Bonding in alkanes

Definition

Alkane

A saturated hydrocarbon in which every bond between atoms is a single covalent bond.

Alkanes are a family of hydrocarbons, so their molecules contain carbon and hydrogen atoms only.
Every bond in an alkane molecule is a single covalent bond.
1. A single covalent bond is one shared pair of electrons between two atoms.
2. The carbon atoms are joined to each other by $C−C\text{C}-\text{C}$ single bonds, and the hydrogen atoms are joined to carbon by $C−H\text{C}-\text{H}$ single bonds.
Each carbon atom forms four single bonds and each hydrogen atom forms one.
Alkanes are saturated hydrocarbons, because they contain only single bonds between their carbon atoms.
1. Saturated means that every carbon atom already holds the maximum number of atoms it can bond to, so no more atoms can be added to the molecule.
The names of alkanes all end in -ane, such as methane, ethane and propane.

Example

Bonds in ethane and propane

Ethane, $C2H6\text{C}_2\text{H}_6$ , has one $C−C\text{C}-\text{C}$ bond and six $C−H\text{C}-\text{H}$ bonds, all of them single.

\begin{array}{ccccccc} & & \text{H} & & \text{H} & & \\ & & | & & | & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{H} \\ & & | & & | & & \\ & & \text{H} & & \text{H} & & \end{array}

Propane, $C3H8\text{C}_3\text{H}_8$ , has two $C−C\text{C}-\text{C}$ bonds and eight $C−H\text{C}-\text{H}$ bonds.

\begin{array}{ccccccccc} & & \text{H} & & \text{H} & & \text{H} & & \\ & & | & & | & & | & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{C} & - & \text{H} \\ & & | & & | & & | & & \\ & & \text{H} & & \text{H} & & \text{H} & & \end{array}

Counting the lines around any carbon atom gives four, which confirms that every bond is single and the molecule is saturated.

Reactivity of alkanes

Alkanes are generally unreactive, so they do not react with most common substances.
1. They do not react with acids, with alkalis or with most metals, even when warmed.
2. Their $C−C\text{C}-\text{C}$ and $C−H\text{C}-\text{H}$ bonds are strong single bonds that are hard to break.
Alkanes take part in only two types of reaction.
1. Combustion, in which the alkane burns in oxygen and releases a large amount of thermal energy.
2. Substitution by chlorine, in which chlorine atoms replace hydrogen atoms, but only when ultraviolet light is present.
Because they are unreactive apart from burning, alkanes can be stored and transported safely, which makes them useful as fuels.

Complete combustion

Definition

Complete combustion

The burning of a fuel in a plentiful supply of oxygen, so that all of the carbon in the fuel is converted into carbon dioxide.

Complete combustion happens when an alkane burns in a plentiful supply of oxygen, such as plenty of air.
The products of complete combustion are carbon dioxide and water.
$\text{alkane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
All the carbon atoms end up in $CO2\text{CO}_2$ and all the hydrogen atoms end up in $H2O\text{H}_2\text{O}$ .
Combustion is exothermic, so it gives out thermal energy and the surroundings get hotter.
Methane burns completely with a clean blue flame.
$\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$

Example

Balancing the combustion of propane

Write the formulae of the reactants and products first.

\text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

Balance carbon first, so three carbon atoms give $3CO23\text{CO}_2$ .
Balance hydrogen next, so eight hydrogen atoms give $4H2O4\text{H}_2\text{O}$ .
Balance oxygen last, so the products contain $6 + 4 = 10$ oxygen atoms, which need $5O25\text{O}_2$ .

\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

Octane, $C8H18\text{C}_8\text{H}_{18}$ , needs $121212\tfrac{1}{2}$ oxygen molecules, so you double every number to remove the fraction.

2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}

Incomplete combustion

Definition

Incomplete combustion

The burning of a fuel in a limited supply of oxygen, which forms carbon monoxide or carbon instead of carbon dioxide.

Incomplete combustion happens when an alkane burns in a limited supply of oxygen.
There is not enough oxygen to turn all the carbon into carbon dioxide, so carbon monoxide, $CO\text{CO}$ , or carbon, $C\text{C}$ , forms instead.
1. Water is still formed, because the hydrogen atoms are still converted into $H2O\text{H}_2\text{O}$ .
2. The carbon forms as tiny solid particles called soot, which blacken surfaces.
Methane can burn incompletely to form carbon monoxide or carbon.
$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$
1. With even less oxygen, solid carbon forms.
  $\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O}$
Incomplete combustion releases less thermal energy than complete combustion of the same amount of fuel.
The flame is yellow and smoky because glowing soot particles are present.
Carbon monoxide is a toxic gas with no colour or smell, so faulty gas heaters in poorly ventilated rooms are dangerous.

Example

Incomplete combustion of propane

With a limited supply of oxygen, propane forms carbon monoxide and water.

2\text{C}_3\text{H}_8 + 7\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}

There are $6 + 8 = 14$ oxygen atoms on the right, so $7O27\text{O}_2$ is needed on the left.
With even less oxygen, the carbon is released as soot.

\text{C}_3\text{H}_8 + 2\text{O}_2 \rightarrow 3\text{C} + 4\text{H}_2\text{O}

The hydrogen still ends up as water in every case, so only the carbon product changes.

Practical

Testing the products of combustion

Aim: to show that burning a hydrocarbon fuel produces water and carbon dioxide.
Apparatus: small spirit burner or candle, inverted glass funnel, rubber tubing, U-tube standing in a beaker of ice, boiling tube of limewater, water pump or filter pump, anhydrous copper(II) sulfate, clamp stands and eye protection.
Method:
- Clamp the funnel upside down a few centimetres above the burner and connect it by tubing to the U-tube in the ice.
- Connect the outlet of the U-tube to a delivery tube dipping into about $cm35\ \text{cm}^3$ of limewater, and connect the limewater tube to the pump.
- Light the burner and switch on the pump, so that the gases from the flame are drawn through the cold U-tube and then through the limewater.
- Run the apparatus for about five minutes and watch the U-tube and the limewater.
- Add a small amount of the liquid from the U-tube to some white anhydrous copper(II) sulfate.
Results:
- A colourless liquid collects in the cold U-tube and turns white anhydrous copper(II) sulfate blue.
- The limewater turns milky.
Conclusion: the colourless liquid is water and the gas is carbon dioxide, so the fuel contains both hydrogen and carbon.
Safety: wear eye protection, keep loose hair and paper away from the flame, keep the funnel high enough that it does not overheat, and switch off the pump only after the flame is out.

Common Mistake

Do not call alkanes completely unreactive, because they do undergo combustion and substitution with chlorine.
Do not leave water out of the products of incomplete combustion, because the hydrogen still forms $H2O\text{H}_2\text{O}$ .
Do not say that carbon monoxide causes acid rain, because carbon monoxide is toxic and acid rain is caused by other oxides.

Exam technique

Writing about combustion

State the condition as "plentiful oxygen" or "limited oxygen", because just saying "oxygen" does not tell the two types apart.
Name the products in full, writing carbon dioxide and water for complete combustion and carbon monoxide or carbon plus water for incomplete combustion.
Balance a combustion equation in the order carbon, hydrogen, oxygen, and double everything if you end up with half an oxygen molecule.
Describe alkanes as saturated because they have only single bonds, using both key words.

Self review

What type of bond joins the atoms in an alkane?
Why are alkanes described as saturated hydrocarbons?
Name the two types of reaction that alkanes undergo.
What are the products of the complete combustion of methane?
Write a balanced equation for the incomplete combustion of methane to carbon monoxide.

Bonding in alkanes

Definition

Alkane

A saturated hydrocarbon in which every bond between atoms is a single covalent bond.

Alkanes are a family of hydrocarbons, so their molecules contain carbon and hydrogen atoms only.
Every bond in an alkane molecule is a single covalent bond.
1. A single covalent bond is one shared pair of electrons between two atoms.
2. The carbon atoms are joined to each other by $C−C\text{C}-\text{C}$ single bonds, and the hydrogen atoms are joined to carbon by $C−H\text{C}-\text{H}$ single bonds.
Each carbon atom forms four single bonds and each hydrogen atom forms one.
Alkanes are saturated hydrocarbons, because they contain only single bonds between their carbon atoms.
1. Saturated means that every carbon atom already holds the maximum number of atoms it can bond to, so no more atoms can be added to the molecule.
The names of alkanes all end in -ane, such as methane, ethane and propane.

Example

Bonds in ethane and propane

Ethane, $C2H6\text{C}_2\text{H}_6$ , has one $C−C\text{C}-\text{C}$ bond and six $C−H\text{C}-\text{H}$ bonds, all of them single.

\begin{array}{ccccccc} & & \text{H} & & \text{H} & & \\ & & | & & | & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{H} \\ & & | & & | & & \\ & & \text{H} & & \text{H} & & \end{array}

Propane, $C3H8\text{C}_3\text{H}_8$ , has two $C−C\text{C}-\text{C}$ bonds and eight $C−H\text{C}-\text{H}$ bonds.

\begin{array}{ccccccccc} & & \text{H} & & \text{H} & & \text{H} & & \\ & & | & & | & & | & & \\ \text{H} & - & \text{C} & - & \text{C} & - & \text{C} & - & \text{H} \\ & & | & & | & & | & & \\ & & \text{H} & & \text{H} & & \text{H} & & \end{array}

Counting the lines around any carbon atom gives four, which confirms that every bond is single and the molecule is saturated.

Reactivity of alkanes

Alkanes are generally unreactive, so they do not react with most common substances.
1. They do not react with acids, with alkalis or with most metals, even when warmed.
2. Their $C−C\text{C}-\text{C}$ and $C−H\text{C}-\text{H}$ bonds are strong single bonds that are hard to break.
Alkanes take part in only two types of reaction.
1. Combustion, in which the alkane burns in oxygen and releases a large amount of thermal energy.
2. Substitution by chlorine, in which chlorine atoms replace hydrogen atoms, but only when ultraviolet light is present.
Because they are unreactive apart from burning, alkanes can be stored and transported safely, which makes them useful as fuels.

Complete combustion

Definition

Complete combustion

The burning of a fuel in a plentiful supply of oxygen, so that all of the carbon in the fuel is converted into carbon dioxide.

Complete combustion happens when an alkane burns in a plentiful supply of oxygen, such as plenty of air.
The products of complete combustion are carbon dioxide and water.
$\text{alkane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
All the carbon atoms end up in $CO2\text{CO}_2$ and all the hydrogen atoms end up in $H2O\text{H}_2\text{O}$ .
Combustion is exothermic, so it gives out thermal energy and the surroundings get hotter.
Methane burns completely with a clean blue flame.
$\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$

Example

Balancing the combustion of propane

Write the formulae of the reactants and products first.

\text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

Balance carbon first, so three carbon atoms give $3CO23\text{CO}_2$ .
Balance hydrogen next, so eight hydrogen atoms give $4H2O4\text{H}_2\text{O}$ .
Balance oxygen last, so the products contain $6 + 4 = 10$ oxygen atoms, which need $5O25\text{O}_2$ .

\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

Octane, $C8H18\text{C}_8\text{H}_{18}$ , needs $121212\tfrac{1}{2}$ oxygen molecules, so you double every number to remove the fraction.

2\text{C}_8\text{H}_{18} + 25\text{O}_2 \rightarrow 16\text{CO}_2 + 18\text{H}_2\text{O}

Incomplete combustion

Definition

Incomplete combustion

The burning of a fuel in a limited supply of oxygen, which forms carbon monoxide or carbon instead of carbon dioxide.

Incomplete combustion happens when an alkane burns in a limited supply of oxygen.
There is not enough oxygen to turn all the carbon into carbon dioxide, so carbon monoxide, $CO\text{CO}$ , or carbon, $C\text{C}$ , forms instead.
1. Water is still formed, because the hydrogen atoms are still converted into $H2O\text{H}_2\text{O}$ .
2. The carbon forms as tiny solid particles called soot, which blacken surfaces.
Methane can burn incompletely to form carbon monoxide or carbon.
$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$
1. With even less oxygen, solid carbon forms.
  $\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O}$
Incomplete combustion releases less thermal energy than complete combustion of the same amount of fuel.
The flame is yellow and smoky because glowing soot particles are present.
Carbon monoxide is a toxic gas with no colour or smell, so faulty gas heaters in poorly ventilated rooms are dangerous.

Example

Incomplete combustion of propane

With a limited supply of oxygen, propane forms carbon monoxide and water.

2\text{C}_3\text{H}_8 + 7\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}

There are $6 + 8 = 14$ oxygen atoms on the right, so $7O27\text{O}_2$ is needed on the left.
With even less oxygen, the carbon is released as soot.

\text{C}_3\text{H}_8 + 2\text{O}_2 \rightarrow 3\text{C} + 4\text{H}_2\text{O}

The hydrogen still ends up as water in every case, so only the carbon product changes.

Practical

Testing the products of combustion

Aim: to show that burning a hydrocarbon fuel produces water and carbon dioxide.
Apparatus: small spirit burner or candle, inverted glass funnel, rubber tubing, U-tube standing in a beaker of ice, boiling tube of limewater, water pump or filter pump, anhydrous copper(II) sulfate, clamp stands and eye protection.
Method:
- Clamp the funnel upside down a few centimetres above the burner and connect it by tubing to the U-tube in the ice.
- Connect the outlet of the U-tube to a delivery tube dipping into about $cm35\ \text{cm}^3$ of limewater, and connect the limewater tube to the pump.
- Light the burner and switch on the pump, so that the gases from the flame are drawn through the cold U-tube and then through the limewater.
- Run the apparatus for about five minutes and watch the U-tube and the limewater.
- Add a small amount of the liquid from the U-tube to some white anhydrous copper(II) sulfate.
Results:
- A colourless liquid collects in the cold U-tube and turns white anhydrous copper(II) sulfate blue.
- The limewater turns milky.
Conclusion: the colourless liquid is water and the gas is carbon dioxide, so the fuel contains both hydrogen and carbon.
Safety: wear eye protection, keep loose hair and paper away from the flame, keep the funnel high enough that it does not overheat, and switch off the pump only after the flame is out.

Common Mistake

Do not call alkanes completely unreactive, because they do undergo combustion and substitution with chlorine.
Do not leave water out of the products of incomplete combustion, because the hydrogen still forms $H2O\text{H}_2\text{O}$ .
Do not say that carbon monoxide causes acid rain, because carbon monoxide is toxic and acid rain is caused by other oxides.

Exam technique

Writing about combustion

State the condition as "plentiful oxygen" or "limited oxygen", because just saying "oxygen" does not tell the two types apart.
Name the products in full, writing carbon dioxide and water for complete combustion and carbon monoxide or carbon plus water for incomplete combustion.
Balance a combustion equation in the order carbon, hydrogen, oxygen, and double everything if you end up with half an oxygen molecule.
Describe alkanes as saturated because they have only single bonds, using both key words.

Self review

What type of bond joins the atoms in an alkane?
Why are alkanes described as saturated hydrocarbons?
Name the two types of reaction that alkanes undergo.
What are the products of the complete combustion of methane?
Write a balanced equation for the incomplete combustion of methane to carbon monoxide.

11.4.1 Alkanes and their properties

Bonding in alkanes

Bonds in ethane and propane

Reactivity of alkanes

Complete combustion

Balancing the combustion of propane

Incomplete combustion

Incomplete combustion of propane

Testing the products of combustion

Writing about combustion

Review 11.4.1 Alkanes and their properties by teaching Genie

11.4.1 Alkanes and their properties

Bonding in alkanes

Bonds in ethane and propane

Reactivity of alkanes

Complete combustion

Balancing the combustion of propane

Incomplete combustion

Incomplete combustion of propane

Testing the products of combustion

Writing about combustion

Review 11.4.1 Alkanes and their properties by teaching Genie

11.4.1 Alkanes and their properties

Bonding in alkanes

Bonds in ethane and propane

Reactivity of alkanes

Complete combustion

Balancing the combustion of propane

Incomplete combustion

Incomplete combustion of propane

Testing the products of combustion

Writing about combustion

Review 11.4.1 Alkanes and their properties by teaching Genie

11.4.1 Alkanes and their properties Revision Guide

11.4.1 Alkanes and their properties

Bonding in alkanes

Bonds in ethane and propane

Reactivity of alkanes

Complete combustion

Balancing the combustion of propane

Incomplete combustion

Incomplete combustion of propane

Testing the products of combustion

Writing about combustion

Review 11.4.1 Alkanes and their properties by teaching Genie

11.4.1 Alkanes and their properties Revision Guide