ExpertMedium3 marksStructured
AQA GCSE · Question 08.4 · Organic chemistry
Compound A has the formula C₄H₆O₂. Compound A is flammable.
Write a balanced equation for the complete combustion of compound A.
Compound A has the formula C₄H₆O₂. Compound A is flammable.
Write a balanced equation for the complete combustion of compound A.
How to approach this question
This question asks for a balanced equation for complete combustion.
1. Write down the general form of a complete combustion reaction for a compound containing C, H, and O: Compound + O₂ → CO₂ + H₂O.
2. Start with the reactants: C₄H₆O₂ + O₂ → CO₂ + H₂O.
3. Balance the atoms one element at a time.
a. **Carbon:** There are 4 C atoms on the left, so you need 4CO₂ on the right.
b. **Hydrogen:** There are 6 H atoms on the left, so you need 3H₂O on the right (since 3 x 2 = 6).
c. **Oxygen:** Now count the total O atoms on the right: (4 x 2 in CO₂) + (3 x 1 in H₂O) = 8 + 3 = 11. On the left, you already have 2 O atoms in C₄H₆O₂. So you need 11 - 2 = 9 more O atoms from the O₂ molecules. This means you need 4.5 O₂ molecules (since 4.5 x 2 = 9).
4. Write the final equation: C₄H₆O₂ + 4.5O₂ → 4CO₂ + 3H₂O.
5. (Optional but good practice) If you want to avoid fractions, multiply the entire equation by 2.
Full Answer
C₄H₆O₂ + 4.5O₂ → 4CO₂ + 3H₂O
OR (multiplying by 2 to remove the fraction)
2C₄H₆O₂ + 9O₂ → 8CO₂ + 6H₂O
Complete combustion means the substance reacts completely with oxygen to produce carbon dioxide and water.
The unbalanced equation is:
C₄H₆O₂ + O₂ → CO₂ + H₂O
To balance it, we follow these steps:
1. **Balance Carbon (C):** There are 4 carbon atoms on the left in C₄H₆O₂. To get 4 carbon atoms on the right, we need 4 molecules of CO₂.
C₄H₆O₂ + O₂ → 4CO₂ + H₂O
2. **Balance Hydrogen (H):** There are 6 hydrogen atoms on the left. To get 6 hydrogen atoms on the right, we need 3 molecules of H₂O (since each H₂O has 2 H atoms, 3 × 2 = 6).
C₄H₆O₂ + O₂ → 4CO₂ + 3H₂O
3. **Balance Oxygen (O):** Now, we count the total oxygen atoms on the right side. In 4CO₂, there are 4 × 2 = 8 oxygen atoms. In 3H₂O, there are 3 × 1 = 3 oxygen atoms. The total on the right is 8 + 3 = 11 oxygen atoms.
On the left side, we have 2 oxygen atoms in the C₄H₆O₂ molecule. We need a total of 11, so we need 11 - 2 = 9 more oxygen atoms from the O₂ molecules. Since O₂ contains two oxygen atoms, we need 9 / 2 = 4.5 molecules of O₂.
C₄H₆O₂ + 4.5O₂ → 4CO₂ + 3H₂O
4. **Remove Fractions (Optional):** Chemical equations are conventionally written with whole number coefficients. To remove the 4.5, we can multiply the entire equation by 2.
2C₄H₆O₂ + 9O₂ → 8CO₂ + 6H₂O
Both forms of the balanced equation are correct.
Common mistakes
✗ Forgetting that complete combustion produces CO₂ and H₂O.
✗ Errors in counting atoms, especially oxygen.
✗ Forgetting to account for the oxygen atoms already present in the C₄H₆O₂ molecule when balancing the O₂ on the left.
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
48 questions · hints · full answers · grading
More questions from this exam
Q01.1A student investigated an aqueous solution of a salt. The student identified that the salt soluti...EasyQ01.2Describe a test to identify chloride ions. Give the result of the test.EasyQ01.3The student determined the concentration of sodium chloride in the salt solution using this metho...EasyQ01.4How did the student calculate the mass of solid sodium chloride remaining after steps 1 to 5?EasyQ01.5The student calculated the concentration of sodium chloride in the salt solution. Table 1 shows t...Medium