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Chemistry - Fuels

Grade 9IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Fuels are substances that release energy, usually in the form of heat, when they are burned. Common examples include coal, natural gas, and petroleum.

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Fossil fuels (coal, oil, and natural gas) are non-renewable resources formed from the remains of plants and animals over millions of years under high pressure and temperature.

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Natural gas is composed mainly of methane, which has the chemical formula CH4CH_{4}.

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Petroleum (crude oil) is a complex mixture of hydrocarbons. It is separated into useful fractions by the process of fractional distillation based on different boiling points.

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The main fractions of petroleum and their uses are: Refinery gas (heating and cooking), Gasoline/Petrol (fuel for cars), Naphtha (making chemicals), Kerosene/Paraffin (jet fuel), Diesel oil/Gas oil (fuel for diesel engines), Fuel oil (fuel for ships and home heating systems), Lubricating oil (lubricants, waxes, and polishes), and Bitumen (making roads).

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As the carbon chain length increases in petroleum fractions: the boiling point increases, the viscosity increases (becomes thicker), and the flammability decreases.

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Complete combustion occurs when there is a plentiful supply of oxygen, producing carbon dioxide (CO2CO_{2}) and water (H2OH_{2}O).

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Incomplete combustion occurs when oxygen supply is limited, producing carbon monoxide (COCO) and/or carbon (soot) along with water (H2OH_{2}O).

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Pollutants from fuel combustion: CO2CO_{2} is a greenhouse gas contributing to climate change; COCO is a toxic gas that binds to hemoglobin; SO2SO_{2} (from sulfur impurities) and nitrogen oxides (NOxNO_{x}) cause acid rain.

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Hydrogen is used as a fuel in rockets and fuel cells. The only product of its combustion is water: 2H2+O2→2H2O2H_{2} + O_{2} \rightarrow 2H_{2}O.

📐Formulae

CH4+2O2→CO2+2H2OCH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O

2CH4+3O2→2CO+4H2O2CH_{4} + 3O_{2} \rightarrow 2CO + 4H_{2}O

CnH2n+2C_{n}H_{2n+2}

S+O2→SO2S + O_{2} \rightarrow SO_{2}

2H2+O2→2H2O2H_{2} + O_{2} \rightarrow 2H_{2}O

💡Examples

Problem 1:

A sample of 32 g32\text{ g} of methane (CH4CH_{4}) undergoes complete combustion. Calculate the mass of carbon dioxide (CO2CO_{2}) produced. (Relative atomic masses: C=12C = 12, H=1H = 1, O=16O = 16)

Solution:

  1. Molar mass of CH4=12+(4×1)=16 g/molCH_{4} = 12 + (4 \times 1) = 16\text{ g/mol}.
  2. Moles of CH4=3216=2 molCH_{4} = \frac{32}{16} = 2\text{ mol}.
  3. Balanced equation: CH4+2O2→CO2+2H2OCH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O.
  4. Mole ratio CH4:CO2=1:1CH_{4} : CO_{2} = 1 : 1, so 2 moles2\text{ moles} of CO2CO_{2} are produced.
  5. Molar mass of CO2=12+(2×16)=44 g/molCO_{2} = 12 + (2 \times 16) = 44\text{ g/mol}.
  6. Mass of CO2=2×44=88 gCO_{2} = 2 \times 44 = 88\text{ g}.

Explanation:

We first convert the mass of the fuel to moles using the molar mass. Based on the stoichiometry of the combustion reaction, we determine the moles of the product and then convert those moles back to mass.

Problem 2:

Explain why carbon monoxide is considered a dangerous pollutant when fuels are burned in confined spaces.

Solution:

COCO is a colorless, odorless, and toxic gas. It reacts with the hemoglobin in the blood to form carboxyhemoglobin, which prevents the blood from carrying oxygen to the body's cells, leading to suffocation.

Explanation:

This is a qualitative concept regarding the physiological impact of incomplete combustion products.