometer is a small sensor, today usually a MEMS (micro-electro-mechanical system) chip, that senses changes in speed and the direction of gravity.
Read the full explanation →Consider the following statements :
1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Show answer
Carbon fibre is widely used in cars and aircraft for its strength and low weight, and it can be recovered and recycled.
Read the full explanation →Which one of the following cell organelles contains DNA?
- A.Golgi apparatus
- B.Mitochondrion
- C.Lysosome
- D.Endoplasmic reticulum
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Mitochondria carry their own small, circular DNA and their own ribosomes. Mitochondria are the powerhouses of the cell, where aerobic respiration makes ATP.
Read the full explanation →Which set of vegetables is underground stem or modified stem of a plant?
- A.Potato, artichoke, round gourd
- B.Onion, garlic, carrot
- C.Potato, onion, garlic
- D.Potato, garlic, turnip
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All three are underground modified stems: the potato is a stem tuber, and onion and garlic are bulbs. A stem can be told from a root by its nodes and buds.
Read the full explanation →A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :
- A.0.5 s
- B.1 s
- C.1.5 s
- D.2 s
Show answer
A wave moves forward by one wavelength in each cycle, so 150 wavelengths take 150 cycles, and 150 cycles at 300 per second take half a second.
Read the full explanation →