9.2 Hydrocarbons

This question is about the reactions of alkanes.

Decane can be cracked to form heptane and one other product in the following reaction:

C₁₀H₂₂   →   C₇H₁₆   + ____________

Name and draw the displayed formula of the other product formed.

Describe and give the result of a chemical test that would distinguish between the two products.

Explain why decane is a saturated hydrocarbon.

Another alkane, butane, can undergo the following combustion reactions:

   Equation 1:  2C₄H₁₀ + 13O₂ ⟶ 8CO₂ + 10H₂O 

   Equation 2:     2C₄H₁₀ + 5O₂ ⟶ 8C + 10H₂O 

Explain why different products are made.

Alkanes can be obtained from the fractional distillation of crude oil.

Figure 1 shows some alkanes that are found in crude oil.

What is the general formula for alkanes?

Write the balanced symbol equation for the complete combustion of C₇H₁₆. 

Explain the trend in boiling points of the alkanes.

A student was investigating the properties of the alkanes shown in Figure 2 in part (a) using the following method:

1) Pour 20 cm³ of C₅H₁₂ into a funnel 

2) Open the tap and start a stopwatch 

3) When the level of C₅H₁₂ reaches line X stop the stopwatch 

4) Repeat steps with the other two alkanes 

The diagram in Figure 2 shows the apparatus used.

Figure 2

C₅H₁₂ took 5.3 seconds to reach line X.

Explain the trend you would expect to see when the student repeats the method with the other two alkanes.

Cracking is used break long chain hydrocarbons down into small more useful hydrocarbons. 

Figure 1 shows the reaction that occurs when a long chain hydrocarbon, C₁₈H₃₈ undergoes cracking. 

C₁₈H₃₈       →       C₁₁H₂₄     +      C₃H₆      +       2C₂H₄ 

compound  W         compound X   compound Y    compound Z      

Figure 1 

Give the empirical formula of compound W.

Write a balanced symbol equation for the incomplete combustion of compound Y to produce carbon monoxide and water.

Compound Y undergoes a reaction with bromine water. 

(1)

(1)

Compare the structures and bonding of compounds X and Y. 

Alkanes and alkenes are hydrocarbons.

The structure of a molecule of butane is shown.

The graph in Figure 13 shows the boiling points of some alkanes plotted against the number of carbon atoms in one molecule of each alkane.

Figure 13

A molecule of pentane contains five carbon atoms.

Use the graph to estimate the boiling point of pentane.

boiling point of pentane = .............................................................. °C

Figure 14 shows some information about the alkenes, ethene and propene.

Complete the table. The structure of propene must show all covalent bonds.

name of alkene	molecular formula	structure
ethene
propene	C3H6

Alkenes burn completely to produce carbon dioxide and water.

Balance the equation for the complete combustion of butene gas, C₄H_8.

C₄H₈ + .......... O₂ → .......... CO₂ + .......... H₂

Butene reacts with steam to produce butanol.

C₄H₈ + H₂O → C₄H₉OH

(3)

mass of butanol = .............................................................. kg

A sample of each of three hydrocarbons, X, Y and Z, was shaken with bromine water. Bromine water is orange coloured.

The results are:

X orange mixture becomes colourless
Y orange mixture becomes colourless
Z mixture remains orange

Using the results, comment on the structures of the hydrocarbons X, Y and Z.

Alkenes are used to make polymers.

Figure 15 shows the repeating unit of a polymer.

Figure 15

Draw the structure of a molecule of the monomer that was used to produce this polymer.

Bottles can be made of polymers, such as poly(ethene), and of glass.

Give one advantage of a bottle made of a polymer rather than a bottle made of glass.

Which of the following shows the product of the reaction between propene and bromine?

	☐	A
	☐	B
	☐	C
	☐	D

Figure 8 shows one molecule of each of four different substances, A, B, C and D.

Figure 8

(1)

(1)

The amount of oxygen in the atmosphere has increased since the Earth’s early atmosphere was formed.

Explain what has caused this change.

Carbon dioxide is present in the Earth’s atmosphere.

Some processes increase the amount of carbon dioxide in the atmosphere, other processes decrease it.

Draw one straight line from each change in the amount of carbon dioxide in the atmosphere to the process causing the change.

Figure 9 shows a graph of the amount of carbon dioxide in the Earth’s atmosphere from 1985 to 2005.

(2)

change in amount = .................................... ppm

Which of the following molecules is an alkene?

Propene can be produced by the cracking of some hydrocarbons obtained from crude oil.

 The equation shows the cracking of one molecule of decane to produce one molecule of butene and one molecule of another product.

The structure of a molecule of ethene is shown in Figure 8.

Substance X is an unsaturated hydrocarbon.

 The structure of a molecule of substance X is shown in Figure 10.

Explain how the structure of substance X shows that it is an unsaturated hydrocarbon.

Two liquid hydrocarbons, A and B, were tested with bromine water.

One hydrocarbon was known to be an alkane.

The other hydrocarbon was known to be an alkene.

Each hydrocarbon was shaken with a few drops of bromine water.

 The results of the tests were

Explain these results.

The complete combustion of pentane, C₅H₁₂, takes place in the following unbalanced equation:

_C₅H₁₂   +   _O₂ →   _CO₂   +  _H₂O

What numbers will balance the reaction?

Ethene, C₂H₄, is an unsaturated hydrocarbon.

A sample of ethene is burned completely in oxygen.

Write the balanced equation for this reaction.

Ethene can be polymerised to form poly(ethene).

Describe what you would see when a sample of ethene and a sample of poly(ethene) are shaken with separate, small volumes of bromine water.

A different hydrocarbon has a relative formula mass of 84.
It has an empirical formula of CH₂.

Deduce the molecular formula of this hydrocarbon.

You must show your working.
(relative atomic masses : H=1, C=12)

Bromine water is an orange-brown solution.

When shaken with bromine water, which of the following compounds would cause the solution to decolourise?