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Boiling Points of Alkanes

Reminder about Alkanes:

Alkanes are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) in proportions according to the general formula: CnH(2n+2) where the letter n represents the number of carbon atoms in each molecule. The atoms that form alkanes are linked exclusively by single bonds, hence alkanes are saturated hydrocarbons.

In general there are three basic types of alkanes: Linear alkanes, branched alkanes and cyclic alkanes (which may also be referred to as cycloalkanes). Of these, linear alkanes are the simplest to draw and explain and are therefore usually the first to be introduced in chemistry lessons.

Definitions of Boiling Point:

A simple definition of a boiling point is the temperature at which a substance (e.g. an element or compound) changes state from liquid to gas.

There are also more technical (scientific) definitions of the term "boiling point",
and also some more specific definitions of "boiling points".

*

(Stricter) Physical Chemistry
Definition of "Boiling Point":

... the temperature at which the vapor pressure of a liquid equals the environmental pressure surrounding the liquid.

 

 

(Hence the boiling point of a substance varies according to its circumstances.
A memorable example may be that of boiling water to make tea while up a very high mountain: Due to the lower air pressure, the water boils at a slightly lower temperature, hence the tea may not brew as well as at sea level.
)

*

= "normal boiling point"
= "atmospheric boiling point"
= "atmospheric pressure boiling point"

... the special case of the temperature at which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level.

*

The Standard Boiling Point defined by the IUPAC

... the temperature at which boiling occurs under a pressure of 1 bar.

As boiling points are temperatures they may be expressed in degrees Fahrenheit, degrees Celsius or degrees Kelvin.
When stating boiling points remember to specify the units - without which a number alone is meaningless!

Boiling versus Evapouration :
Liquids may change state to vapors (gases) at temperatures below the boiling point of the substance via the process of evaporation, which is a surface effect in which molecules located near the vapor/liquid surface escape into the vapor (gas) phase. This is in contrast to the process of boiling in which molecules of the substance located anywhere in the liquid - including well below its surface - escape, by forming vapor bubbles within the liquid, which then rise to the surface to leave the liquid phase.

 

Table of Boiling Points of Linear Alkanes:

Name of Alkane
Chemical Formula

Methane

1

C H4

-162

gas

-183

Ethane

2

C2H6

-89

gas

-172

Propane

3

C3H8

-42

gas

-188

n-Butane

4

C4H10

0

gas

-138

n-Pentane

5

C5H12

36

liquid

-130

n-Hexane

6

C6H14

69

liquid

-95

n-Heptane

7

C7H16

98

liquid

-91

n-Octane

8

C8H18

126

liquid

-57

n-Nonane

9

C9H20

151

liquid

-54

n-Decane

10

C10H22

174

liquid

-30


Chart of Boiling Points plotted against Number of Carbon Atoms in Linear Alkanes:

lt may be easier to see the trend in boiling points across the homologous series of alkanes by plotting the temperatures of the boiling points of the alkanes against the number of carbon atoms in each (linear) alkane. The following chart of boiling points of alkanes against the number of carbon atoms in each includes values for larger alkanes with up to 50 carbon atoms.

Chart showing the boiling points of alkanes and also the melting points of alkanes.

Discussion of Chart of melting and boiling points of alkanes:

  1. Actual values plotted on the above chart are represented by red squares (in the case of boiling points) and green triangles (in the case of melting points). Values have been plotted for each of the first 12 linear alkanes and also for linear alkanes with each of 20, 30, 40 and 50 carbon atoms.
  2. The dotted lines in red (between the points representing boiling points) and in green (between the points representing melting points) indicate likely approximate values for the alkanes with numbers of carbon atoms in their chains for which specific values were not available with which to draw the chart.
  3. Why include both melting points and boiling points ?
    There are three main states of matter: solid, liquid and gas.
    Plotting both melting points and boiling points on the same graph enables one to use the graph to decide if a particular linear alkane (position along the x-axis) is likely* to be a solid, liquid or gas at any particular temperature (position aling the y-axis).
    * We cannot draw absolutely accurate conclusions about states of matter from this graph because boiling points vary according to pressure as well as temperature and the information on the above graph does not mention pressures, or even include actual values for all linear alkanes.

    Example
    : What is the state (solid, liquid or gas) of each alkane compound at 20oC ?
    A dashed line is shown to represent the condition "temperature = 20oC", which is a useful temperature to discuss because room temperature is typically around 20oC.

    At 20oC :
    • Both the red line (representing boiling points) and the green line (representing melting points) are below the dashed black line (representing temperature = 20oC) for values of n="number of carbon atoms forming the linear alkane chain" < 5.
      Hence for values n=1 to 4 (methane, ethane, propane and butane), the linear alkane is likely to be a gas at approx. 20oC.
    • The dashed black line (representing temperature = 20oC) is vertically between the red line (representing boiling points) above it, and the green line (representing melting points) below it, for values of n="number of carbon atoms forming the linear alkane chain" of between n=5 (pentane) and n=17.
      Hence for values of n=5 to 17, the linear alkane is likely to be a liquid at approx. 20oC.
    • Both the red line (representing boiling points) and the green line (representing melting points) are above the dashed black line (representing temperature = 20oC) for values of n="number of carbon atoms forming the linear alkane chain" > approximately 17.
      Hence for values of n=18 and above, the linear alkane is likely to be a solid at approx. 20oC.

  4. The shapes of the red line (representing boiling points) and the green line (representing melting points)
    describe visually the trends, that is - how boiling points and melting points vary with n="number of carbon atoms forming the linear alkane chain" in the range of values shown. In general such trends can also be described mathematically by equations describing the curve that best fits each series of points - not required for UK GCSE Chemistry.
  5. Why only plot resuts for Linear Alkanes ?
    This is a simplification to make the chart easier to explain and discuss.
    The boiling (and melting) points of branched alkanes differ from those of linear alkanes. Recall that alkanes that include 4 or more carbon atoms show structural isomerism, meaning that there are two or more different structural formulae that you can draw for each molecular formula. Structural isomers of alkanes may include cycloalkanes and various branched alkanes. Each isomer has a different melting and boiling point. As the number of carbon atoms in alkanes increase the number of different isomers increases dramatically. To give an indication of the variation in boiling points of different isomers of the same alkane, cycloalkanes have boiling points typically around 10oC to 20oC higher than the corresponding linear alkane.

 

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