Alkene addition: bromine, hydrogen and steam products
Draw the three required ethene products, use the correct catalyst conditions and distinguish one-product addition from chlorine substitution.
Section C12.5 · Alkenes
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Draw ethene with the C=C bond and account for four bonds at each carbon.
- Distinguish substitution's replacement and separate HCl product.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many products form in the required addition model?
What you will learn
- Explain addition as forming only one product from the stated reactants.
- Describe alkene addition with bromine, hydrogen/nickel and steam/acid catalyst, and draw the structural or displayed products.
The carbon double bond becomes a single bond
In the required addition model, only one product is formed: reactant atoms combine across the alkene's C=C bond, which becomes C–C. With bromine, CH₂=CH₂+Br₂→CH₂BrCH₂Br, molecular equation C₂H₄+Br₂→C₂H₄Br₂. Each carbon gains one Br and keeps its two H atoms. The displayed 1,2-dibromoethane diagram shows every atom and bond. Bromine or aqueous bromine is the specified reagent; disappearance of its colour connects this reaction with the Core unsaturation test.
With hydrogen and a nickel catalyst, CH₂=CH₂+H₂→CH₃CH₃, molecular equation C₂H₄+H₂→C₂H₆. Each carbon gains one hydrogen and the product is ethane. State nickel for hydrogenation, not the acid catalyst used for the steam reaction. The catalyst is stated alongside the arrow and does not add atoms to the product formula.
Steam gives an alcohol under the stated acid-catalyst condition
With steam and an acid catalyst, CH₂=CH₂+H₂O→CH₃CH₂OH, molecular equation C₂H₄+H₂O→C₂H₅OH. One carbon gains H and the other OH, giving ethanol with six H atoms in total. The displayed ethanol product retains the O–H bond; adding two H atoms while losing the O would instead describe hydrogenation, not steam addition. Detailed industrial temperature/pressure and manufacture comparisons follow in the Alcohols section.
All three required equations combine the reactants into one stated organic product. Chlorine substitution in methane instead replaces a hydrogen and gives both CH₃Cl and HCl. Do not carry its UV activation-energy condition or separate HCl product into these addition examples. The drawings are connectivity schemes, not measured molecular shapes; the school bromine-addition model gives the specified dibromo product without claiming a complete account of every possible real aqueous side reaction.
Sources for this lesson
Pause and explain
Which pairing correctly describes ethene hydrogenation?
Worked example
Give ethene's three required addition products and match the stated catalyst or reagent condition to each.
Show the worked solution
- Bromine/bromine water gives CH₂BrCH₂Br, 1,2-dibromoethane, by adding one Br to each C.
- H₂ with nickel gives CH₃CH₃, ethane, by adding one H to each C.
- Steam with an acid catalyst gives CH₃CH₂OH, ethanol, by adding H and OH across the double bond.
Answer Bromine → CH₂BrCH₂Br; hydrogen/Ni → CH₃CH₃; steam/acid catalyst → CH₃CH₂OH. Each stated addition forms one product.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw ethene's bromine addition product and explain why no separate HBr product belongs to this equation.
Give me a hint
Keep the four starting H atoms and attach one Br to each carbon.
Compare my reasoning
- Change C=C to C–C and keep two separately shown H atoms on each C.
- Add one Br to each C to form CH₂BrCH₂Br; every carbon has four single bonds.
- C₂H₄+Br₂→C₂H₄Br₂ uses all atoms in one product, with no hydrogen removed to make HBr.
Displayed Br–CH₂–CH₂–Br with every H shown; CH₂BrCH₂Br is the only stated product and no HBr is lost.
Look for these in your work
- The double bond becomes single and every atom is accounted for.
- One-product addition is distinguished from substitution.
Give the structural product, balanced equation and required catalyst for ethene with hydrogen, then for ethene with steam.
Give me a hint
Track the oxygen input only in the steam reaction.
Compare my reasoning
- Hydrogen adds across C=C to give CH₃CH₃: C₂H₄+H₂→C₂H₆, with nickel.
- Steam adds H and OH to give CH₃CH₂OH: C₂H₄+H₂O→C₂H₅OH, with an acid catalyst.
- Each conserves all atoms in one product; ethanol totals C₂H₆O including its OH hydrogen.
Hydrogen/Ni: CH₃CH₃, C₂H₄+H₂→C₂H₆. Steam/acid: CH₃CH₂OH, C₂H₄+H₂O→C₂H₅OH (C₂H₆O).
Look for these in your work
- Both structures and equations conserve atoms.
- Nickel and acid catalyst are matched correctly.
A drawing of steam addition keeps ethene's C=C, puts H and OH on its carbons, and predicts a separate HCl molecule under UV. Correct the bond, product count and conditions.
Give me a hint
Check the carbon bond totals and whether any chlorine entered.
Compare my reasoning
- Convert C=C to C–C before adding H and OH; otherwise the proposed carbon bond counts exceed four.
- The single product is ethanol CH₃CH₂OH, and no chlorine input exists to supply HCl.
- Use steam and an acid catalyst; UV and HCl came from a different chlorine-substitution example.
C–C replaces C=C; one ethanol product CH₃CH₂OH forms with steam/acid catalyst, without HCl or a UV requirement.
Look for these in your work
- Atom sources and ordinary carbon valence are respected.
- Addition conditions are separated from chlorine substitution.
Common mistakes
- Keeping a C=C double bond while adding atoms and exceeding carbon's four bonds.
- Giving HBr as a separate product in the specified bromine addition.
- Swapping nickel and acid catalysts.
- Losing the oxygen or OH hydrogen when drawing ethanol from steam addition.
What can you explain now?
Which product and catalyst are required when steam adds to ethene?
Compare with the explanation
Ethanol, CH₃CH₂OH, with an acid catalyst.
Steam contributes H and OH; the product retains its oxygen and O–H hydrogen.
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Make it stick
Draw all three ethene addition products with every atom and bond. Label bromine, hydrogen/Ni and steam/acid, then contrast the product count with methane substitution.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.