LESSON 100: You’re Fired!

LESSON 100
You’re Fired!
Combustion

THINK ABOUT IT

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An automobile engine burns gasoline or ethanol. In a fireplace, you burn paper and wood. In a barbecue grill, you burn charcoal, lighter fluid, and an occasional marshmallow. While many substances burn, others—like sodium chloride, table salt, and water—do not.

What types of substances burn?

To answer this question, you will explore

Substances That Burn

EXPLORING THE TOPIC

Substances That Burn

HISTORY CONNECTION

HISTORY

CONNECTION

From the 1880s until the 1930s, photographers would ignite a sample of magnesium to produce the flash needed to take a picture. For safety and simplicity, flash bulbs, which contained magnesium wire or thin sheets of magnesium foil, were then invented. In the 1980s, these bulbs were replaced with electronic flashes.

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© Steve Hamblin/Alamy

A wide variety of substances can burn, or combust. Solids, such as wood and coal, liquids, such as methanol or kerosene, and gases, such as methane or hydrogen, all combust. Something besides phase must determine whether or not a material can burn.

Use the table to compare substances that combust with those that do not combust.

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Using the information in the table, it is possible to make some generalizations about whether or not a substance will combust.

Combustion Reactions

Combustion Reactions

Combustion is a chemical reaction in which an element or a compound reacts with oxygen and releases energy in the form of heat and light. When molecular covalent substances containing mainly carbon and hydrogen burn, they produce carbon dioxide and water. When metallic substances combust, they produce a metal oxide.

These are the chemical equations for the combustion of methane and of magnesium.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat + light

2Mg(s) + O2(g) → 2MgO(s) + heat + light

Oxygen is always a reactant in a combustion reaction. The other reactant in a combustion reaction is usually called a fuel. In these reactions, methane and magnesium are considered the fuels.

Notice that both reactions have heat and light on the product side of the equation. Remember, heat is not a substance, but chemists sometimes include it in the equation to keep track of energy changes.

HISTORY CONNECTION

HISTORY

CONNECTION

Around 1680, Irish physicist Robert Boyle experimented with creating a ready source of fire by coating a small piece of wood with sulfur to be struck against phosphorus-coated paper. Yet it was not till the mid-1800s that efficient manufacturing of the safety match was developed by Johan Lundstrom of Sweden.

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© webking/iStockphoto

If a compound is already combined with oxygen, it is not likely to combust and combine with more oxygen. This is why some molecular covalent compounds like water, H2O, and carbon dioxide, CO2, do not burn. Other examples include sulfur trioxide, SO3(g), and nitrogen dioxide, NO2(g).

FIRE—A SPECIAL TYPE OF COMBUSTION

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© Tobias Machhaus/iStockphoto

The photo shows the flame produced when methane burns. When we call something a fire, it is because a flame is present during the combustion reaction. The combustion of methane produces a fire.

Not all combustion reactions produce fire. The burning of magnesium produces a bright light but no flame. The light produced is so bright that it can damage your eyes. Most elemental metals are combustible but do not produce fire.

Ash is often left over after a fire. For molecular compounds, ash is composed of unburned or partially burned reactants. After combustion of metallic substances, the product may look like ash, but it is actually the metal oxide product of the reaction.

LESSON SUMMARY

LESSON SUMMARY

What types of substances burn?

KEY TERM

combustion

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The process of burning is also referred to as combustion. Combustion is the combining of compounds or elements with oxygen through a chemical reaction that produces energy in the form of heat and light. Most ionic compounds do not combust. Most molecular covalent compounds are combustible. When molecular substances that are made mostly of hydrogen and carbon combust, they produce carbon dioxide and water. When metals combust, they form metal oxides, producing heat and light but no flame.

Exercises

Reading Questions

  1. What should you look for in a chemical formula to decide if a compound or element will not combust?

  2. Name three things that are true of every combustion reaction.

Reason and Apply

  1. Which of the substances listed below will combust? Explain your reasoning.

    1. Na2SO4(s)

    2. Cu(s)

    3. MgO(s)

    4. Na(s)

    5. C2H6O(l)

    6. Ar(g)

  2. Balance the following equations for combustion reactions. Circle the fuel for each reaction.

    1. C2H6(g) + O2(g) → CO2(g) + H2O(l)

    2. C6H12O6(s) + O2(g) → CO2(g) + H2O(l)

    3. C2H5OH(l) + O2(g) → CO2(g) + H2O(l)

    4. C21H24N2O4(s) + O2(g) → CO2(g) + H2O(l) + NO2(g)

    5. C2H5SH(l) + O2(g) → CO2(g) + H2O(l) + SO2(g)

  3. What are the products of these combustion reactions? Write balanced chemical equations for each reaction.

    1. C7H6O(l) + O2(g) →

    2. CH3COCH3(l) + O2(g) →

    3. H2C2O4(s) + O2(g) →

    4. Ca(s) + O2(g) →

    5. Li(s) + O2(g) →

    6. Si(s) + O2(g) →