CIE iGCSE Co-ordinated Sciences-C11.5 Alkenes- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C11.5 Alkenes – Study Notes
CIE iGCSE Co-ordinated Sciences-C11.5 Alkenes – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- State that the bonding in alkenes includes a double carbon–carbon covalent bond and that alkenes are unsaturated hydrocarbons
- Describe the test to distinguish between saturated and unsaturated hydrocarbons by their reaction with aqueous bromine
Supplement
- Describe the manufacture of alkenes and hydrogen by the cracking of larger alkane molecules using a high temperature and a catalyst
- Describe the properties of alkenes in terms of addition reactions with:
(a) bromine
(b) hydrogen in the presence of a nickel catalyst
(c) steam in the presence of an acid catalyst
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Bonding in Alkenes
Alkenes have molecules that contain a double carbon–carbon covalent bond (C=C). They are therefore called unsaturated hydrocarbons.
Explanation:
- Each carbon atom in an alkene forms four covalent bonds in total.
- Two of the carbon atoms share two pairs of electrons — this forms the double bond (C=C).
- The presence of the double bond means that other atoms can be added to the molecule, so alkenes are unsaturated.
Definition of Terms:
- Covalent bond: a shared pair of electrons between two atoms.
- Double bond: two shared pairs of electrons between two carbon atoms.
- Unsaturated: can undergo addition reactions because atoms can be added across the double bond.
General Formula of Alkenes:
\(\mathrm{C_nH_{2n}}\)
Key Idea: Alkenes are called unsaturated hydrocarbons because they contain a double carbon–carbon bond (C=C) and are composed only of carbon and hydrogen atoms.
Examples of Simple Alkenes:
| Name | Molecular Formula | Displayed Formula |
|---|---|---|
| Ethene | C₂H₄ |  |
| Propene | C₃H₆ |  |
Example :
Why are alkenes described as unsaturated hydrocarbons?
▶️ Answer/Explanation
Step 1: Alkenes contain a carbon–carbon double bond (C=C).
Step 2: This double bond can open up to allow more atoms to add to the molecule.
Final Answer: Alkenes are unsaturated hydrocarbons because they contain a C=C double bond and consist of only carbon and hydrogen atoms.
Test for Saturation and Unsaturation Using Aqueous Bromine (Bromine Water)
The bromine water test is used to distinguish between saturated and unsaturated hydrocarbons.
- This test relies on the reaction of bromine (Br₂) with a carbon–carbon double bond (C=C).
Test Procedure:
- Add a few drops of aqueous bromine (bromine water) — orange or brown in colour — to the hydrocarbon sample.
- Shake the mixture gently and observe any colour change.
Observations and Results:
| Type of Hydrocarbon | Example | Observation with Bromine Water | Explanation |
|---|---|---|---|
| Saturated Hydrocarbon | Alkane (e.g. Ethane) | No colour change — bromine water remains orange/brown. | No reaction occurs because alkanes contain only single (C–C) bonds. |
| Unsaturated Hydrocarbon | Alkene (e.g. Ethene) | Orange/brown colour of bromine water disappears (decolourises). | Bromine adds across the double bond (C=C) in an addition reaction. |
Chemical Equation (Example with Ethene):
\(\mathrm{C_2H_4 + Br_2 → C_2H_4Br_2}\)
Ethene + Bromine → 1,2-dibromoethane
Summary:
- Alkanes (saturated): No colour change — no reaction.
- Alkenes (unsaturated): Bromine water decolourises — addition reaction occurs.
- This test confirms the presence of a carbon–carbon double bond (C=C).
Example :
How can bromine water be used to distinguish between ethane and ethene?
▶️ Answer/Explanation
Step 1: Add a few drops of bromine water (orange) to each hydrocarbon and shake.
Step 2: Ethane (an alkane) will not react — bromine water stays orange.
Step 3: Ethene (an alkene) reacts — bromine water becomes colourless.
Final Answer: Ethene decolourises bromine water, while ethane does not.
Cracking of Alkanes — Manufacture of Alkenes and Hydrogen
Cracking is the process of breaking down large alkane molecules into smaller, more useful molecules such as alkenes and hydrogen gas.
- Cracking is a form of thermal decomposition — it occurs when large hydrocarbon molecules are broken apart by heat.
- The process requires a high temperature and often a catalyst to speed up the reaction.
Conditions for Cracking:
- High temperature: typically between 600°C and 700°C.
- Catalyst: such as alumina (Al₂O₃) or silica (SiO₂).
- Sometimes cracking can also be done without a catalyst, called thermal cracking, but higher temperatures are needed.
Why Cracking is Important:
- Large hydrocarbons from crude oil are not very useful (e.g. heavy fuel oils).
- Cracking produces:
- Smaller alkanes — used as fuels (e.g. petrol, LPG).
- Alkenes — used to make plastics and other chemicals.
- Hydrogen gas — used in industry (e.g. ammonia production, hydrogenation of oils).
Chemical Example of Cracking:
\(\mathrm{C_{10}H_{22} → C_8H_{18} + C_2H_4}\)
Decane → Octane + Ethene
\(\mathrm{C_2H_6 → C_2H_4 + H_2}\)
Ethane → Ethene + Hydrogen
Large alkane molecule ───(heat/catalyst)──▶ Smaller alkane + Alkene (+ Hydrogen)
Example :
Explain how ethene and hydrogen can be produced from ethane.
▶️ Answer/Explanation
Step 1: Ethane (\(\mathrm{C_2H_6}\)) is heated to a high temperature in the presence of a catalyst such as alumina.
Step 2: The large alkane molecule breaks down into smaller molecules.
Step 3: Ethane decomposes to form ethene (\(\mathrm{C_2H_4}\)) and hydrogen (\(\mathrm{H_2}\)).
Final Answer: \(\mathrm{C_2H_6 → C_2H_4 + H_2}\)
Properties of Alkenes — Addition Reactions
Key Idea:
- Alkenes are unsaturated hydrocarbons — they contain a carbon–carbon double bond (C=C).
- The C=C double bond makes alkenes much more reactive than alkanes.
- They undergo addition reactions — where atoms add across the double bond to form a single-bonded (saturated) product.
(a) Reaction of Alkenes with Bromine Test for Unsaturation
- When an alkene reacts with bromine (Br₂), the bromine adds across the double bond.
- The orange colour of bromine water disappears (decolourises), confirming the presence of a double bond.
- This is an addition reaction forming a dihaloalkane.
Word Equation:
Ethene + Bromine → 1,2-Dibromoethane
Chemical Equation:
\(\mathrm{C_2H_4 + Br_2 → C_2H_4Br_2}\)
The bromine atoms add across the C=C bond, causing decolourisation.
(b) Reaction of Alkenes with Hydrogen Hydrogenation
- When an alkene reacts with hydrogen (H₂) in the presence of a nickel catalyst at about 150°C, it forms an alkane.
- This process is called hydrogenation — hydrogen adds across the double bond.
- It converts unsaturated hydrocarbons into saturated ones.
Word Equation:
Ethene + Hydrogen → Ethane
Chemical Equation:
\(\mathrm{C_2H_4 + H_2 \xrightarrow{Ni} C_2H_6}\)
Hydrogen adds across the C=C bond to form a saturated alkane.
Industrial use: Hydrogenation is used to convert unsaturated vegetable oils into saturated fats (margarine production).
(c) Reaction of Alkenes with Steam Hydration
- When an alkene reacts with steam (water vapour) in the presence of an acid catalyst such as phosphoric acid (H₃PO₄), an alcohol is formed.
- This reaction is known as hydration.
Word Equation:
Ethene + Steam → Ethanol
Chemical Equation:
\(\mathrm{C_2H_4 + H_2O \xrightarrow{H_3PO_4} C_2H_5OH}\)
Steam adds across the C=C bond to form an alcohol (ethanol).
Industrial use: This is how ethanol is manufactured from ethene in the chemical industry.
| Reagent | Conditions | Type of Reaction | Product | Example |
|---|---|---|---|---|
| Bromine (Br₂) | Room temperature | Addition | Dibromoalkane | Ethene → 1,2-dibromoethane |
| Hydrogen (H₂) | Nickel catalyst, 150°C | Addition (Hydrogenation) | Alkane | Ethene → Ethane |
| Steam (H₂O) | Acid catalyst (H₃PO₄), high temp & pressure | Addition (Hydration) | Alcohol | Ethene → Ethanol |
Example :
Describe what happens when ethene reacts with hydrogen in the presence of a nickel catalyst.
▶️ Answer/Explanation
Step 1: Ethene reacts with hydrogen gas at 150°C using a nickel catalyst.
Step 2: The hydrogen atoms add across the double bond, turning it into a single bond.
Final Answer: Ethene undergoes hydrogenation to form ethane (\(\mathrm{C_2H_4 + H_2 → C_2H_6}\)).