Investigating Chemical Reactions

Learning Objectives

In this lesson we will learn how chemical reactions can be classified, based on the way atoms are rearranged, or according to the energy changes that take place. We will also learn how to test for oxygen, hydrogen and carbon dioxide gas produced during chemical reactions.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the general rearrangement of atoms during combination, decomposition, displacement and combustion reactions.

  • Distinguish between exothermic and endothermic chemical reactions.

  • Distinguish between spontaneous and non-spontaneous chemical reactions.

  • Describe how to test for the presence of oxygen, hydrogen and carbon dioxide gas.

 

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Introduction

  • Chemical reactions involve the rearrangement of atoms to form new substances.
  • Chemical reactions can be categorised into general types, based on the way these atoms are rearranged.
  • Chemical reactions can also be classified based on the energy changes that take place.

 
chemical reaction particle collision

Chemical reactions involve the rearrangement of atoms.

 

Types of Chemical Reactions

  • There are countless different chemical reactions that take place in nature and in the laboratory.
    The following types of chemical reactions group many of these into categories according to generalised ways in which atoms are rearranged.

 

Combination Reactions

  • Combination reactions involve two or more reactants combining to form a single product.

 
combination synthesis reaction diagram

 

  • They can be generalised by the following equation:

 
combination synthesis reaction general formula

 

  • Example 1
  •     hydrogen  +  oxygen    water
        H2  +  O2    H2O

    Example 2
        carbon monoxide  +  oxygen    carbon dioxide
        CO  +  O2    CO2
    Example 3
        lithium oxide  +  carbon dioxide    lithium carbonate
        Li2O  +  CO2    Li2CO3

 

Decomposition Reactions

  • Decomposition reactions involve a single reactant breaking down to form two or more products.

 
decomposition reaction diagram

 

  • They can be generalised by the following equation:

 
decomposition reaction formula

 

  • Example 1
  •     magnesium nitride    magnesium  +  nitrogen
        Mg3N2    Mg  +  N2

    Example 2
        potassium chlorate    potassium chloride  +  oxygen
        KClO3    KCl  +  O2
    Example 3
        ammonium nitrate    nitrous oxide  +  water
        NH4NO3    N2O  +  H2O

 

Displacement Reactions

  • Displacement reactions involve the displacing (swapping) of atoms.
  • There are two types of types of displacement reactions – single displacement and double displacement.

 

Single Displacement Reactions

  • Single displacement reactions involve an element and a compound reacting to form a different element and compound:
  • element-1  +  compound-1    compound-2  +  element-2

 
single displacement reaction diagram

 

  • They can be generalised by the following equation:

 
single displacement reaction general formula

 

  • Example 1
  •     zinc  +  copper nitrate    zinc nitrate  +  copper
        Zn  +  Cu(NO3)2    Zn(NO3)2  +  Cu

    Example 2
        zinc  +  hydrobromic acid    zinc bromide  +  hydrogen
        Zn  +  HBr    ZnBr2  +  H2
    Example 3
        chlorine  +  potassium iodide    potassium chloride  +  iodine
        Cl2  +  KI    KCl  +  I2

 

Double Displacement Reactions

  • Double displacement reactions involve two compounds reacting to form two different compounds:
  • compound-1  +  compound-2    compound-3  +  compound-4

 
double displacement reaction diagram

 

  • They can be generalised by the following equation:

 
double displacement reaction general formula

 

  • Example 1
  •     silver nitrate  +  lithium chloride    silver chloride  +  lithium nitrate
        AgNO3  +  LiCl    AgCl  +  LiNO3

    Example 2
        potassium carbonate  +  magnesium fluoride    potassium fluoride  +  magnesium carbonate
        K2CO3  +  MgF2    KF  +  MgCO3
    Example 3
        sodium hydroxide  +  sulfuric acid    sodium sulfate  +  water
        NaOH  +  H2SO4    Na2SO4  +  H2O

 

Combustion Reactions

  • Combustion reactions involve the reaction of a hydrocarbon with oxygen.
    Hydrocarbons are a group of fuels containing only hydrogen and carbon atoms, which burn when reacting with oxygen.

 
complete combustion reaction diagram

 

  • Combustion reactions can be generalised by the following equation:

 
complete combustion reaction general formula

 

  • Example 1
  •     methane  +  oxygen    carbon dioxide  +  water
        CH4  +  O2    CO2  +  H2O

    Example 2
        butane  +  oxygen    carbon dioxide  +  water
        C4H10  +  O2    CO2  +  H2O
    Example 3
        ethylene  +  oxygen    carbon dioxide  +  water
        C2H4  +  O2    CO2  +  H2O

 

Energy and Chemical Reactions

  • The rearrangement of atoms during chemical reactions requires the breaking and forming of chemical bonds.
  • Breaking bonds absorbs energy, which is absorbed from the surrounding environment.

 
breaking chemical bonds energy endothermic

Breaking chemical bonds absorbs energy.

 

  • Forming bonds releases energy, which is transferred to the surrounding environment.

 
forming chemical bonds energy exothermic

Forming chemical bonds releases energy.

 

Exothermic and Endothermic Chemical Reactions

  • Chemical reactions can be classified based on whether they release heat or absorb heat.
  • An exothermic reaction is a chemical reaction that releases heat energy, causing the surrounding environment to increase in temperature.
    During these reactions, more energy is released from the formation of bonds than is absorbed from the breaking of bonds.
    Example
    Combustion reactions, which involve the burning of fuels, are highly exothermic reactions, as they release large amounts of heat.

 
exothermic chemical reaction

Exothermic chemical reactions release energy, heating up the surrounding environment.

 

  • An endothermic reaction is a chemical reaction that absorbs heat energy, causing the surrounding environment to decrease in temperature.
    During these reactions, more energy is absorbed from the breaking of bonds than is released from the formation of bonds.
    Example
    The reaction between barium hydroxide and ammonium chloride results in a dramatic drop in temperature, to well below 0 °C.

 
endothermic chemical reaction

Endothermic chemical reactions absorb energy, cooling down the surrounding environment.

 

Spontaneous and Non-Spontaneous Chemical Reactions

  • Chemical reactions can also be classified based on whether they occur spontaneously or not.
  • A spontaneous reaction is a chemical reaction that, once started, does not require a constant supply of energy to continue to completion.
    Example
    Once a candle is lit, it will continue to burn. It doesn’t need to be constantly relit.

 
candle burning spontaneous chemical reaction

Once lit, a candle will continue to burn.

(Image: Myriams-Fotos, Pixabay)

 

  • A non-spontaneous reaction is a chemical reaction that does require a constant supply of energy to continue.
    Example
    Recharging batteries involves a chemical reaction that requires a constant supply of electrical energy, If the electricity is turned off, the battery will stop recharging.
  • Spontaneous reactions are usually exothermic, but not always.
    For example, the endothermic reaction described above is spontaneous.

 
battery recharge non-spontaneous chemical reaction

Recharging a battery requires a constant supply of electrical energy.

(Image: rawpixel, Pixabay)

 

Testing for the Production of Gas during a Chemical Reaction

  • We have learned previously that the production of a gas is one sign that a chemical reaction may have taken place.
    Three gases that are common products of chemical reactions are oxygen, hydrogen and carbon dioxide.
    However, they are all colourless, odourless gases, which makes it impossible to distinguish between them without further investigation.
  • A commonly used test for distinguishing between these three gases involves collecting the gas produced during a chemical reaction in a test tube, then placing a lit splint into the test tube.
    The presence of each gas will result in a different outcome.

 

Testing for Oxygen Gas

  • If a lit splint is placed inside a test tube containing oxygen gas, the flame will become brighter.
    This is because the burning splint is a chemical reaction between the wood and oxygen in the air. Adding more oxygen ‘feeds’ the chemical reaction, creating a bigger and brighter flame.

 
flame test oxygen gas

Test for oxygen gas: oxygen will cause a flame to burn brighter.

 

Testing for Hydrogen Gas

  • If a lit splint is placed inside a test tube containing hydrogen gas, there will be a “pop” sound.
    This is because the hydrogen gas, in the presence of a flame, reacts rapidly with oxygen in the air to form water.
    This reaction can be represented by the following chemical equation:
    hydrogen  +  oxygen    water

 
pop test hydrogen gas

Test for hydrogen gas: exposing hydrogen to a flame will result in a “pop”.

 

Testing for Carbon Dioxide Gas

  • If a lit splint is placed inside a test tube containing carbon dioxide gas, the flame will be extinguished.
    This is because carbon dioxide does not react with oxygen, but instead smothers the flame by preventing oxygen in the air from getting near the flame.
    For this reason, carbon dioxide is used in certain types of fire extinguishers.

 
snuff test carbon dioxide gas

Test for carbon dioxide gas: carbon dioxide will extinguish a flame.

 

  • Another method for testing whether a gas is carbon dioxide is to bubble the gas through limewater (calcium hydroxide solution).
    If the limewater turns cloudy, the gas is carbon dioxide.
    The limewater turns cloudy because carbon dioxide gas reacts with the limewater to form a calcium carbonate precipitate (solid). Over time, this fine white solid will settle to the bottom of the tube/beaker.
    This reaction can be represented by the following chemical equation:
    calcium hydroxide  +  carbon dioxide    calcium carbonate  +  water

 

Summary

  • There are many different chemical reactions, which can be categorised into general types, based on the way atoms and molecules are rearranged.
  • Combination reactions involve two or more reactants combining to form a single product.
    They can be represented as: A + B AB
  • Decomposition reactions involve a single reactant breaking down to form two or more products.
    They can be represented as: AB A + B
  • Displacement reactions involve the displacing or swapping of atoms.
    Single displacement reactions can be represented as: A + BC AC + B
    Double displacement reactions can be represented as: AB + CD AD + CB
  • Combustion reactions involve the reaction of hydrocarbons with oxygen.
    They can be represented as: CxHy + O2 CO2 + H2O
  • Chemical reactions can also be classified based on energy changes that take place.
    An exothermic reaction releases heat energy into the surrounding environment.
    An endothermic reaction absorbs heat energy from the surrounding environment.
  • A spontaneous reaction does not require a constant supply of energy to continue.
    A non-spontaneous reaction does require a constant supply of energy to continue.
  • A lit splint can be used to test for and distinguish between oxygen, hydrogen and carbon dioxide gas.
    • Oxygen gas will cause a flame to burn brighter.
    • Hydrogen gas will cause a “pop” sound.
    • Carbon dioxide gas will extinguish a flame.
    The presence of carbon dioxide can also be confirmed if the gas turns limewater cloudy, when bubbled through it.

 
 
chemical reactions combination decomposition displacement

(Image: Daniele Pugliesi, Wikimedia Commons)

 

(Header image: Spiff, Wikimedia Commons)

 

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