Carbon and sulfur burn in oxygen to form acidic oxides: C(s) + O2(g) → CO2(g) and S(s) + O2(g) → SO2(g); both oxides dissolve in water to form acids.
Carbon and sulfur are non-metals, and when they burn in oxygen they form the acidic oxides carbon dioxide and sulfur dioxide. This reaction is used in the Periodic Table chapter to show that non-metals form acidic oxides, in contrast with the basic oxides of metals, so it appears regularly in 4541.
The balanced equation
The general pattern is: non-metal + oxygen → acidic oxide. With correct formulae and state symbols:
C(s) + O2(g) → CO2(g)
S(s) + O2(g) → SO2(g)
When oxygen is limited, carbon burns incompletely to give the poisonous gas carbon monoxide instead:
2C(s) + O2(g) → 2CO(g)
Both dioxides are acidic oxides, and they dissolve in water to form acids. Carbon dioxide forms carbonic acid and sulfur dioxide forms sulfurous acid:
CO2(g) + H2O(l) → H2CO3(aq)
SO2(g) + H2O(l) → H2SO3(aq)
These equations connect the combustion directly to the acidic behaviour that the exam asks you to explain and to the problem of acid rain.
Conditions required
Each element must be heated to its ignition temperature and burned in a good supply of oxygen or air. Carbon (charcoal) is heated until it is red hot and then lowered into a gas jar of oxygen; sulfur is heated in a deflagrating spoon until it melts and burns. A plentiful supply of oxygen ensures complete combustion of carbon to carbon dioxide rather than carbon monoxide.
What you observe
Carbon glows red as it burns and produces a colourless gas, carbon dioxide, with no visible flame. Sulfur first melts to an amber liquid, then burns with a characteristic blue flame and gives off a colourless gas with a sharp, choking, pungent smell, sulfur dioxide. Both gases turn moist blue litmus paper red, confirming they are acidic. Carbon dioxide also turns limewater milky, the standard confirmatory test, because it reacts with calcium hydroxide:
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Where it appears in the SPM exam
In 4541/1 you classify these oxides as acidic and identify the products of combustion. In 4541/2 you write the balanced equations, explain why non-metal oxides are acidic, and connect sulfur dioxide and the oxides of nitrogen to acid rain and its effects. The limewater test for carbon dioxide and the pungent smell of sulfur dioxide are common gas-test answers, and sulfur dioxide reappears as the starting oxide in the Contact process for making sulfuric acid.
How we teach it
Our teachers make sure you can state the type of oxide and back it up: carbon dioxide and sulfur dioxide are acidic because they are non-metal oxides that dissolve to form acids. Students most often lose marks by forgetting the state symbols, by writing the incomplete-combustion product when the question specifies plenty of oxygen, or by mixing up the limewater test (for carbon dioxide) with the glowing-splint test (for oxygen). Keeping the products and their tests firmly paired makes these questions quick and reliable.
Quick summary
Carbon and sulfur burn in oxygen to give the acidic oxides carbon dioxide and sulfur dioxide. Learn both balanced equations with state symbols, the acidic behaviour shown by moist litmus and by dissolving to form acids, the limewater test for carbon dioxide and the pungent smell of sulfur dioxide, and the link to acid rain and the Contact process, and this topic is fully covered.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply