S B C9 C O a F, Slides of Chemistry

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Lesson 1

Hydrocarbons and alkanes

What are hydrocarbons and alkanes?

Hydrocarbons are compounds that are made of carbon and hydrogen atoms only ; these atoms are covalently bonded together. Many hydrocarbons are used as fuels as they are a chemical store of energy and they burn in a controlled way.

Alkanes are an example of a homologous series that have similar chemical properties and have the same general formula of C n H 2 n +2 , where n is the number of carbon atoms. This means that if there are 4 carbon atoms, there will be (2x 4 )+2 hydrogen atoms giving a formula of C 4 H 10. If there are 6 carbon atoms, there will be (2x 6 )+2 hydrogen atoms giving a formula of C 6 H 14.

Alkanes contain the functional group of a carbon – carbon single bond.

Examples of alkanes

All of the alkanes have the suffix ‘-ane’

1. Methane (top tip! Remember the prefix ‘Me-’ as there is only one of you!)

Methane is the first alkane within the homologous series and has the formula CH 4. This formula shows there is one carbon atom and 4 hydrogen atoms.

**2. Ethane – C 2 H 6

3. Propane – C 3 H 8
4. Butane – C 4 H 10**

BBC bitesize – hydrocarbons and alkanes

Properties of alkanes

The first four alkanes (methane, ethane, propane and butane) are gases at room temperature.

They have very low boiling points because there are weak intermolecular forces of attraction between the molecules. These forces of attraction do not require much energy to be overcome for melting and boiling to happen.

As the number of carbon atoms in a hydrocarbon molecule increases, the length of the carbon chain increases, so does the boiling point of the alkane. This is because there is more surface area of the molecule for the intermolecular forces to form.

Example: State of Pentane and methane at room temperature

Methane is a much smaller molecule than pentane, so there is less surface area for intermolecular forces of attraction in methane. This means that less energy is needed to move molecules of methane apart compared to pentane. Therefore, methane is a gas at room temperature and pentane is a liquid.

You will learn more about the properties of different length hydrocarbons in the next lesson.

Lesson 1 Glossary

Keyword Definition

1. Alkane A homologous series of saturated (contain carbon-carbon single bond) hydrocarbons
2. Boiling point The temperature at which a liquid boils and turns to vapour.
3. Covalent bond A shared pair of electrons between non- metal atoms.
4. Functional group The group of atoms and bonds which characterises a group of organic chemicals.
5. Homologous series A family of compounds that will have the same general formula and similar properties but differ by the number of carbon atoms. Homologous series:
   • differ by CH 2 in molecular formulae from neighbouring compounds
   • show a gradual variation in physical properties, such as their boiling points
   • have similar chemical properties
6. Hydrocarbon A compound made up of carbon and hydrogen atoms only.
7. Intermolecular forces Interactions between molecules that determine a compounds melting/boiling point.

Exam questions

Q1.

This question is about hydrocarbons.

(a) The names and formulae of three hydrocarbons in the same homologous series are:

Ethane C 2 H 6

Propane C 3 H 8

Butane C 4 H 10

The next member in the series is pentane.

What is the formula of pentane? (1)

(b) Which homologous series contains ethane, propane and butane? (1)

Tick one box.

Alcohols

Alkanes

Alkenes

Carboxylic acids

(c) Octane (C 8 H 18 ) is a hydrocarbon found in petrol.

Explain why octane is a hydrocarbon. (2)

Q2.

This question is about compounds produced from crude oil.

The table below shows four of these compounds.

Compound Melting point in °C Boiling point in °C

methane (CH 4 ) −183 −

ethene (C 2 H 4 ) −169 −

decane (C 10 H 22 ) −30 +

icosane (C 20 H 42 ) +37 +

(a) Tick ( ) two correct statements about the four compounds. (2)

Statement Tick ( )

Methane has the lowest melting point and icosane has the highest boiling point.

Ethene and methane are alkanes.

Methane and decane are gases at room temperature (20°C).

Decane and icosane are liquid at 100°C.

Q4.

The table shows some information about alkanes.

Name Formula Relative formula mass Boiling point in °C

methane CH 4 16 –

ethane C 2 H 6 30 –

propane 44 –

butane C 4 H 10 58

pentane C 5 H 12 72 36

hexane C 6 H 14 86 68

(a) Give the formula of propane. (1)

(b) What happens to the boiling points of the alkanes as the relative formula mass increases? (1)

(c) Draw a graph. Plot the points and draw a best fit line. (3)

(d) What is the boiling point of butane? (1)

(e) Show clearly on the graph how you found the boiling point of butane. (1)

(f) Circle which of the following is not an alkane. (1)

C 71116 C 9 H 18 C 11 H 24 C 24 H 50

1. What is the correct formula of butane? a. C 4 b. C 4 H 8 c. C 4 H 10 2. Which compound could be described as a hydrocarbon and an alkane? a.

b.

c.

3. What does this line represent? (see purple arrow)

a. An ionic bond between carbon and hydrogen atoms b. A single covalent bond between carbon and hydrogen atoms c. A double covalent bond between carbon and hydrogen atoms

If you answered A , you are mistakenly thinking that butane only contains carbon atoms. Butane is a hydrocarbon and contains carbon and hydrogen atoms only. There are 4 carbon atoms, you are correct in thinking this but there are also 10 hydrogen atoms. To ‘fix it’, write the formula of ethane, pentane and heptane.

If you answered B , you have used the general formula CnH2n+2 incorrectly and have doubled the number of carbon atoms but have not added two additional carbon atoms after doubling. To ‘fix it’, if pentane has 5 carbon atoms use the general formula CnH2n+2 to calculate the number of hydrogen atoms.

If you answered C , you think that the formula of butane is C 4 H 10. This is correct! Well done for correctly using the general formula CnH2n+2 to determine the formula. To stretch your knowledge further, explain why the formula of dodecane is C 12 H 26.