Chemical Equations – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain the difference between a word equation and a chemical equation using the burning of magnesium as an example.
Answer:
- A word equation uses names of substances. Example: Magnesium + Oxygen → Magnesium oxide.
- A chemical equation uses symbols and formulae. Example: 2 Mg + O₂ → 2 MgO.
- In both, substances on the left are reactants; on the right are products.
- The arrow (→) shows the direction of the reaction.
- Chemical equations are shorter and more precise than word equations.
- They also allow checking if the equation is balanced or skeletal.
Q2. Show how to convert a word equation into a balanced chemical equation. Use sodium reacting with chlorine as an example.
Answer:
- Start with the word equation: Sodium + Chlorine → Sodium chloride.
- Write the skeletal equation: Na + Cl₂ → NaCl.
- Count atoms: Left has 1 Na and 2 Cl; right has 1 Na and 1 Cl. It is unbalanced.
- Add coefficients to balance: 2 Na + Cl₂ → 2 NaCl.
- Do not change subscripts inside formulas. Only adjust coefficients.
- Recount atoms: Both sides now have 2 Na and 2 Cl. It is balanced.
Q3. Using the law of conservation of mass, explain why balancing equations is necessary. Illustrate with Zn + H₂SO₄ → ZnSO₄ + H₂.
Answer:
- The law of conservation of mass says mass is neither created nor destroyed.
- So the number of atoms of each element must be equal on both sides.
- For Zn + H₂SO₄ → ZnSO₄ + H₂, count atoms on both sides.
- Zn: 1 each; S: 1 each; O: 4 each; H: 2 each. It is already balanced.
- A balanced equation reflects equal mass of reactants and products.
- It also helps in calculations of amounts in reactions.
Q4. Describe a systematic method to balance equations. Demonstrate with Fe + H₂O → Fe₃O₄ + H₂.
Answer:
- Step 1: Box each formula to avoid changing subscripts.
- Step 2: Count atoms on both sides for Fe, H, and O.
- Step 3: Start with the most complex formula, here Fe₃O₄.
- Step 4: Balance Fe by putting 3 before Fe: 3 Fe.
- Step 5: Balance O by putting 4 before H₂O: 4 H₂O; then balance H with 4 H₂.
- Final balanced equation: 3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂.
Q5. Why should we add physical states and conditions to chemical equations? Use examples to support your answer.
Answer:
- Physical states show if a substance is solid (s), liquid (l), gas (g), or aqueous (aq).
- They help predict observations like gas release or precipitate.
- Example: 3 Fe(s) + 4 H₂O(g) → Fe₃O₄(s) + 4 H₂(g) shows iron reacts with steam.
- Reaction conditions like temperature and pressure affect reactions.
- Example: CO(g) + 2 H₂(g) → CH₃OH(l) at about 340°C and 300 atm.
- States and conditions make equations more informative and realistic.
High Complexity (Analysis & Scenario-Based)
Q6. A classmate writes Mg + O₂ → MgO and says it is balanced. Analyze the mistakes and correct the equation.
Answer:
- The equation is a skeletal equation. It is not balanced.
- Left side: 1 Mg, 2 O atoms; right side: 1 Mg, 1 O atom.
- Oxygen atoms are unequal, so the equation breaks the conservation law.
- Balance MgO by putting 2 before MgO: Mg + O₂ → 2 MgO.
- Now Mg is unbalanced. Put 2 before Mg: 2 Mg + O₂ → 2 MgO.
- Final balanced equation: 2 Mg + O₂ → 2 MgO. All atoms now match.
Q7. You need to explain combustion of a hydrocarbon to your friend. Use propane (C₃H₈) as an example and show a balanced equation with reasoning.
Answer:
- In complete combustion, hydrocarbons form CO₂ and H₂O.
- Start with skeletal form: C₃H₈ + O₂ → CO₂ + H₂O.
- Balance C: put 3 before CO₂. Balance H: put 4 before H₂O.
- Now oxygen on right = 3×2 + 4×1 = 10 O atoms.
- Put 5 before O₂ to give 10 O atoms: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
- This shows products are carbon dioxide and water, as in typical combustion.
Q8. In a lab demo, you show why state symbols and conditions matter. Compare iron with steam versus iron with liquid water.
Answer:
- With steam, iron reacts: 3 Fe(s) + 4 H₂O(g) → Fe₃O₄(s) + 4 H₂(g).
- The (g) state tells us water is steam, which is more reactive here.
- Hydrogen gas is formed, so we expect bubbles and a magnetic oxide solid.
- With liquid water (l), iron reacts very slowly at room temperature.
- State symbols show which conditions make a reaction occur or speed up.
- They help us predict what we will see and collect in the lab.
Q9. Your teacher asks you to balance 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O from the skeletal form. Explain your steps and checks.
Answer:
- Start with NH₃ + O₂ → NO + H₂O (skeletal).
- Balance N first: Put 4 before NH₃ and 4 before NO.
- Now H: 4 NH₃ has 12 H, so put 6 before H₂O to make 12 H.
- Count O on right: 4 NO gives 4 O; 6 H₂O gives 6 O; total 10 O.
- Put 5 before O₂ to get 10 O atoms: 5 O₂.
- Final check: N, H, and O are balanced. Equation is correct.
Q10. A student changes subscripts to balance “Hydrogen + Oxygen → Water,” writing H₂ + O₂ → H₂O₂. Critically evaluate and correct this.
Answer:
- Changing subscripts changes the substance. H₂O₂ is hydrogen peroxide, not water.
- We should only change coefficients, not the formula itself.
- Start with skeletal equation: H₂ + O₂ → H₂O.
- Balance H by putting 2 before H₂O: H₂ + O₂ → 2 H₂O.
- Now O atoms on right = 2; put 2 before H₂ on left: 2 H₂ + O₂ → 2 H₂O.
- Final equation is balanced and keeps the correct compound (water).