Organic Families

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Fossil Fuels & Fractional Distillation

Fossil fuels (coal, natural gas, petroleum) release heat energy when burned.
Petroleum (crude oil) is a mixture of hydrocarbons separated by fractional distillation.
Fractional distillation separates fractions by boiling point in a column with a temperature gradient (hot at bottom, cool at top).
Fractions include: refinery gas (heating/cooking), gasoline (car fuel), naphtha (chemicals), diesel (diesel engines), bitumen (road surfacing).
As chain length increases down the column, boiling point increases due to stronger intermolecular forces.

Fractional distillation of petroleum

Alkanes

Alkanes are saturated hydrocarbons with only single C–C bonds and general formula CₙH₂ₙ₊₂.
They are colourless, generally unreactive due to strong single covalent bonds.
Complete combustion of alkanes produces CO₂ and H₂O (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O).
First five members: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂).

Butane, an alkane

Alkenes & Cracking

Alkenes are unsaturated hydrocarbons with at least one C=C double bond and general formula CₙH₂ₙ.
The C=C bond makes alkenes more reactive than alkanes; it can open to form new bonds.
Cracking breaks long-chain alkanes into shorter alkanes, alkenes, and H₂ using high temperature (600–700 °C) and a catalyst (Al₂O₃ or SiO₂).
Example: decane → octane + ethene (C₁₀H₂₂ → C₈H₁₈ + C₂H₄).
Cracking increases supply of useful fuels (e.g., petrol) and produces alkenes for polymers.

But-1-ene, an alkene

Test for Alkenes

Alkenes are identified by adding aqueous bromine (bromine water), which is orange.
Alkane: no colour change (remains orange).
Alkene: decolourises (orange → colourless) because bromine adds across the C=C bond.

Bromine water test for alkanes and alkenes

Addition Reactions (Extended)

Addition reactions involve atoms adding across the C=C double bond, forming a single saturated product.
Alkene + bromine → dibromoalkane (e.g., ethene + Br₂ → 1,2-dibromoethane).
Alkene + hydrogen (with Ni catalyst) → alkane (e.g., ethene + H₂ → ethane).
Alkene + steam (with acid catalyst) → alcohol (hydration; e.g., ethene + H₂O → ethanol).

Addition reaction of ethene and bromine

Alcohols

Alcohols contain the hydroxyl (–OH) functional group and have general formula CₙH₂ₙ₊₁OH.
First three: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH).
Ethanol is used in alcoholic drinks, as a fuel, and as a solvent.
Ethanol undergoes complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Ethanol, an alcohol

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Câu hỏi luyện tập

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1.What is the main constituent of natural gas?
Easy
- Aethane
- Bpropane
- Cmethane
- Dbutane
2.State the general formula of the alkanes.
Easy
3.Alkenes are saturated hydrocarbons.
Easy
True or false?
4.Complete the sentence.
Easy
The process of breaking down long-chain hydrocarbons into shorter, more useful molecules is called ____.
5.How many carbon atoms are in a molecule of butane?
Easy
- A3
- B5
- C4
- D6
6.Which of the following is the product of the complete combustion of methane?
Easy
- Acarbon monoxide and water
- Bcarbon dioxide and water
- Ccarbon and hydrogen
- Dcarbon dioxide and hydrogen
7.What colour change is observed when bromine water is added to an alkene?
Easy
8.Cracking requires a high temperature of around 600–700 °C and a catalyst such as aluminium oxide.
Medium
True or false?

Thêm câu hỏi

9.In the addition reaction of ethene with steam, what is the product?
Medium
- Aethane
- Bethanol
- Cethanoic acid
- Dethene oxide
10.State the functional group present in alcohols.
Easy
11.Arrange the following alkanes in order of increasing boiling point (lowest first).
Medium
- ethane
- methane
- propane
- butane
12.The diagram shows the structure of ethanol. Name the functional group shown in red.
Easy
13.Ethanol can be produced by the hydration of ethene using an acid catalyst.
Medium
True or false?
14.Complete the sentence.
Medium
The addition of hydrogen to an alkene to form an alkane requires a ____ catalyst.
15.How many hydrogen atoms are in a molecule of pentane?
Easy
- A11
- B12
- C13
- D14
16.Alkanes are unsaturated hydrocarbons.
Easy
True or false?
17.Which of the following are products of the complete combustion of an alkane? (Select all that apply.)
Medium
- Acarbon dioxide
- Bwater
- Ccarbon monoxide
- Dsoot
18.A sample of hydrocarbon X is mixed with bromine water and shaken. The bromine water remains orange. Which statement about X is correct?
Medium
- AX is an alkene
- BX is a saturated hydrocarbon
- CX contains a carbon-carbon double bond
- DX is an unsaturated hydrocarbon
19.Which of the following are products of the complete combustion of ethanol? (Select all that apply.)
Medium
- Acarbon dioxide
- Bcarbon monoxide
- Cwater
- Doxygen
- Enitrogen
20.In the fractional distillation of petroleum, the fraction with the highest boiling point condenses at the top of the column.
Medium
True or false?
21.Match each alkane to its molecular formula.
Easy
- methane
- ethane
- propane
- C2H6
- C3H8
- CH4
22.Arrange the following fractions of petroleum in order of increasing boiling point (lowest first).
Easy
- gasoline
- bitumen
- refinery gas
- diesel
23.The hydration of ethene to produce ethanol is an addition reaction. Which conditions are required?
Medium
- Ahigh temperature and a nickel catalyst
- Bhigh temperature and an acid catalyst
- Clow temperature and a nickel catalyst
- Dlow temperature and an acid catalyst
24.Which of the following is the correct general formula for alkenes?
Easy
- ACnH2n+2
- BCnH2n
- CCnH2n-2
- DCnH2n+1OH
25.A student carries out a cracking experiment using a long-chain alkane. Which piece of apparatus is most suitable to collect the gaseous products?
Easy
- Abeaker
- Bmeasuring cylinder
- Cgas syringe
- Dconical flask
26.In an experiment to test for unsaturation, a student adds bromine water to two hydrocarbons, X and Y. X decolourises bromine water, Y does not. What can be concluded about X and Y?
Medium
- AX is an alkane, Y is an alkene
- BX is an alkene, Y is an alkane
- CBoth X and Y are alkanes
- DBoth X and Y are alkenes
27.Which of the following are required for the cracking of long-chain alkanes? (Select all that apply.)
Medium
- Ahigh temperature
- Blow pressure
- Ccatalyst such as aluminium oxide
- Dwater
- Eoxygen
28.In an experiment to investigate the effect of chain length on boiling point, a student measures the boiling points of pentane, hexane, and heptane. Which of the following is the most likely result?
Medium
- Apentane has the highest boiling point
- Bheptane has the highest boiling point
- Call three have the same boiling point
- Dhexane has the highest boiling point
29.What is the general formula of alkanes?
Easy
- ACnH2n+2
- BCnH2n
- CCnH2n-2
- DCnH2n+1
30.Methane is the main constituent of natural gas.
Easy
True or false?
31.Match each fraction of petroleum to its correct use.
Easy
- Refinery gas
- Gasoline
- Bitumen
- Heating and cooking
- Fuel for cars (petrol)
- Surfacing roads
32.The table shows the boiling points of four alkanes. Which alkane has the longest carbon chain? | Alkane | Boiling point (°C) | |---|---| | A | -164 | | B | -42 | | C | -0.5 | | D | 36 |
Medium
- AA
- BB
- CC
- DD
33.Complete combustion of an alkane produces carbon dioxide and water only.
Easy
True or false?
34.How many carbon atoms are in a molecule of pentane?
Easy
35.Complete the sentence: In the addition reaction of ethene with steam, the product is ___.
Easy
In the addition reaction of ethene with steam, the product is ____.
36.A student cracks a sample of decane (C10H22) and collects 0.50 g of ethene (C2H4). Assuming the only reaction is C10H22 → C8H18 + C2H4, what mass of decane was cracked? Give your answer in grams to two decimal places.
Hard
37.Complete the sentences about cracking.
Medium
Cracking is the thermal decomposition of long-chain ____ to produce shorter-chain alkanes and ____. The process requires a high temperature and a catalyst such as ____.
38.Calculate the number of hydrogen atoms in a molecule of pentane.
Easy
39.Complete the sentence about the test for alkenes.
Easy
When bromine water is added to an alkene, the colour changes from ____ to ____.
40.Explain why alkenes are more reactive than alkanes.
Medium
41.How many carbon atoms are in a molecule of pentane?
Easy
- A5
- B7
- C6
- D4
42.A student cracks a sample of decane (C10H22) and collects 0.50 g of ethene (C2H4). Assuming the only reaction is C10H22 → C8H18 + C2H4, what mass of decane was cracked? Give your answer in grams to two decimal places.
Hard
- A4.54 g
- B1.54 g
- C2.54 g
- D3.54 g

Thẻ ghi nhớ

What is a hydrocarbon?

A compound made only from hydrogen and carbon atoms.

What is the main constituent of natural gas?

Methane (CH₄).

What is petroleum (crude oil)?

A thick, sticky, black liquid mixture of hydrocarbons found under porous rock.

How are the fractions of petroleum separated?

By fractional distillation, which separates them according to their boiling points.

Describe the temperature gradient in a fractionating column.

It is very hot at the bottom and cooler at the top.

State one use of refinery gas.

Heating and cooking.

State one use of bitumen.

For surfacing roads.

What is the trend in boiling point as chain length increases in petroleum fractions?

Boiling point increases as chain length increases.

What is an alkane?

A saturated hydrocarbon with only single carbon-carbon bonds.

What is the general formula of alkanes?

CₙH₂ₙ₊₂.

Write the balanced equation for the complete combustion of methane.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).

What is an alkene?

An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C).

What is the general formula of alkenes?

CₙH₂ₙ.

Why are alkenes more reactive than alkanes?

The C=C double bond can open up and form new bonds, whereas alkanes have only strong single bonds.

What is cracking?

The thermal decomposition of long-chain alkanes into shorter-chain alkanes, alkenes, and hydrogen.

State the conditions required for cracking.

A high temperature (600–700 °C) and a catalyst (aluminium oxide or silicon dioxide).

Write an equation for the cracking of decane.

C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene).

How can you distinguish an alkane from an alkene using aqueous bromine?

Alkane: no colour change (orange remains). Alkene: orange to colourless (decolourises).

What type of reaction occurs when bromine adds across a C=C bond?

An addition reaction.

What is formed when ethene reacts with hydrogen?

Ethane (C₂H₆).

What is formed when ethene reacts with steam?

Ethanol (C₂H₅OH).

What functional group do alcohols contain?

The hydroxyl group (-OH).

What is the general formula of alcohols?

CₙH₂ₙ₊₁OH.

State two uses of ethanol.

As a fuel for cars and as a solvent.

Write the balanced equation for the complete combustion of ethanol.

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

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