Physical Properties of Alkanes

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:10 PM IST

Now suppose you are lighting your gas stove to cook something tasty. The fuel that you are using is probably a simple alkane, such as butane or propane. Alkanes are sometimes casually or roughly called paraffins. This is an entity flippantly found in most common commercial products derived from petroleum, like natural gas, gasoline, etc., and sometimes even in candles. Whether it is an automobile engine, a warming fire, or merely the reassuring light of a camping candle, all rely upon the special features of alkanes.

This Story also Contains

Main Concept: Definitions and Explanations
Different Features of Alkanes:
State of Matter and Molecular Size:
The various important physical properties of alkanes are discussed below:
Applications and significance:
Some Solved Examples
Summary

Physical Properties of Alkanes

Main Concept: Definitions and Explanations

Alkanes are saturated hydrocarbons; each carbon atom is joined to other carbon atoms with single covalent bonds and has the general formula: $\mathrm{CnH} 2 \mathrm{n}+2$ from which a homologous series of saturated hydrocarbons may be derived. The simplest alkane is methane, CH₄ , the next in the series being ethane$(\mathrm{CH} 4 \mathrm{CH} 4)$, followed by ethane $(\mathrm{C} 2 \mathrm{H} 6 \mathrm{C} 2 \mathrm{H} 6)$, propane $(\mathrm{C} 3 \mathrm{H} 8 \mathrm{C} 3 \mathrm{H} 8)$,etc. This chapter shall cover some of the fundamental properties of alkane liquid states, melting and boiling points solubility, and density, etc. Alkanes are relatively unreactive, due to the strong carbon-carbon and carbon-hydrogen bonds which also make them ideal for use as fuels and solvents respectively.

Different Features of Alkanes:

State of Matter and Molecular Size:

The nature of alkanes with normal temperature can give information on the molecular size. Lower alkanes a gas- methane /ethane/propane/butane- while with its five to sixteen carbon alkanes are liquids, and for those above it are solids. The difference is attributed to the increasing strength of the van der Walls forces with an increase in molecular size.
Alkanes normally have low melting and boiling points which are found to increase with increasing molecular weight. This can be further explained by the fact that the rate of increase in the size of alkanes is making more van der Wal forces operate on their increased surface areas. Also, it requires more energy for alkanes to undergo a state change. For instance, methane is said to have a boiling point of -161.5°C while that of octane is 125.7°C. Solubility and Density:
Being a nonpolar molecule, alkanes are not soluble in water, but they are soluble in the use of another nonpolar solvent like hexane. The density of oil is lesser than that of water; hence making the oil, which is also a mixture of different alkanes, float above the water.

The various important physical properties of alkanes are discussed below:

State: Due to weak forces, the alkanes up to four carbon atoms, i.e., methane, ethane, propane, and butane are colorless, odorless gases but the next thirteen members are colorless, odorless liquids. Alkanes from C₁₈ onwards are colorless and odorless solids.
Density: The density of alkanes increases very slowly with the rise of molecular mass until it becomes constant at about 0.8. Thus, all alkanes are lighter than water.
Solubility: They are generally insoluble in polar solvents such as water but soluble in non-polar solvents like ether, carbon tetrachloride, benzene, etc. The solubility decreases with an increase in molecular mass.
Boiling point: The boiling points of straight-chain or n-alkanes increase regularly with the increasing number of carbon atoms. Among the isomeric alkanes, the normal isomer has a higher boiling point than the branched-chain isomers. The greater the branching of the chain, the lower will be the boiling point.
Melting point: The melting points of alkanes do not follow a very smooth gradation with the increase of molecular size. Alkanes with an even number of carbon atoms have a higher melting point than the next lower and next higher alkanes having an odd number of carbon atoms.

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Applications and significance:

Real World Applications :
We will come to know about what alkanes are, what benefits the human race gets from them, and what losses also it has to bear so that we are able to enjoy the benefits. Methane forms a significant proportion of the natural gas that is used at home in providing heating services for buildings as well as cooking purposes. Propane is also another typical burning gas used in portable stoves and heating systems. The liquid alkanes, in this case octane, were the critical component of the gasoline, the most vital component in the powering of the internal combustion oils. Other important applications are in solid alkanes; the paraffin wax, which is employed in the making of candles and sealing agents.

Commercial and Industrial Significance:

On the other hand, as far as the academic work is concerned, the alkanes rank among the first examples of the organic compounds study and are utilized in creating a basis for ensuring the understanding of more complicated classes of the hydrocarbons. Apart from the fact that their properties are quite predictable, there is a feature of enables them to obtain the most informative description of molecular relations and phase transitions. Alkanes have found a lot of industrial use, mainly in the manufacture of fuels, lubricants, and petrochemicals. The understanding of their properties is very useful in the study of process refinements and the development of sources for material and energy use.

Some Solved Examples

Example 1

Question: Which of the following exists in both gaseous as well as liquid state?

1) Ethane
2) Propane
3) Pentane
4) Butyl decane

Solution: Pentane exists in both gaseous and liquid states. Ethane and propane exist in a gaseous state, while butyl decane exists only in a liquid state.

Hence, the answer is option (3).

Example 2

Question: Arrange the following in decreasing order of their melting point: NeoPentane, n-Pentane, Isopentane.

1) $1>2>3$
2) $3>2>1$
3) $1>3>2$
4) $3>1>2$

Solution: For different isomers of an alkane, an alkane with a spherical shape molecule has the highest melting point due to the most compact packing in the solid state. Thus, the order is:

NeoPentane > n-Pentane > Isopentane.

Therefore, option (1) is correct.

Example 3

Question: Which one of the following has the minimum boiling point?

1) n-butane
2) 1-butyne
3) 1-butene
4) Isobutene

Solution: The boiling point follows the order:

1-butyne > 1-butene > n-butane > Isobutene.

Hence, the answer is option (4).

Summary

Their states of matter, melting and boiling points, solubility, and density are a result of molecular size and structure. The knowledge of properties is basically of practical significance in everyday life and industry. An understanding of the basic laws in chemistry is yielded from the knowledge gained through alkane research, and what they mean in practical outputs or generally in real life.

Frequently Asked Questions (FAQs)

1. What is the relationship between the number of carbon atoms in an alkane and its heat of combustion?

The heat of combustion of alkanes increases as the number of carbon atoms increases. This is because larger alkanes contain more C-C and C-H bonds, which release energy when broken during combustion. The relationship is approximately linear, with each additional CH2 group contributing about 650 kJ/mol to the heat of combustion.

2. Why do alkanes have lower electrical conductivity compared to other organic compounds?

Alkanes have very low electrical conductivity because they are composed entirely of nonpolar covalent bonds between carbon and hydrogen atoms. There are no free electrons or ions to carry an electric current. The electrons in alkanes are tightly held in sigma bonds, making it difficult for them to move and conduct electricity. This property makes alkanes excellent electrical insulators.

3. What is the relationship between the structure of alkanes and their freezing point depression in solutions?

The freezing point depression of a solution depends on the number of particles dissolved, not their identity. When alkanes are dissolved in a solvent, they do not dissociate into smaller particles. Therefore, alkanes generally cause less freezing point depression compared to ionic compounds or molecules that can hydrogen bond with the solvent. The magnitude of freezing point depression is primarily influenced by the concentration of the alkane in solution.

4. How does the refractive index of alkanes change as the number of carbon atoms increases?

The refractive index of alkanes generally increases as the number of carbon atoms increases. This is because larger alkane molecules have more electrons, which can interact more with light, causing greater refraction. The increase in refractive index is not linear and tends to level off for very large alkanes. This property is useful in identifying and characterizing different alkanes.

5. How does the surface tension of alkanes compare to that of water, and why?

The surface tension of alkanes is significantly lower than that of water. This is because alkane molecules are held together by weak van der Waals forces, while water molecules form strong hydrogen bonds. The stronger intermolecular forces in water create a greater inward pull on surface molecules, resulting in higher surface tension. The lower surface tension of alkanes contributes to their spreading behavior on surfaces.

6. Why do alkanes have lower dielectric constants compared to polar solvents?

Alkanes have lower dielectric constants compared to polar solvents because they are nonpolar molecules. The dielectric constant is a measure of a substance's ability to store electrical energy in an electric field. Polar molecules, like water, can align their dipoles with an external electric field, storing more energy and resulting in higher dielectric constants. Alkanes, being nonpolar, cannot align in this way and thus have low dielectric constants, making them poor solvents for ionic compounds.

7. Why are alkanes insoluble in polar solvents but soluble in nonpolar solvents?

Alkanes are insoluble in polar solvents but soluble in nonpolar solvents due to the "like dissolves like" principle. Alkanes, being nonpolar, cannot form strong interactions with polar solvent molecules. However, they can form weak van der Waals interactions with nonpolar solvent molecules, allowing them to dissolve. This is why alkanes are soluble in solvents like hexane or carbon tetrachloride but not in water or ethanol.

8. Why do alkanes have lower specific heat capacities compared to water?

Alkanes have lower specific heat capacities compared to water because they lack the ability to form hydrogen bonds. Water molecules can form extensive hydrogen bonding networks, which require significant energy to break, resulting in a high specific heat capacity. Alkanes, with only weak van der Waals forces between molecules, require less energy to increase their temperature, leading to lower specific heat capacities.

9. What is the relationship between the structure of alkanes and their compressibility?

The compressibility of alkanes is related to their molecular structure. Generally, alkanes are more compressible than many other liquids due to the weak intermolecular forces between molecules. Branched alkanes tend to be slightly more compressible than their straight-chain isomers because they pack less efficiently, leaving more intermolecular space that can be reduced under pressure.

10. Why do alkanes have lower heats of vaporization compared to alcohols with similar molecular weights?

Alkanes have lower heats of vaporization compared to alcohols with similar molecular weights because they lack hydrogen bonding. Alcohols form hydrogen bonds between molecules, which require more energy to break during vaporization. Alkanes only have weak van der Waals forces between molecules, which require less energy to overcome. This results in lower heats of vaporization for alkanes compared to alcohols of similar size.

11. How does the optical rotation of alkanes compare to that of other organic compounds?

Alkanes do not exhibit optical rotation because they lack chiral centers (unless they have a chiral substituent). Optical rotation occurs when a molecule is chiral, meaning it has a non-superimposable mirror image. Simple alkanes have highly symmetrical structures and do not possess this property. This is in contrast to many other organic compounds, such as sugars or amino acids, which can rotate plane-polarized light due to their chiral nature.

12. Why do alkanes have lower enthalpies of fusion compared to compounds capable of hydrogen bonding?

Alkanes have lower enthalpies of fusion compared to compounds capable of hydrogen bonding because they only have weak van der Waals forces between molecules. The enthalpy of fusion is the energy required to convert a substance from solid to liquid at its melting point. Compounds that can form hydrogen bonds, like water or alcohols, require more energy to break these strong intermolecular forces during melting, resulting in higher enthalpies of fusion.

13. What is the relationship between the structure of alkanes and their sound transmission properties?

The sound transmission properties of alkanes are related to their molecular structure and density. Alkanes generally have lower sound transmission speeds compared to water or metals due to their lower density and weaker intermolecular forces. The speed of sound in alkanes increases slightly with increasing molecular weight due to the increase in density. This property is relevant in applications such as ultrasound imaging and acoustic sensors.

14. How does the diffusion rate of alkanes in air change with increasing molecular weight?

The diffusion rate of alkanes in air decreases with increasing molecular weight. This is because larger alkane molecules have greater mass and larger cross-sectional areas, which results in more frequent collisions with air molecules. These factors reduce the average speed and mean free path of the alkane molecules, leading to slower diffusion rates. This trend is consistent with Graham's law of diffusion, which states that the rate of diffusion is inversely proportional to the square root of molecular mass.

15. What is the relationship between the structure of alkanes and their ability to form clathrates?

Alkanes, particularly smaller ones like methane, can form clathrates (inclusion compounds) with water under specific conditions of low temperature and high pressure. The water molecules form cage-like structures around the alkane molecules. Larger alkanes are less likely to form clathrates due to their size. The ability to form clathrates is related to the molecular size and shape of the alkane, with smaller, more spherical molecules being more conducive to clathrate formation.

16. How does branching affect the boiling point of alkanes?

Branching in alkanes generally lowers their boiling point compared to their straight-chain isomers with the same number of carbon atoms. This is because branched alkanes have a more compact shape, reducing their surface area and weakening intermolecular van der Waals forces. As a result, less energy is required to overcome these forces, leading to lower boiling points.

17. How does the volatility of alkanes change with increasing molecular weight?

The volatility of alkanes decreases with increasing molecular weight. This is because larger alkane molecules have stronger intermolecular forces, requiring more energy to overcome these forces and enter the gas phase. As a result, smaller alkanes like methane and ethane are highly volatile gases at room temperature, while larger alkanes like octane are less volatile liquids.

18. What is the trend in density for alkanes as molecular weight increases?

The density of alkanes generally increases as molecular weight increases. This trend is due to the greater mass of larger alkane molecules relative to their volume. However, the increase in density is not linear and tends to level off for very large alkanes. Liquid alkanes have densities less than water, which is why they float on water.

19. How does the melting point of alkanes change with increasing chain length?

The melting point of alkanes increases with increasing chain length. This is because longer alkane chains have stronger van der Waals forces between molecules due to their larger surface area. More energy is required to overcome these intermolecular forces and change the solid to a liquid, resulting in higher melting points for longer-chain alkanes.

20. How does the viscosity of alkanes change as molecular weight increases?

The viscosity of alkanes increases as molecular weight increases. This is because larger alkane molecules have stronger intermolecular forces (van der Waals) due to their increased surface area. These stronger forces result in greater resistance to flow, making the liquid more viscous. This trend is observed in the progression from light, low-viscosity alkanes like pentane to heavier, more viscous alkanes like motor oil.

21. What are the main physical properties that distinguish alkanes from other hydrocarbons?

The main physical properties that distinguish alkanes are their nonpolar nature, low reactivity, and relatively low melting and boiling points compared to other hydrocarbons. Alkanes are insoluble in water due to their nonpolar C-C and C-H bonds, have low reactivity because of their stable single bonds, and their melting and boiling points increase with molecular weight due to increasing van der Waals forces.

22. Why do alkanes have low solubility in water?

Alkanes have low solubility in water because they are nonpolar molecules, while water is polar. The nonpolar nature of alkanes is due to their C-C and C-H bonds having similar electronegativity values, resulting in an even distribution of electrons. Water molecules, being polar, cannot form strong interactions with nonpolar alkanes, leading to their low solubility in water.

23. How does the boiling point of alkanes change as the number of carbon atoms increases?

The boiling point of alkanes increases as the number of carbon atoms increases. This is because larger alkane molecules have stronger van der Waals forces between them due to their increased surface area. More energy is required to overcome these intermolecular forces, resulting in higher boiling points for alkanes with more carbon atoms.

24. What is the relationship between the physical state of alkanes and their molecular weight?

The physical state of alkanes is directly related to their molecular weight. At room temperature, alkanes with 1-4 carbon atoms are gases, those with 5-17 carbon atoms are liquids, and those with 18 or more carbon atoms are solids. This trend is due to the increasing strength of intermolecular forces as molecular weight increases, affecting their melting and boiling points.

25. Why are alkanes considered nonpolar molecules?

Alkanes are considered nonpolar molecules because the electronegativity difference between carbon and hydrogen atoms in C-H bonds is very small. This results in an even distribution of electrons throughout the molecule, with no significant charge separation. The symmetrical arrangement of atoms in alkanes also contributes to their nonpolar nature.

26. How does the speed of sound in alkanes change with increasing molecular weight?

The speed of sound in alkanes generally increases slightly with increasing molecular weight. This is primarily due to the increase in density as molecular weight increases. Sound waves travel faster in denser media because the molecules are closer together, allowing for quicker transfer of vibrations. However, the increase is not dramatic because the intermolecular forces in alkanes remain relatively weak regardless of size.

27. How does the vapor pressure of alkanes change with increasing molecular weight?

The vapor pressure of alkanes decreases with increasing molecular weight. This is because larger alkane molecules have stronger intermolecular forces (van der Waals) due to their increased surface area. More energy is required for these molecules to overcome these forces and enter the gas phase. Consequently, smaller alkanes like butane have higher vapor pressures than larger alkanes like octane at the same temperature.

28. How does the thermal conductivity of alkanes compare to that of metals, and why?

The thermal conductivity of alkanes is much lower than that of metals. This is because heat transfer in alkanes occurs primarily through molecular vibrations and collisions, which are less efficient than the movement of free electrons in metals. The weak intermolecular forces in alkanes limit their ability to quickly transfer thermal energy. This property makes alkanes good thermal insulators, while metals are good thermal conductors.

29. How does the critical temperature of alkanes change as the number of carbon atoms increases?

The critical temperature of alkanes increases as the number of carbon atoms increases. This is because larger alkane molecules have stronger intermolecular forces due to their increased surface area. More energy is required to overcome these forces and prevent condensation at high pressures. As a result, the critical temperature, above which a gas cannot be liquefied by pressure alone, increases for larger alkanes.

30. How does the molar volume of alkanes change as the number of carbon atoms increases?

The molar volume of alkanes increases as the number of carbon atoms increases. This is because each additional carbon atom adds to the overall size of the molecule. However, the increase is not perfectly linear due to factors such as molecular shape and packing efficiency. Branched alkanes tend to have slightly smaller molar volumes than their straight-chain isomers with the same number of carbon atoms due to more efficient packing.

31. What is the relationship between the structure of alkanes and their coefficient of thermal expansion?

The coefficient of thermal expansion for alkanes is generally higher than that of many other substances due to their weak intermolecular forces. As temperature increases, the increased molecular motion easily overcomes these weak forces, leading to significant expansion. Branched alkanes typically have slightly higher coefficients of thermal expansion than their straight-chain isomers because their less efficient packing allows for more expansion when heated.

32. How does the heat capacity ratio (γ = Cp/Cv) of alkanes change with increasing molecular weight?

The heat capacity ratio (γ) of alkanes generally decreases with increasing molecular weight. This ratio is the relationship between heat capacity at constant pressure (Cp) and constant volume (Cv). As molecular weight increases, alkanes have more degrees of freedom for energy storage, particularly in vibrational modes. This leads to a larger increase in Cv relative to Cp, causing the ratio to decrease. The trend is most noticeable when comparing small alkanes (like methane) to larger ones.

33. Why do alkanes have lower heats of solution in water compared to polar organic compounds?

Alkanes have lower heats of solution in water compared to polar organic compounds because they cannot form hydrogen bonds or strong dipole-dipole interactions with water molecules. When an alkane dissolves in water, it disrupts the hydrogen bonding network of water without forming new, strong interactions. This process is energetically unfavorable, resulting in low solubility and low heats of solution. Polar organic compounds, in contrast, can form new hydrogen bonds or dipole-dipole interactions with water, leading to higher heats of solution.

34. How does the polarizability of alkanes change as the number of carbon atoms increases?

The polarizability of alkanes increases as the number of carbon atoms increases. Polarizability refers to the ease with which the electron cloud of a molecule can be distorted by an external electric field. Larger alkane molecules have more electrons and a larger electron cloud, making them more susceptible to distortion. This increased polarizability contributes to stronger van der Waals forces between larger alkane molecules, affecting properties like boiling point and viscosity.

35. What is the relationship between the structure of alkanes and their acoustic absorption properties?

The acoustic absorption properties of alkanes are related to their molecular structure and viscosity. Generally, alkanes have relatively low acoustic absorption compared to more polar liquids. As the molecular weight of alkanes increases, their viscosity also increases, which can lead to slightly higher acoustic absorption due to increased internal friction. However, the effect is not dramatic because the intermolecular forces in alkanes remain relatively weak.

36. How does the surface energy of alkanes compare to that of metals, and why?

The surface energy of alkanes is much lower than that of metals. This is because alkanes have weak intermolecular forces (van der Waals) compared to the strong metallic bonds in metals. Surface energy is a measure of the excess energy at the surface of a material due to the imbalance of molecular forces. In metals, breaking the strong metallic bonds to create a surface requires significant energy, resulting in high surface energy. Alkanes, with their weak intermolecular forces, require much less energy to create a surface, leading to lower surface energy.

37. Why do alkanes have lower dipole moments compared to other organic compounds?

Alkanes have very low or zero dipole moments compared to other organic compounds because they are composed of nonpolar C-C and C-H bonds. The electronegativity difference between carbon and hydrogen is small, resulting in an even distribution of electron