Chemistry · Chemistry of the elements · Grade 9

Carbon and its compounds

Diamond and graphite are both carbon — so why is carbon monoxide so dangerous?

The situation

Diamond is the hardest stone in a ring, while the graphite in a pencil is soft enough to leave a mark on paper — and yet both consist of carbon alone. In winter many homes light a sandal (a traditional table heated by embers) or a coal stove; every year someone is poisoned by carbon monoxide, even though this gas is colourless and odourless. The bubbles in fizzy water, the stalactites in caves and the limescale in a kettle are also carbon compounds. How can a single element form such different substances, and how do we protect ourselves from carbon monoxide?

Topic

Carbon and its compounds. Diamond, graphite and activated carbon; the danger of carbon monoxide from a sandal and a stove; fizzy water and stalactites in caves

Key ideas

Allotropy and adsorption
CO and CO₂
Carbonates and hydrogencarbonates

Formulas

C + O_{2} \overset{}{\to} CO_{2} 2 C + O_{2} \overset{}{\to} 2 CO

Complete combustion gives CO₂; with too little oxygen, carbon monoxide CO is formed. In a layer of hot coal: $C + CO_{2} \overset{}{\to} 2 CO$ .

\frac{[COHb]}{[O_{2} H b]} = M \frac{p (CO)}{p (O_{2})}, M \approx 218

Haldane equation: haemoglobin binds CO about 200–250 times more strongly than oxygen.

CaCO_{3} + 2 HCl \overset{}{\to} CaCl_{2} + H_{2} O + CO_{2} ↑

Preparing CO₂ in the laboratory (Practical 1): marble + hydrochloric acid.

Ca (OH)_{2} + CO_{2} \overset{}{\to} CaCO_{3} ↓ + H_{2} O CaCO_{3} + CO_{2} + H_{2} O \overset{}{\to} Ca (HCO_{3})_{2}

Limewater test: it first turns cloudy; with excess CO₂ the precipitate dissolves.

2 NaHCO_{3} \overset{}{\to} Na_{2} CO_{3} + H_{2} O + CO_{2} ↑

A hydrogencarbonate decomposes on heating; qualitative test for the carbonate ion: $CO_{3}^{2 -} + 2 H^{+} \overset{}{\to} H_{2} O + CO_{2} ↑$ .

Theory

The carbon group and the chemical properties of carbon

Group IVA: carbon, silicon, germanium, tin, lead. The outer shell has 4 electrons ( $n s^{2} n p^{2}$ ), and oxidation states range from −4 to +4. Going down the group the atomic radius increases and non-metallic character decreases: C and Si are non-metals, Ge is a semiconductor, Sn and Pb are metals; the stability of the hydrides ( $EH_{4}$ ) decreases. Carbon occurs in nature in the free state (diamond, graphite), in coal, oil, gas, carbonates (limestone, marble, chalk) and as CO₂ in the air (≈ 0.04 %); the largest coal deposit in Uzbekistan is the Angren brown coal (lignite) deposit, mined by the open-cast method. Carbon is a reducing agent: $C + O_{2} \overset{}{\to} CO_{2}$ , $C + 2 CuO \overset{}{\to} 2 Cu + CO_{2}$ ; with too little oxygen, $2 C + O_{2} \overset{}{\to} 2 CO$ . At high temperature it forms carbides with metals ( $4 Al + 3 C \overset{}{\to} Al_{4} C_{3}$ ) and methane with hydrogen ( $C + 2 H_{2} \overset{}{\to} CH_{4}$ ).

Allotropy and adsorption

Allotropy is the ability of one element to form several simple substances. In diamond each carbon atom is bonded to 4 neighbours in a tetrahedron (sp³, C–C 154 pm) — a strong three-dimensional network: the hardest natural substance (10 on the Mohs scale), density 3.51 g/cm³, and it does not conduct electricity. In graphite the atoms form flat hexagonal layers (sp², C–C 142 pm); the layers are 335 pm apart and held by weak forces — the layers slide easily, so a pencil leaves a mark; the fourth electron of each atom moves freely along the layer — graphite conducts electricity, density ≈ 2.26 g/cm³. Fullerene C₆₀ is a “football” molecule of 12 pentagons and 20 hexagons (1985). Graphene is a layer of graphite one atom thick. Coal, soot and coke are “amorphous” carbon, close to finely crystalline graphite. Activated charcoal is highly porous: the internal surface of 1 g is typically 500–1500 m² — it takes up gas and dye molecules on its surface (adsorption): water filters, gas masks, medicine taken in cases of poisoning.

Carbon monoxide CO: formation and danger

Carbon(II) oxide is a colourless, odourless gas, sparingly soluble in water and slightly lighter than air (ρ = 1.25 g/L at STP), a non-salt-forming oxide. It forms in incomplete combustion of fuel: in a sandal the embers burn under the blanket with too little oxygen, and if a stove flue is closed before the embers die down, $C + CO_{2} \overset{}{\to} 2 CO$ takes place in the layer of hot coal. CO burns with a blue flame ( $2 CO + O_{2} \overset{}{\to} 2 CO_{2}$ ) and is a reducing agent in metallurgy: $Fe_{2} O_{3} + 3 CO \overset{}{\to} 2 Fe + 3 CO_{2}$ . Toxicity: CO binds to the haemoglobin in blood about 200–250 times more strongly than oxygen does (Haldane equation, $M \approx 218$ ) — carboxyhaemoglobin COHb forms, and the blood can no longer carry oxygen. With only 100 ppm (0.01 %) of CO in the air, prolonged breathing brings COHb to ≈ 15 %. At 10–20 % there is a headache, at 30–40 % a person may lose consciousness, and above 50–60 % it is life-threatening. First aid: take the person into fresh air immediately and call an ambulance (103); in fresh air half of the COHb is cleared in about 4–5 hours, in pure oxygen in ≈ 1–1.5 hours. Prevention: do not sleep in a closed room with a sandal or a brazier, do not close the flue until the embers have completely died down, ventilate the room and install a CO detector.

CO₂ and carbonic acid (Practical 1)

Carbon(IV) oxide is prepared in the laboratory from marble and hydrochloric acid: $CaCO_{3} + 2 HCl \overset{}{\to} CaCl_{2} + H_{2} O + CO_{2} ↑$ . It is 1.5 times heavier than air (1.98 g/L at STP, air 1.29 g/L), so it is collected in a vessel with its mouth upwards by displacing the air, and it fills a beaker from the bottom — the lowest candle goes out first. CO₂ does not support combustion (fire extinguishers), and is colourless and odourless. Detection: a burning splint goes out; limewater turns cloudy: $Ca (OH)_{2} + CO_{2} \overset{}{\to} CaCO_{3} ↓ + H_{2} O$ . At 20 °C about 1.7 g of CO₂ dissolves in 1 L of water; the solution is weakly acidic — litmus turns red (pH ≈ 4): $CO_{2} + H_{2} O \overset{}{⇌} H_{2} CO_{3}$ , $H_{2} CO_{3} \overset{}{⇌} H^{+} + HCO_{3}^{-}$ . Carbonic acid is unstable and weak and exists only in solution; when fizzy water is opened, CO₂ escapes. Solid CO₂ — “dry ice” — sublimes at −78.5 °C at normal pressure without melting. In nature plants absorb CO₂ in photosynthesis: $6 CO_{2} + 6 H_{2} O \overset{}{\to} C_{6} H_{12} O_{6} + 6 O_{2}$ ; respiration and combustion return it — the carbon cycle. Safety: wear safety goggles when working with acid; tie back your hair near a burning candle.

Carbonates and hydrogencarbonates (Lab work 1)

Carbonic acid gives two kinds of salt: carbonates (CaCO₃, Na₂CO₃) and hydrogencarbonates (Ca(HCO₃)₂, NaHCO₃). The carbonates of alkali metals and ammonium, and all hydrogencarbonates, dissolve in water; other carbonates (CaCO₃, MgCO₃, BaCO₃) are practically insoluble. They convert into one another: $CaCO_{3} + CO_{2} + H_{2} O \overset{}{\to} Ca (HCO_{3})_{2}$ — groundwater containing CO₂ dissolves limestone; on heating or when the pressure drops the reverse reaction takes place: $Ca (HCO_{3})_{2} \overset{}{\to} CaCO_{3} ↓ + CO_{2} ↑ + H_{2} O$ — this is how stalactites and stalagmites in caves and limescale in a kettle form. Baking soda decomposes on heating: $2 NaHCO_{3} \overset{}{\to} Na_{2} CO_{3} + H_{2} O + CO_{2} ↑$ (dough rising), while calcined soda Na₂CO₃ is stable (melts at 851 °C). Qualitative test for the carbonate ion: under the action of acid CO₂ is released, and it turns limewater cloudy: $CO_{3}^{2 -} + 2 H^{+} \overset{}{\to} H_{2} O + CO_{2} ↑$ . Chlorides and sulfates do not “fizz” like this.

Safety and the model

Safety: light the spirit lamp with a match and put it out with the cap; do not point the mouth of a heated test tube at yourself or others; first take the gas delivery tube out of the limewater, and only then stop heating (otherwise liquid is sucked back into the hot test tube). Wear goggles when working with hydrochloric acid. Model: the CO in the room is taken to be uniformly mixed (a single “box” model); the amounts of CO from the heaters (sandal ≈ 8 g/h, stove with the flue closed early ≈ 25 g/h) are approximate model values — in reality they depend greatly on the fuel, the room and ventilation; COHb approaches the Haldane equilibrium with a half-time of ≈ 4 hours (at rest). Time in the room is sped up: 1 s ≈ 10 min. The dots are a schematic picture of CO: the real gas is invisible. CO₂ in the beaker is assumed to collect as a layer without mixing with the air; limewater and litmus solution absorb CO₂ completely (litmus solution until saturation), and ≈ 10 % of the gas from heated soda is absorbed in the limewater. The lattice models are at real distances, magnified ≈ 10⁸ times.

Did you know?

The fullerene C₆₀ was discovered in 1985 by Harry Kroto, Robert Curl and Richard Smalley (Nobel Prize 1996); its molecule resembles a football — 12 pentagons and 20 hexagons.
Graphene — a layer of graphite one atom thick — was separated in 2004 by Andre Geim and Konstantin Novoselov using ordinary sticky tape; they were awarded the Nobel Prize in Physics in 2010.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8
Phosphorus and mineral fertilisers — Grade 8
Silicon and silicates — Grade 9
Making cast iron and steel — Grade 9
Metals and alloys — Grade 9
Alkali metals and soda — Grade 9
Aluminium and its compounds — Grade 9