CIE iGCSE Co-ordinated Sciences-C11.4 Alkanes- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C11.4 Alkanes – Study Notes
CIE iGCSE Co-ordinated Sciences-C11.4 Alkanes – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons
- Describe the properties of alkanes as being generally unreactive, except in terms of combustion
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Bonding in Alkanes
Alkanes have single covalent bonds between all their atoms. They are therefore called saturated hydrocarbons.
Explanation:
- Each carbon atom in an alkane forms four single covalent bonds — either with hydrogen atoms or with other carbon atoms.
- A covalent bond is formed when two atoms share a pair of electrons.
- Because there are only single bonds, no more atoms can be added to the molecule without breaking existing bonds — this makes alkanes saturated.
Example: Methane (\(\mathrm{CH_4}\))
Example: Ethane (\(\mathrm{C_2H_6}\))
Key Idea: Alkanes are called saturated hydrocarbons because they contain only single C–C bonds and are composed entirely of carbon and hydrogen atoms.
- “Hydrocarbon” → made of carbon and hydrogen only.
- “Saturated” → contains only single bonds; cannot add more hydrogen.
Example :
Why are alkanes described as saturated hydrocarbons?
▶️ Answer/Explanation
Step 1: Alkanes contain only single covalent bonds between carbon atoms.
Step 2: This means no additional atoms can be added without breaking existing bonds.
Final Answer: Alkanes are saturated hydrocarbons because they contain only single C–C bonds and are made of carbon and hydrogen only.
Properties and Reactivity of Alkanes
Alkanes are generally unreactive compounds, except when they undergo combustion.
Explanation:
- Alkanes contain only single covalent bonds (C–C and C–H), which are strong and stable.
- They do not easily react with acids, alkalis, or oxidising agents at room temperature.
- This makes alkanes chemically unreactive under normal conditions.
- However, alkanes readily undergo combustion (burning in oxygen), producing energy in the form of heat and light.
1. Complete Combustion of Alkanes
- When alkanes burn in excess oxygen, they undergo complete combustion.
- This produces carbon dioxide (CO₂) and water (H₂O).
Word Equation:
Alkane + Oxygen → Carbon dioxide + Water
Chemical Equation Example:
\(\mathrm{CH_4 + 2O_2 → CO_2 + 2H_2O}\)
(Methane + Oxygen → Carbon dioxide + Water)
Key Idea: Complete combustion releases a large amount of energy, which is why alkanes (e.g. methane, propane, butane) are used as fuels.
2. Incomplete Combustion of Alkanes
- When alkanes burn in a limited supply of oxygen, they undergo incomplete combustion.
- This produces carbon monoxide (CO) or carbon (soot) instead of carbon dioxide.
- Carbon monoxide is a toxic gas it reduces the blood’s ability to carry oxygen.
Chemical Equations:
\(\mathrm{2CH_4 + 3O_2 → 2CO + 4H_2O}\)
\(\mathrm{CH_4 + O_2 → C + 2H_2O}\)
Observation: A yellow, smoky flame is produced due to unburned carbon particles.
Summary of Reactivity:
| Property | Explanation |
|---|---|
| Chemical Reactivity | Generally unreactive due to strong C–C and C–H single bonds. |
| Combustion | Burns in oxygen to produce CO₂ and H₂O (complete combustion) or CO/soot (incomplete combustion). |
| Flammability | Highly flammable — good fuels. |
Example :
Why are alkanes described as being generally unreactive, and what is their main type of reaction?
▶️ Answer/Explanation
Step 1: Alkanes have strong single covalent bonds (C–C and C–H) that are difficult to break.
Step 2: This makes them unreactive under normal conditions.
Step 3: However, they burn easily in oxygen (combustion reaction).
Final Answer: Alkanes are generally unreactive but undergo combustion to produce carbon dioxide and water.