7.2 Chemical Reactions and Representations

7.2 Rearranging Atoms and Molecules

7.2.1 Chemical Reaction

  • A chemical reaction can be observed in various everyday phenomena, such as:
      - A cake rising in an oven.
      - A bath bomb fizzing in a bathtub.
      - An old car rusting.

  • These reactions provide evidence of chemical changes taking place.

7.2.2 Reactants and Products

  • Definitions:
      - Reactants: The original substances involved in a chemical reaction.
      - Products: The new substances formed as a result of the chemical reaction.

  • Example of Reactants and Products:
      - When hydrogen gas (H₂) is added to oxygen gas (O₂) and ignited, the product formed is water (H₂O).
      - The reaction can be summarized:
        - Reactants: H₂ + O₂
        - Product: H₂O

  • Understanding the Reaction Process:
      - The bonds between hydrogen atoms and oxygen atoms are broken during the reaction.
      - New bonds are formed between oxygen and hydrogen to create water.
      - It's important to notice that hydrogen and oxygen atoms are conserved in the reaction—no atoms are lost or gained.
      - Molecules can consist of identical atoms (like hydrogen gas and oxygen gas) or various atoms (like in water).

LESSON 7.2 LEARNING INTENTION

  • Goals:
      - Describe the difference between reactants and products.
      - Understand how chemical reactions are represented.

  • Chemical reactions involve bond breaking and formation, leading to a new arrangement of atoms and at least one new substance.

  • Observable changes during reactions may include:
      - Changes in temperature or color.
      - Formation of a visible gas or new solid.
      - Emission of odor.

7.2.3 Representing Chemical Reactions

  • Importance of Models:
      - Models help in understanding what occurs during chemical reactions.

  • Types of Representations:
      - Simple Particle Models: Use colored spheres to represent atoms.
        - Common conventions include:
          - Dark grey spheres for carbon.
          - Red spheres for oxygen.
          - White spheres for hydrogen.
      - Examples of Chemical Models:
        - Figure 7.3a shows a simple representation of a chemical reaction.
        - Figure 7.3b displays a molecular model set, illustrating how reactants combine to form products.

  • Significance of Models:
      - Visual models aid in comprehending the properties and arrangements of reactants and products.
      - Differentiation between ions, atoms, and molecules is facilitated by models.
      - Notable chemical models, such as DNA, assist in various fields including biology and medicine.

7.2.4 Word Equations

  • Definition:
      - A word equation uses the names of the substances involved in a reaction instead of chemical symbols or formulae.

  • Structure of Word Equations:
      - The format follows:
        - Reactants → Products
        - Reactants are listed on the left side, with products on the right, separated by an arrow (→) indicating reaction.
        - A plus sign (+) is used to separate multiple reactants or products.

  • Example:
      - The word equation for the reaction between hydrogen and oxygen forming water is:
        - Oxygen + Hydrogen → Water

  • Activity Suggestion:
      - Construct a model of the reaction using a molecular modeling kit or materials like plasticine and toothpicks.
        - For hydrogen, use four white atoms, forming two H₂ molecules.
        - For oxygen, use two red atoms to create O₂.
        - Rearrange to form two H₂O molecules.

7.2.5 Chemical Equations

  • Importance of Standardization:
      - Scientific communication about chemical reactions necessitates a common language.

  • Characteristics of Chemical Equations:
      - Chemical symbols and formulae provide detailed representations of reactions.

  • Writing Chemical Equations:
      - Requires knowledge of chemical formulae and involves basic mathematics for balancing.
      - Chemical Formulae: Quickly express the elements in a compound and the number of each atom.
      - Examples of Non-Metal Elements:
        - Common non-metals (C, H, O) can bond to form molecules.
        - Methane ( ext{CH}_4) shows 1 carbon atom and 4 hydrogen atoms bonded together.
      - Noble gases (He, Ne, Ar) exist as single atoms, while other gaseous elements (O₂, Cl₂) exist as diatomic molecules in reactions.

  • Ionic Compounds:
      - Ionic compounds are made up of metal and non-metal atoms forming giant lattices.
      - Example: Sodium chloride (NaCl) indicates a 1:1 ratio of sodium to chloride atoms.

  • Metals are represented solely by their elemental symbol (e.g., Al, Na, Mg).

  • Structure of Chemical Equations:
      - Similar to word equations:
        - Reactants on the left, products on the right.
      - Differences include:
        - Use of chemical formulae.
        - Physical states indicated as:
          - (g) for gas,
          - (l) for liquid,
          - (aq) for aqueous solution,
          - (s) for solid.
        - Balancing atoms requires the inclusion of coefficients in front of the formulae.
      - Example Chemical Reaction:
        - The reaction between sodium (Na) and chlorine gas (Cl₂) can be written as:
          - 2Na (s) + Cl₂ (g) → 2NaCl (s)

7.2 Common Formulae for Compounds

Table 7.1 - Formulae of Common Ionic Compounds

  • Sodium hydroxide: NaOH

  • Sodium chloride: NaCl

  • Sodium sulfate: Na₂SO₄

  • Sodium citrate: C₆H₅O₇Na₃

  • Sodium hydrogen carbonate: NaHCO₃

  • Copper(II) hydroxide: Cu(OH)₂

  • Copper(II) sulfate: CuSO₄

  • Magnesium chloride: MgCl₂

  • Mercury(II) oxide: HgO

Table 7.2 - Formulae of Common Covalent Compounds

  • Water: H₂O

  • Citric acid: C₆H₈O₇

  • Carbon dioxide: CO₂

  • Oxygen: O₂

  • Hydrochloric acid: HCl

  • Carbon monoxide: CO

  • Hydrogen: H₂

  • Methane: CH₄

  • Ammonia: NH₃

7.2 Quick Quiz

Q1 - Complete the Passage

  • Reactions take place when the bonds between atoms are broken and new bonds are formed, creating a new arrangement of atoms and at least one observable substance.

Q2 - Chemical Representation Terms

  • What is the term for a representation of a reaction using symbols and subscripts? Chemical Equation

Q3 - Molecular Formula Query

  • What is the molecular formula of water? H₂O

Q4 - Match Definitions to Terms

  • Terms to match:
      - Molecule: Group of atoms bonded together covalently.
      - Aqueous Solutions: Mixtures in which substances are dissolved in water.
      - Reactants: The original substances present in a chemical reaction.
      - Products: Chemical substances that result from a chemical reaction.
      - Diatomic Molecules: Substance containing two atoms only.
      - Statement Describing a Chemical Reaction: Chemical equation.