Chemical Reactions

A chemical reaction is a process that results in the interconversion of chemical species. In simpler terms, chemical reactions consist of making and/or breaking chemical bonds to form new products.

Chemical Reactions Chemical Reactions

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Table of contents

    Have you ever tried to make soap at home? The art of soap-making is a type of chemical reaction! First, look at the structure of a soap molecule.

    Chemical Reactions Structure of Soap StudySmarterFigure 1. Soap molecule structure

    Soap is a type of salt made up of fatty acids. The structure of soap consists of a nonpolar hydrocarbon chain containing a carboxylic acid group. The carboxylic acid group forms an ionic bond with a sodium or potassium ion. So, soap has a hydrophobic end that does not dissolve in water (hydrocarbon chain) and a hydrophilic end (ionic salt). When soap comes into contact with grease and water, the hydrophobic end of the soap interacts with the nonpolar molecules of grease and suspends the grease/oil, allowing it to be washed away by water!

    Soap forms by the reaction between the triglycerides in fat/oil and an alkali such as aqueous sodium hydroxide or potassium hydroxide.

    The chemical reaction for the making of soap is:

    Chemical Reactions saponification chemical process StudySmarterFigure 2. The saponification process

    • First, we will discuss chemical reactions and how to balance them.
    • Then, we will look at the different types of chemical reactions and give some examples.

    Chemical Reaction Definition


    During a chemical reaction, the atoms of the reactants rearrange themselves, forming new bonds and breaking existing ones to create one or more different substances, known as products.

    A chemical reaction is a process that results in the conversion of one or more chemical substances into one or more different substances.

    When this happens, we can see changes in colour, gas formation, temperature changes, or even the formation of a precipitate. Or, in some cases, the result looks the same as before.

    Chemists use chemical equations to show how what happens to the reactants and products involved in a chemical reaction.

    • Reactants are substances that react together to form products.
    • Products are the new substances formed after the chemical reaction takes place.

    Chemical Reaction Balance

    A very important skill to have when it comes to chemical reactions is the ability to balance chemical equations.

    A chemical equation is balanced when the number of atoms is the same on both sides.

    To balance chemical equations, there are some steps we need to follow. As an example, take a look at the chemical equation below: $$ \text{K + H}_{2}\text{O} \to \text{KOH + H}_{2} $$

    Step 1: Count the number of atoms present on each side of the chemical equation.

    First, we need to count the number of atoms for each type of atom present in the reactant and on the product side. To find out the number of atoms for each type, we can multiply the coefficient (which, in this case, is 1) by the number on the subscript.

    To make it easier to remember, we can make a simple table below the chemical equation.

    Chemical reactions Counting the numbers of atoms presents on each side of the equation Chemical reaction balance StudySmarterFigure 4. Counting the number of atoms present on each side of the equation

    If the number of atoms on each side is the same, then it means that your equation is already balanced. Now, if the number of atoms on each side is different, then you need to go to step two.

    Step 2: Determine how many atoms you must add in order to balance the chemical equation.

    In order to balance this chemical equation, we have to look for multiples of the coefficient. In this example, the coefficients are 1.

    The only way to get the same amount of H atoms on both sides you be to add a coefficient of 2 to the H2O molecules, and a coefficient of 2 to the KOH molecules. This way, we would get 4 H atoms on each side.

    However, this would also change the number of K atoms on the product side, and the number of oxygen atoms on both sides. To overcome this, we can add a coefficient of 2 to the K on the reactant side. Now, the reaction should be balanced!

    Chemical reactions Adding coefficients to balance a chemical equation Chemical reaction balance StudySmarterFigure 5. Adding coefficients to balance a chemical equation

    Types of Chemical Reactions

    There are five basic types of chemical reactions:

    • Combination or synthesis reactions
    • Decomposition reactions
    • Combustion reactions
    • Single-replacement reactions
    • Double-replacement reactions

    Replacement reactions are also called "displacement reactions". Neutralization reactions are an example of replacement reactions. Synthesis and single-replacement reactions can be redox reactions, which are chemical reactions where electrons are transferred from one species to another.

    Synthesis Reactions

    Synthesis reactions are reactions in which two or more substances combine to form a new substance. The general formula for synthesis reactions is: \( A + B \to C \).

    For example, the formation of hydrogen bromide (HBr) is the result of a synthesis reaction between hydrogen and bromine.

    $$ H_{2}\text{ (g)} \text{ }+ \text{ }Br_{2} \text{ (l)}\to 2\text{ }HBr \text{ (g)} $$

    Oxidation-Reduction (redox) Reactions

    In redox reactions, electrons are transferred from one species (oxidized) to another species (reduced). A change in oxidation state indicates that a redox reaction has taken place.

    • Oxidation is the loss of electrons
    • Reduction is the gain of electrons

    For example, the reaction between zinc (Zn) and manganese oxide (MnO2) is a type of oxidation-reduction reaction.

    $$ Zn\text{ }( s)\text{ + }2\text{ }MnO_{2}\text{ }(s)\text{ }\to \text{ }ZnO \text{ }(s)\text{ + }Mn_{2}O_{3}\text{ }(s) $$

    For an in-depth explanation of oxidation-reduction chemical reactions, check out "Redox Reactions"!

    Decomposition Reactions

    Next, we have decomposition reactions. Decomposition reactions are chemical reactions in which one substance gets converted into 2 or more substances, and the general formula for this is: \( AB \to A + B \).

    The decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) is an example of a decomposition reaction.

    $$ CaCO_{3}\text{ } (s)\to \text{ } CaO \text{ (s)} \text{ + } CO_{2} \text{ (g)} $$

    Combustion Reactions

    Combustion reactions are reactions involving a hydrocarbon reacting with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). These reactions give out energy as heat and light.

    A common combustion reaction is the reaction between the hydrocarbon methane (CH4) and oxygen (O2).

    $$ CH_{4}\text{ }(g) + O_{2}\text{ } (g) \text{ }\to\text{ } CO_{2} \text{ }(g)\text{ }+\text{ }H_{2}O \text{ }(g) $$

    Single Replacement Reactions

    Single replacement reactions occur when one element in a compound is replaced by another element. The general formula for single displacement reactions is: \( \color{Teal} A \color{orchid} B +\color{Blue} C \color{black} \to \color{Teal} A \color{black} + \color{orchid} B \color{Blue}C \)

    In single replacement reactions, the element that tends to form cations will replace the cation in a compound. Similarly, the element that tend to form anions will replace the anion in a compound.

    For example, in a reaction between zinc chloride (ZnCl2) and copper (Cu), copper replaces the Zn to form cooper chloride (CuCl2).

    $$ ZnCl_{2}\text{ } (s)\text{ + } Cu \text{ } (s) \text{ } \to \text{ }CuCl_{2}\text{ } (s)\text{ } + \text{ } Zn\text{ } (s) $$

    Double Replacement Reactions

    Double replacement reactions occur when two compounds reacts in aqueous solutions, causing the cations and anions of the two reactants to "trade partners or switch places" to form two new compounds. The general formula is this case is: \( \color{Teal} A\color{Orchid} B\color{black}\text{ } + \text{ }\color{Blue} C\color{Orange} D\color{black} \text{ }\to \text{ }\color{Teal} A\color{Orange} D\color{black}\text{ } + \text{ }\color{Orchid} B\color{Blue} C \).

    For example, the reaction between mercury (II) nitrate and diammonium sulfide in aqueous solutions to yield mercuric sulfide and ammonium nitrate is a type of double replacement reaction.

    $$ Hg(NO_{3})_{2} \text{ }(aq)\text{ + }(NH_{4})_{2}S\text{ }(aq) \text{ }\to HgS \text{ }(s) \text{ + } 2NH_{4}NO_{3} \text{ } (aq) $$

    Neutralization Reactions

    Neutralization reactions are a special type of double replacement reactions in which one reactant is an acid and the other is a base, giving salt and water as products. The equation below is an example of a neutralization reaction.

    $$ HCl \text{ (acid) + }LiOH\text{ (base) } \to LiCl \text{ (salt)}+ H_{2}O \text{ (water)} $$

    Chemical Reaction of Photosynthesis

    Now that you are aware of the different types of chemical reactions that can occur around you, let's take a look at the chemical reaction happens in photosynthesis.

    Photosynthesis is the process of using solar energy to convert carbon dioxide (CO2) and H2O into carbohydrates (glucose) and oxygen (O2).

    The process and chemical reaction of photosynthesis is shown in the figure below.

    Chemical Reaction Examples

    Lastly, let's look at some examples involving chemical reactions. Let's start with the following reaction: \( \text{Zn + AgNO}_{3}\to \text{Ag + Zn(NO}_{3}\text{)}_{2} \).

    Can you guess which type of chemical reaction it is? Well, if you guessed that it is a decomposition reaction, you are right!

    Now, let's amplify the difficulty a bit and solve a problem similar to what you might see in your exam.

    What type of chemical reaction occurs between CoCl3 and Ba(OH)2?

    $$ 2\text{ }COCl_{3}\text{ + } 3\text{ }Ba(OH)_{2}\text{ }\to\text{ }3\text{ }BaCl_{2} \text{ + }2\text{ }Co(OH)_{3} $$

    1. Combustion
    2. Single replacement
    3. Double replacement
    4. Decomposition
    5. Synthesis

    The chemical reaction above is an example of a double replacement reaction because cations and anions of the two reactants "switched places" to form two new compounds.

    Now, I hope that you feel more confident in your understanding of chemical reactions!

    Chemical Reactions - Key takeaways

    • A chemical reaction is a process that results in the interconversion of chemical species.
    • Synthesis reactions are reactions in which two or more substances combine to form a new substance, whereas decomposition reactions are chemical reactions in which one substance gets converted into 2 or more substances.
    • Redox reactions are reactions in which electrons are transferred from one species (oxidized) to another species (reduced).
    • Combustion reactions are reactions involving a hydrocarbon reacting with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).
    • Single replacement reactions occur when one element in a compound is replaced by another element.
    • Double replacement reactions occur when two compounds reacts in aqueous solutions, causing the cations and anions of the two reactants to "trade partners or switch places" to form two new compounds.

    References

    1. AP ® Chemistry COURSE AND EXAM DESCRIPTION Effective Fall 2020. (n.d.). https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf
    2. David, M., Howe, E., & Scott, S. (2015). Head-Start to A-level Chemistry. Cordination Group Publications (Cgp) Ltd.
    3. Jespersen, N. D., & Kerrigan, P. (2021). AP chemistry premium 2022-2023. Kaplan, Inc., D/B/A Barron’s Educational Series.
    4. Moore, J. T., & Langley, R. (2021). McGraw Hill : AP chemistry, 2022. Mcgraw-Hill Education.
    5. Princeton Review (Firm. (2020). Fast track chemistry : essential review for AP, honors, and other advanced study. The Princeton Review.
    6. Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
    Frequently Asked Questions about Chemical Reactions

    What happens during a chemical reaction?

    During a chemical reaction, different substances combine to form a new substance. When this happens, we see changes in color, gas formation, temperature changes, or even the formation of a precipitate.

    What will increase the rate of a chemical reaction?

    Increasing temperature, concentration, and surface area will increase the rate of a chemical reaction. 

    What is a chemical reaction?

    A chemical reaction is when different chemicals are combined to produce a new substance.

    How do enzymes speed up chemical reactions?

    Enzymes are catalysts that work by increasing the rate of a reaction. They achieve this by lowering the activation energy needed to start a reaction.

    What are the reactants in a chemical reaction?

    In a chemical reaction, reactants are the chemicals on the left side of the equation. Reactants will react with each other to form the product.

    Test your knowledge with multiple choice flashcards

    True or false: chemical reactions consist of reactants and products.

    ____ are the chemicals on the left side of the chemical equation.

    ____ are the chemicals on the right side of the equation.

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