Alkanes

1. Overview

Alkanes are the simplest family of hydrocarbons and form a homologous series with the general formula CₙH₂ₙ₊₂. They are the primary components of fossil fuels like natural gas and petroleum, making them essential for global energy production.

Key Definitions

• Hydrocarbon: A compound containing only carbon and hydrogen atoms.
• Saturated Hydrocarbon: A hydrocarbon in which all carbon-carbon bonds are single covalent bonds.
• Alkanes: A homologous series of saturated hydrocarbons.
• Homologous Series: A family of similar compounds with the same functional group and general formula, showing a gradation in physical properties.
• Substitution Reaction: A reaction in which one atom or group of atoms is replaced by another atom or group of atoms.

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Core Content

Bonding and Structure

• Alkanes consist of carbon and hydrogen atoms held together by single covalent bonds.
• They are described as saturated because they contain no carbon-carbon double bonds (C=C). Every carbon atom is bonded to the maximum possible number of hydrogen atoms.
• 📊Displayed formula of Methane (CH₄) showing four single lines from C to H, and Ethane (C₂H₆) showing a single line between two C atoms and three H atoms on each C.

Chemical Properties

Alkanes are generally unreactive compounds. This is because the C-C and C-H bonds are very strong and difficult to break. However, they undergo two key types of reactions:

A. Combustion Alkanes react with oxygen to release energy (exothermic).

1. Complete Combustion: Occurs in excess oxygen, producing carbon dioxide and water.
   • Word Equation: methane + oxygen → carbon dioxide + water
   • Symbol Equation: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
2. Incomplete Combustion: Occurs when oxygen is limited, producing carbon monoxide and/or soot (carbon) and water.
   • Word Equation: ethane + oxygen (limited) → carbon monoxide + water
   • Symbol Equation: 2C₂H₆(g) + 5O₂(g) → 4CO(g) + 6H₂O(l)

B. Substitution (General) Alkanes can react with halogens (like chlorine) under specific conditions to swap a hydrogen atom for a halogen atom.

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Extended Content (Extended Curriculum Only)

Photochemical Substitution

The reaction between alkanes and halogens (e.g., chlorine) is a photochemical reaction.

• Requirement: Ultraviolet (UV) light.
• Role of UV: The UV light provides the activation energy ($E_a$) required to break the Cl-Cl bond to start the reaction.

Monosubstitution of Methane

In a monosubstitution reaction, only one hydrogen atom is replaced.

• Word Equation: methane + chlorine → chloromethane + hydrogen chloride
• Symbol Equation: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)

Structural Formulas of Products:

• Chloromethane (CH₃Cl): H | H—C—Cl | H

• Hydrogen Chloride (HCl): H—Cl

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Key Equations

Reaction Type	General/Specific Equation	Symbols/Units
General Formula	CₙH₂ₙ₊₂	n = number of carbon atoms
Complete Combustion	C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)	Propane combustion
Substitution	CH₄(g) + Cl₂(g) $\xrightarrow{UV}$ CH₃Cl(g) + HCl(g)	Requires UV light

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Common Mistakes to Avoid

• ❌ Wrong: Defining a hydrocarbon as a molecule made of carbon and hydrogen.
• ✓ Right: A molecule made of only carbon and hydrogen (the word "only" is essential for marks).
• ❌ Wrong: Forgetting to include the second product (HCl) in substitution reactions.
• ✓ Right: Always show the inorganic byproduct (e.g., HCl or HBr) in substitution equations.
• ❌ Wrong: Writing the formula for ethane as C₂H₄.
• ✓ Right: Alkanes follow CₙH₂ₙ₊₂, so ethane is C₂H₆ (C₂H₄ is an alkene).

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Exam Tips

• Command Word "State": If a question asks you to "state the bonding in alkanes," simply write "single covalent bonds."
• Command Word "Name": If asked to name the condition for substitution, "UV light" or "ultraviolet radiation" is the required answer.
• Contextual Questions: Be prepared to discuss the environmental impact of combustion (CO₂ as a greenhouse gas or CO as a toxic gas).
• High Frequency: Substitution reactions and the definition of "saturated" appear very frequently. Memorize the "one atom replaced by another" definition for substitution exactly.
• Numerical Check: If asked to identify an unknown alkane from its mass or formula, always check if it fits the $C_nH_{2n+2}$ pattern. (e.g., If n=5, H must be 12).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term 'hydrocarbon'. [1]

(b) Alkanes are described as saturated hydrocarbons. Explain what is meant by the terms 'saturated' and 'hydrocarbon' in this context. [2]

(c) State one property of alkanes, other than flammability. [1]

(d) Name the alkane with three carbon atoms. [1]

Worked Solution:

(a)

1. A compound containing carbon and hydrogen atoms only. This is the definition of a hydrocarbon.

How to earn full marks:

• Must mention carbon AND hydrogen, AND that these are the ONLY elements present.

(b)

1. 'Hydrocarbon' means the compound contains carbon and hydrogen atoms only. This is the definition of a hydrocarbon.
2. 'Saturated' means that all the bonds between carbon atoms are single bonds. This explains the saturation of the hydrocarbon chain.

How to earn full marks:

• 1 mark for correctly defining 'hydrocarbon'.
• 1 mark for stating that 'saturated' means only single bonds between carbon atoms.

(c)

1. Generally unreactive. This is a key property of alkanes.

How to earn full marks:

• Acceptable answers include: Generally unreactive, insoluble in water, etc.

(d)

1. Propane This is the name of the alkane with three carbon atoms.

How to earn full marks:

• The spelling must be correct (Propane).

Common Pitfall: Make sure you know the definitions of key terms like "hydrocarbon" and "saturated." Many students lose marks by not being precise enough in their definitions. Also, double-check your spelling of alkane names!

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State what is meant by the term 'substitution reaction'. [1]

(b) Methane ($CH_4$) reacts with chlorine ($Cl_2$) in the presence of ultraviolet light. State the role of ultraviolet light in this reaction. [1]

(c) Write the balanced chemical equation for the reaction between methane and chlorine to produce chloromethane ($CH_3Cl$) and hydrogen chloride ($HCl$). [2]

(d) Explain why this reaction is classified as a substitution reaction. [2]

Worked Solution:

(a)

1. A reaction in which one atom or group of atoms is replaced by another atom or group of atoms. This is the definition of a substitution reaction.

How to earn full marks:

• Must mention the replacement of one atom/group by another.

(b)

1. Provides the activation energy. Ultraviolet light provides the energy needed for the reaction to start.

How to earn full marks:

• The answer must refer to activation energy or breaking bonds.

(c)

1. $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$ This is the balanced equation for the reaction.

How to earn full marks:

• 1 mark for the correct reactants and products.
• 1 mark for the correct balancing.

(d)

1. A hydrogen atom in methane is replaced by a chlorine atom. This explains why the reaction is a substitution.
2. This replacement of one atom by another defines a substitution reaction. This reinforces the definition in the context of the reaction.

How to earn full marks:

• 1 mark for stating that a hydrogen atom is replaced by a chlorine atom.
• 1 mark for relating this to the definition of substitution.

Common Pitfall: Remember that ultraviolet light provides the activation energy needed to break the bonds in the chlorine molecule, initiating the substitution reaction. Also, be clear and specific when explaining why a reaction is classified as substitution.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

(a) Define the term 'alkane'. [1]

(b) Draw the structural formula of butane. [2]

(c) Alkanes can undergo combustion in the presence of oxygen. Write a balanced chemical equation for the complete combustion of butane. [2]

(d) Explain why incomplete combustion of alkanes is dangerous. Include the name of one toxic product formed. [3]

Worked Solution:

(a)

1. An alkane is a saturated hydrocarbon. This is the definition of an alkane.

How to earn full marks:

• Must mention that alkanes are saturated hydrocarbons.

(b)

1. 📊A displayed formula of butane, $CH_3CH_2CH_2CH_3$, showing all the bonds between carbon and hydrogen atoms.
   *This is the structural formula of butane.*

How to earn full marks:

• 1 mark for showing 4 carbon atoms bonded in a chain.
• 1 mark for showing the correct number of hydrogen atoms attached to each carbon atom.

(c)

1. $2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O$ This is the balanced equation for the complete combustion of butane.

How to earn full marks:

• 1 mark for the correct reactants and products.
• 1 mark for the correct balancing (allow multiples of the above).

(d)

1. Incomplete combustion occurs when there is a limited supply of oxygen. This explains the condition for incomplete combustion.
2. This produces carbon monoxide (CO) instead of carbon dioxide ($CO_2$). This identifies the toxic product.
3. Carbon monoxide is toxic because it is a poisonous gas that reduces the capacity of the blood to carry oxygen. This explains why carbon monoxide is dangerous.

How to earn full marks:

• 1 mark for stating that incomplete combustion occurs with limited oxygen.
• 1 mark for identifying carbon monoxide as a product.
• 1 mark for explaining the toxicity of carbon monoxide (e.g., reduces oxygen transport in blood).

Common Pitfall: When balancing combustion equations, start by balancing the carbon and hydrogen atoms, and leave the oxygen until last. Also, remember that incomplete combustion produces carbon monoxide, which is a toxic gas that prevents red blood cells from carrying oxygen effectively.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

(a) Describe the conditions necessary for the substitution reaction between ethane and chlorine to occur. [2]

(b) Write the structural formula of the organic product formed when one chlorine atom substitutes one hydrogen atom in ethane. [2]

(c) Ethane can be cracked to produce ethene and hydrogen. $C_2H_6 \rightarrow C_2H_4 + H_2$ State the conditions needed for cracking to occur. [2]

(d) Suggest why cracking is important in the petrochemical industry. [3]

Worked Solution:

(a)

1. The reaction requires ultraviolet light. This is a necessary condition for the reaction.
2. The reaction is photochemical, meaning it's initiated by light. This further explains the role of UV light.

How to earn full marks:

• 1 mark for stating the need for ultraviolet light.
• 1 mark for mentioning that the reaction is photochemical or light-initiated.

(b)

1. 📊Displayed formula of chloroethane, $CH_3CH_2Cl$, showing all bonds.
   *This is the structural formula of chloroethane.*

How to earn full marks:

• 1 mark for showing 2 carbon atoms bonded to each other.
• 1 mark for showing the correct number of hydrogen atoms and one chlorine atom attached to the carbon atoms.

(c)

1. High temperature, typically between 400°C and 900°C. This is a necessary condition for cracking.
2. A catalyst, such as aluminium oxide ($Al_2O_3$) or silicon dioxide ($SiO_2$). This is also a necessary condition for cracking.

How to earn full marks:

• 1 mark for stating high temperature (accept a temperature range within 400-900 °C).
• 1 mark for stating the need for a catalyst (e.g., aluminium oxide or silicon dioxide).

(d)

1. Cracking converts long-chain alkanes into shorter-chain alkanes and alkenes. This explains the overall outcome of cracking.
2. Shorter-chain alkanes are more useful as fuels because they are more volatile and burn more cleanly. This gives one use of the products.
3. Alkenes are used as monomers to make polymers (plastics). This gives another use of the products.

How to earn full marks:

• 1 mark for stating that cracking converts long-chain alkanes into shorter-chain molecules.
• 1 mark for stating that shorter-chain alkanes are useful as fuels.
• 1 mark for stating that alkenes are used to make polymers.

Common Pitfall: Don't forget that cracking requires both a high temperature AND a catalyst. Many students only mention one of these conditions. Also, be specific about why the products of cracking are useful – mentioning their use as fuels and in polymer production will get you full marks.