Edexcel GCSE Chemistry: Organic Chemistry

Key Takeaways for Organic Chemistry

1. Empirical Formula Calculations

Steps to Calculate Empirical Formula:

1. Convert percentages to grams (assume 100g total).
2. Divide by atomic masses:
   • Carbon (C): 12 g/mol
   • Hydrogen (H): 1 g/mol
   • Oxygen (O): 16 g/mol
3. Find the simplest ratio by dividing all values by the smallest number of moles.
4. Adjust ratios to whole numbers (e.g., multiply by 2 if you get 1.5).

Example (Question 2):

• Given: 37.8% C, 50.4% O, rest H.
  • H = 100 – 37.8 – 50.4 = 11.8%
  • Moles:
    C: 37.812=3.15;O: 50.416=3.15;H: 11.81=11.8C: 1237.8​=3.15;O: 1650.4​=3.15;H: 111.8​=11.8
  • Divide by smallest (3.15):
    C: 1;O: 1;H: 11.83.15≈3.75C: 1;O: 1;H: 3.1511.8​≈3.75
  • Multiply by 4 to get whole numbers: C44​H1515​O44​ (simplifies to C44​H1515​O44​, but check for further simplification).

Tips:

• Always check if oxygen is included.
• Round ratios carefully (e.g., 2.67 ≈ 8/3 → multiply all by 3).

2. Hydrocarbons and Homologous Series

Key Definitions:

• Hydrocarbon: Compounds containing only carbon and hydrogen (e.g., methane CH44​).
• Homologous Series: A family of compounds with:
  • Same general formula (e.g., alkanes: Cnn​H2n+22n+2​).
  • Similar chemical properties.
  • Gradual change in physical properties (e.g., boiling points ↑ with chain length).

Alkanes vs. Alkenes:

Example (Question 5a):

• Hydrocarbon with 85.7% C, 14.3% H:
  • Moles:
    C: 85.712=7.14;H: 14.31=14.3C: 1285.7​=7.14;H: 114.3​=14.3
  • Ratio: C11​H22​ → Empirical formula: CH22​.
  • Molecular formula (mass 112): (CH2)n=14n=112(CH2​)n​=14n=112 → n=8n=8 → C88​H1616​.

Tips:

• For alkenes, always check the position of the double bond in displayed formulae.

3. Combustion of Methane

Products & Conditions:

1. Complete Combustion (excess O22​:
   CH4+2O2→CO2+2H2OCH4​+2O2​→CO2​+2H2​O
2. Incomplete Combustion (limited O22​):
   • Carbon monoxide: CH4+1.5O2→CO+2H2OCH4​+1.5O2​→CO+2H2​O
   • Soot (carbon): CH4+O2→C+2H2OCH4​+O2​→C+2H2​O

Example (Question 3b):

• Black substance = carbon (soot).
• Formation: Insufficient oxygen during combustion.
• Prevention: Use a blue (roaring) flame instead of yellow (safety flame).

4. Ethanol Production

Methods:

1. Hydration of Ethene (from crude oil):
   • Conditions: 300°C, 60 atm, phosphoric acid catalyst.
   • Equation:
     C2H4+H2O→C2H5OHC2​H4​+H2​O→C2​H5​OH
   • Advantages: Fast, pure product.
   • Disadvantages: Non-renewable (uses fossil fuels).
2. Fermentation (from plant sugars):
   • Conditions: 30–40°C, anaerobic, yeast catalyst.
   • Equation:
     C6H12O6→2C2H5OH+2CO2C6​H12​O6​→2C2​H5​OH+2CO2​
   • Advantages: Renewable, sustainable.
   • Disadvantages: Slow, impure product.

Evaluation Tips:

• Compare factors like cost, sustainability, purity, and reaction speed.

5. Isomerism

Definition: Compounds with the same molecular formula but different structural formulae.
Example (Butane, C44​H1010​):

• Straight-chain: Butane
  H−C−C−C−C−HH−C−C−C−C−H
• Branched: 2-methylpropane
  H−C−C(H)−C−HH−C−C(H)−C−H

Tips:

• Alkenes show position isomerism (double bond location).

6. Boiling Points in Alkanes

Trend: Boiling points ↑ as chain length ↑ due to stronger intermolecular forces.
Example (Question 5i):

• Alkane with 6 carbons: C66​H1414​ (hexane).
• Boiling point: ~69°C (from graph).

Graph Tips:

• Draw a curved line showing gradual increase.

Common Mistakes & Tips

• Empirical formula: Forgetting oxygen or miscalculating ratios.
• Hydrocarbons: Confusing with carbohydrates (remember: hydrocarbons = C + H only).
• Combustion equations: Always balance equations step-by-step.
• Isomer drawings: Ensure correct bonding (e.g., 4 bonds for carbon).

Final Tip: Practice laying out calculations neatly (see Student 2’s response) and revise homologous series definitions!

50 GCSE Chemistry Questions (Organic Chemistry)
Answers provided at the end.

Empirical Formula Calculations

1. A compound contains 59.76% carbon, 13.33% hydrogen, and the rest oxygen. Calculate its empirical formula.
2. A compound has 37.8% carbon, 50.4% oxygen, and the remainder hydrogen. Determine its empirical formula.
3. Calculate the empirical formula of a compound with 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen.
4. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen. Its formula mass is 112. Find its empirical and molecular formulas.
5. A compound has 56.8% chlorine, 38.4% carbon, and the rest hydrogen. What is its empirical formula?

Hydrocarbons & Homologous Series

6. Define a hydrocarbon and give two examples.
7. What is a homologous series? State three characteristics.
8. Methane and ethane belong to the same homologous series. Name the series and write their molecular formulas.
9. Draw the displayed formula of ethane.
10. Draw the dot-and-cross diagram for ethene (C22​H44​).
11. Explain why alkenes are called unsaturated hydrocarbons.
12. What is the general formula for alkanes and alkenes?
13. Describe how bromine water can distinguish between an alkane and an alkene.
14. Name the alkene with the molecular formula C33​H66​ and draw its displayed formula.
15. Write the molecular formula of the alkene with seven carbon atoms.

Combustion Reactions

16. Methane burns completely in oxygen. Write the balanced equation.
17. State three possible products of incomplete methane combustion and their conditions.
18. Why does a yellow Bunsen flame produce soot? How can this be prevented?
19. Name the black substance formed during incomplete combustion of methane.
20. Explain why incomplete combustion is harmful.

Ethanol Production

21. Describe how ethanol is made from ethene. Include conditions and a chemical equation.
22. Write the equation for ethanol production via fermentation.
23. Compare the advantages and disadvantages of making ethanol from ethene vs. plant sugars.
24. What catalyst is used in the hydration of ethene?
25. Why is fermentation carried out at 30–40°C?

Isomerism

26. Define isomers.
27. Draw the two isomers of butane (C44​H1010​).
28. Butene (C44​H88​) exhibits position isomerism. Draw two possible structures.
29. Why can’t alkanes with fewer than three carbons have isomers?
30. Name the type of isomerism shown by alkenes.

Boiling Points & Trends

31. Explain why boiling points increase with chain length in alkanes.
32. The boiling point of hexane (C66​H1414​) is 69°C. Estimate the boiling point of heptane (C77​H1616​).
33. What shape of line would you draw on a graph of alkane boiling points vs. carbon number?
34. Name the alkane with three carbon atoms and state its boiling point.
35. Why do branched alkanes have lower boiling points than straight-chain alkanes?

Reactions & Equations

36. Methane reacts with bromine under UV light. Write the equation and name the reaction type.
37. Ethene undergoes addition polymerisation. Write the equation for polyethene formation.
38. Complete the equation:
    CH4+Br2→____+____CH4​+Br2​→____+____
39. What conditions are needed for methane to react with bromine?
40. Name two products of methane combustion in excess oxygen.

Applications & Analysis

41. Why is ethanol used as a fuel?
42. Evaluate the environmental impact of producing ethanol from crude oil vs. crops.
43. Describe how to test a hydrocarbon for unsaturation.
44. A student claims, “Ethanol from plants is carbon-neutral.” Do you agree? Justify.
45. Why might using crops for ethanol production affect food prices?

Structured Questions

46. A compound contains 26.56% oxygen, 59.76% carbon, and 13.68% hydrogen. Calculate its empirical formula.
47. Draw the displayed formula of propane (C33​H88​) and name its homologous series.
48. Explain why alkenes react with bromine water but alkanes do not.
49. Compare the bonding in ethane (C22​H66​) and ethene (C22​H44​).
50. A hydrocarbon decolourises bromine water. What does this tell you about its structure?

Answers

1. Empirical formula: C33​H88​O
   • Oxygen % = 100 – 59.76 – 13.33 = 26.91%
   • Moles:
     C: 59.7612=4.98;H: 13.331=13.33;O: 26.9116=1.68C: 1259.76​=4.98;H: 113.33​=13.33;O: 1626.91​=1.68
   • Ratio: Divide by 1.68 → C2.962.96​H7.937.93​O11​ ≈ C33​H88​O.
2. Empirical formula: C22​H66​O
   • Hydrogen % = 100 – 37.8 – 50.4 = 11.8%
   • Moles:
     C: 37.812=3.15;O: 50.416=3.15;H: 11.81=11.8C: 1237.8​=3.15;O: 1650.4​=3.15;H: 111.8​=11.8
   • Ratio: Divide by 3.15 → C11​H3.753.75​O11​ → Multiply by 4 → C44​H1515​O44​ (Simplifies to C22​H66​O).
3. Empirical formula: CH22​O
   • Moles:
     C: 4012=3.33;H: 6.71=6.7;O: 53.316=3.33C: 1240​=3.33;H: 16.7​=6.7;O: 1653.3​=3.33
   • Ratio: Divide by 3.33 → CH22​O.
4. Empirical: CH22​; Molecular: C88​H1616​
   • Moles:
     C: 85.712=7.14;H: 14.31=14.3C: 1285.7​=7.14;H: 114.3​=14.3
   • Ratio: CH22​.
   • Molecular formula: CH2×8=C8H16CH2​×8=C8​H16​.
5. Empirical formula: C22​H55​Cl
   • Hydrogen % = 100 – 56.8 – 38.4 = 4.8%
   • Moles:
     Cl: 56.835.5=1.6;C: 38.412=3.2;H: 4.81=4.8Cl: 35.556.8​=1.6;C: 1238.4​=3.2;H: 14.8​=4.8
   • Ratio: Divide by 1.6 → Cl11​C22​H33​ → C22​H55​Cl.

Hydrocarbons & Homologous Series

6. Hydrocarbon: Compounds containing only carbon and hydrogen. Examples: methane (CH44​), ethane (C22​H66​).
7. Homologous series:
   • Same general formula.
   • Similar chemical properties.
   • Gradual change in physical properties (e.g., boiling points).
8. Alkanes. Methane: CH44​; Ethane: C22​H66​.
9. Ethane displayed formula:Copy H H | | H-C-C-H | | H H
10. Ethene dot-and-cross:
    • Each carbon shares 4 electrons (2 in double bond).
    • Structure:CopyH H ·· C=C ·· H H
11. Unsaturated: Alkenes have a double bond (C=CC=C) that can open to form single bonds, allowing addition reactions.
12. Alkanes: Cnn​H2n+22n+2​; Alkenes: Cnn​H2n2n​.
13. Bromine water test: Alkenes decolourise bromine water (orange → colourless) due to addition; alkanes no reaction.
14. Propene:CopyH H | | C=C-CH$_3$
15. C77​H1414​.

Combustion Reactions

16. Complete combustion:
    CH4+2O2→CO2+2H2OCH4​+2O2​→CO2​+2H2​O
17. Products:
    • Carbon monoxide (limited O22​).
    • Soot/Carbon (very limited O22​).
    • Water vapour.
18. Yellow flame: Poor air supply → incomplete combustion. Prevent: Use blue flame (air hole open).
19. Soot (carbon).
20. Harmful: CO is toxic; soot causes respiratory issues.

Ethanol Production

21. Hydration of ethene:
    • Conditions: 300°C, 60 atm, phosphoric acid catalyst.
    • Equation:
      C2H4+H2O→C2H5OHC2​H4​+H2​O→C2​H5​OH
22. Fermentation:
    C6H12O6→2C2H5OH+2CO2C6​H12​O6​→2C2​H5​OH+2CO2​
23. Ethene method:
    • Advantages: Fast, pure.
    • Disadvantages: Non-renewable (crude oil).
      Fermentation:
    • Advantages: Renewable (crops).
    • Disadvantages: Slow, low yield.
24. Catalyst: Phosphoric acid.
25. 30–40°C: Optimal temperature for yeast enzymes.

Isomerism

26. Isomers: Same molecular formula, different structural arrangements.
27. Butane isomers:
    • Butane:
      H−C−C−C−C−HH−C−C−C−C−H
    • 2-methylpropane:
      H−C−C(CH3)−HH−C−C(CH3​)−H
28. Butene isomers:
    • 1-butene: CH22​=CHCH22​CH33​.
    • 2-butene: CH33​CH=CHCH33​.
29. <3 carbons: Insufficient carbons for branching (minimum 4 for isomers).
30. Position isomerism (double bond location).

Boiling Points & Trends

31. Longer chains: Increased surface area → stronger London forces.
32. Heptane: ~98°C (higher than hexane).
33. Curved line (gradual increase with carbon number).
34. Propane (C33​H88​), boiling point: -42°C.
35. Branched alkanes: Reduced surface area → weaker intermolecular forces.

Reactions & Equations

36. Substitution reaction:
    CH4+Br2→CH3Br+HBrCH4​+Br2​→CH3​Br+HBr
    Conditions: UV light.
37. Polymerisation:
    nC2H4→(CH2CH2)nnC2​H4​→(CH2​CH2​)n​
38. Equation:
    CH4+Br2→CH3Br+HBrCH4​+Br2​→CH3​Br+HBr
39. Conditions: UV light.
40. Products: CO22​, H22​O.

Applications & Analysis

41. Fuel: Burns cleanly; renewable if from crops.
42. Crude oil: Non-renewable but efficient. Crops: Sustainable but competes with food.
43. Test: Add bromine water; colourless = alkene.
44. Agree: CO22​ absorbed by plants during growth → balanced.
45. Food prices: Crops diverted to ethanol → reduced food supply.

Structured Questions

46. Empirical formula:
    • Moles:
      C: 59.7612=4.98;H: 13.681=13.68;O: 26.5616=1.66C: 1259.76​=4.98;H: 113.68​=13.68;O: 1626.56​=1.66
    • Ratio: Divide by 1.66 → C33​H88​O.
47. Propane structure:CopyH H H | | | H-C-C-C-H | | | H H H Homologous series: Alkanes.
48. Alkenes: Double bond reacts with bromine; alkanes lack reactive double bonds.
49. Ethane: Single bonds (sigma only). Ethene: Double bond (sigma + pi).
50. Unsaturated (contains double bond).