Chemical Reactions and Equations | Types Of Chemical Reactions | Examples Chemical Reactions
Chapter 5:
Chemical Reactions and Equations
Chemical reactions are processes in which substances (reactants) transform into new substances (products) through the breaking and forming of chemical bonds. These reactions are fundamental to all chemical processes in nature and in the lab. The ability to describe these reactions using chemical equations is a vital skill in chemistry, as it helps us understand the changes that take place at the molecular level.
In this chapter, we will:
- Define what a chemical reaction is.
- Explain how to represent chemical reactions using chemical equations.
- Explore different types of chemical reactions.
- Learn about the Law of Conservation of Mass.
- Practice how to balance chemical equations.
1. What is a Chemical Reaction?
A chemical reaction occurs when one or more substances (reactants) undergo a transformation to form one or more different substances (products). This transformation involves changes in the arrangement of atoms as chemical bonds are broken in the reactants and new bonds are formed in the products.
Key Features of Chemical Reactions:
- Reactants: The starting substances that undergo a change.
- Products: The new substances formed as a result of the reaction.
- Energy change: Chemical reactions often involve energy changes, either absorbing or releasing energy (in the form of heat, light, etc.).
- Irreversibility: Many chemical reactions are not easily reversed, especially if products are stable.
Example of a Chemical Reaction:
When hydrogen (H₂) reacts with oxygen (O₂) to form water (H₂O), the hydrogen and oxygen molecules break apart and rearrange themselves into water molecules.
2H2 + O2 → 2H2O
2. Chemical Equations: Representing Chemical Reactions
A chemical equation is a symbolic way of representing a chemical reaction. It shows the reactants on the left-hand side and the products on the right-hand side, with an arrow separating them to indicate the direction of the reaction.
Parts of a Chemical Equation:
- Reactants: Substances present before the reaction (left side of the equation).
- Products: Substances formed as a result of the reaction (right side of the equation).
- Arrow (→): Indicates the direction of the reaction (from reactants to products).
- Coefficients: Numbers placed before the chemical formulas to indicate the number of molecules or moles of each substance involved in the reaction.
- Subscripts: Numbers within the chemical formulas that indicate the number of atoms of each element in a molecule.
Example of a Chemical Equation:
The reaction between nitrogen (N₂) and hydrogen (H₂) to form ammonia (NH₃) can be written as:
N2+3H2 → 2N H3
In this equation:
- N₂ and H₂ are the reactants.
- NH₃ is the product.
- The coefficients (1 for N₂, 3 for H₂, and 2 for NH₃) indicate the ratio of molecules involved in the reaction.
3. Types of Chemical Reactions
There are several types of chemical reactions, each characterized by the way reactants transform into products. Some of the most common types include:
1. Combination Reactions (Synthesis)
In a combination reaction, two or more reactants combine to form a single product.
A+B→AB
Example:
Hydrogen gas reacts with chlorine gas to form hydrogen chloride (HCl).
H2 + Cl2 → 2HCl
2. Decomposition Reactions
In a decomposition reaction, a single compound breaks down into two or more simpler substances.
AB → A + BA
Example:
The decomposition of water into hydrogen and oxygen gases:
2H2O→2H2+O2
3. Single Displacement Reactions (Substitution)
In a single displacement reaction, one element displaces another in a compound.
A+BC→AC+BA + BC \rightarrow AC + B
Example:
Zinc displaces copper from copper sulfate solution to form zinc sulfate and copper metal.
Zn + CuSO4 → ZnSO4
4. Double Displacement Reactions (Metathesis)
In a double displacement reaction, the cations and anions of two compounds switch places, forming two new compounds.
AB+CD→AD+CB
Example:
When sodium chloride (NaCl) reacts with silver nitrate (AgNO₃), silver chloride (AgCl) and sodium nitrate (NaNO₃) are formed.
NaCl + AgNO3→ AgCl + NaNO3NaCl
5. Combustion Reactions
In a combustion reaction, a substance (usually a hydrocarbon) reacts with oxygen to produce carbon dioxide, water, and energy.
CxHy+O2→CO2+H2OCxHy + O₂ \rightarrow CO₂ + H₂O
Example:
The combustion of methane (CH₄) in oxygen:
CH4+2O2→CO2+2H2OCH₄ + 2O₂ \rightarrow CO₂ + 2H₂O
4. Law of Conservation of Mass
One of the fundamental principles of chemistry is the Law of Conservation of Mass, which states that:
- Mass is neither created nor destroyed in a chemical reaction. This means that the total mass of the reactants must equal the total mass of the products.
In terms of atoms, this law implies that the number of atoms of each element must be the same on both sides of a chemical equation. This is why we must balance chemical equations.
5. Balancing Chemical Equations
A chemical equation must be balanced to obey the Law of Conservation of Mass. This means that the number of atoms of each element must be the same on both sides of the equation. To balance an equation, we adjust the coefficients in front of the chemical formulas without changing the subscripts.
Steps to Balance a Chemical Equation:
- Write the unbalanced equation: Identify the reactants and products and write the chemical equation with the correct chemical formulas.
- Count the number of atoms of each element: Count the atoms of each element on both sides of the equation.
- Balance one element at a time: Start with the element that appears in the least number of compounds and adjust the coefficients to balance it.
- Check and adjust coefficients: Continue balancing the other elements, adjusting the coefficients as necessary.
- Verify: Make sure that the equation is balanced by counting the atoms again.
Example of Balancing an Equation:
Consider the combustion of propane (C₃H₈) in oxygen to produce carbon dioxide and water:
C3H8+O2→CO2+H2OC₃H₈ + O₂ \rightarrow CO₂ + H₂O
Step 1: Balance carbon atoms (C):
C3H8+O2→3CO2+H2OC₃H₈ + O₂ \rightarrow 3CO₂ + H₂O
Step 2: Balance hydrogen atoms (H):
C3H8+O2→3CO2+4H2OC₃H₈ + O₂ \rightarrow 3CO₂ + 4H₂O
Step 3: Balance oxygen atoms (O): There are 10 oxygen atoms on the right (6 from CO₂ and 4 from H₂O). To balance the oxygen atoms, place a coefficient of 5 in front of O₂:
C3H8+5O2→3CO2+4H2OC₃H₈ + 5O₂ \rightarrow 3CO₂ + 4H₂O
Now the equation is balanced.
6. Energy Changes in Chemical Reactions
Chemical reactions often involve energy changes. These energy changes occur because the bonds between atoms in the reactants are broken and new bonds are formed in the products.
- Exothermic Reactions: Reactions that release energy, usually in the form of heat. Combustion reactions are typically exothermic.Example: When methane burns, energy is released:CH4+2O2→CO2+2H2O+EnergyCH₄ + 2O₂ \rightarrow CO₂ + 2H₂O + \text{Energy}
- Endothermic Reactions: Reactions that absorb energy. In these reactions, the products have higher energy than the reactants.Example: Decomposition of calcium carbonate (CaCO₃) absorbs heat:CaCO3→CaO+CO2+Energy (absorbed)CaCO₃ \rightarrow CaO + CO₂ + \text{Energy (absorbed)}
Conclusion and Transition to the Next Chapter
In this chapter, we explored the basics of chemical reactions and how to represent them using chemical equations. We learned about different types of reactions and the importance of balancing chemical equations to comply with the Law of Conservation of Mass.
In the next chapter, we will delve deeper into “The Mole Concept and Stoichiometry,” where we will learn how to quantify substances in a reaction, calculate the amounts of reactants and products, and use the mole concept to solve problems related to chemical reactions. This chapter will build upon our understanding of chemical equations and introduce key concepts for analyzing reactions quantitatively.
