Chemical Reactions and Equations

Introduction to Chemical Reactions

Chemical reactions are processes in which substances undergo chemical changes resulting in the formation of new substances with different properties. These changes involve the breaking of old bonds and the formation of new bonds between atoms. In everyday life, many such changes occur around us, such as milk turning sour when left at room temperature, iron objects rusting when exposed to humid air, grapes fermenting, food being cooked and digested, and respiration in living beings. These examples illustrate that the nature and identity of the initial substances have changed, indicating that chemical reactions have taken place. Unlike physical changes, chemical changes result in new substances with properties different from the original substances. To understand chemical reactions better, several activities can be performed, such as burning magnesium ribbon to observe the formation of magnesium oxide, mixing lead nitrate and potassium iodide solutions to observe precipitate formation, and reacting zinc granules with dilute acids to observe gas evolution and temperature change. These observations help identify chemical reactions by changes in state, color, gas evolution, and temperature changes. Overall, chemical reactions are fundamental processes that transform substances into new products, playing a vital role in nature and industry.

• Chemical reactions involve chemical changes producing new substances with different properties.
• Examples include souring of milk, rusting of iron, fermentation, cooking, digestion, and respiration.
• Chemical changes involve breaking and making of chemical bonds.
• Observations such as change in color, state, gas evolution, and temperature indicate chemical reactions.
• Chemical reactions differ from physical changes as they produce new substances.
• Activities like burning magnesium and reacting zinc with acid demonstrate chemical reactions.
• 📌 Chemical reaction: Process where substances change chemically to form new substances.
• 📌 Reactants: Starting substances in a chemical reaction.
• 📌 Products: New substances formed in a chemical reaction.

1.1 Chemical Equations

Chemical equations provide a concise and symbolic way to represent chemical reactions. Instead of describing reactions in long sentences, word equations and then chemical equations using formulae are used. For example, the burning of magnesium in oxygen can be written as a word equation: Magnesium + Oxygen → Magnesium oxide. Here, magnesium and oxygen are reactants, and magnesium oxide is the product. Using chemical formulae, this reaction is represented as Mg + O₂ → MgO. However, this skeletal equation is unbalanced because the number of oxygen atoms on the left (2) does not equal the number on the right (1). According to the law of conservation of mass, matter cannot be created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of the equation. Balancing chemical equations involves adjusting coefficients (numbers before formulas) to ensure equal numbers of atoms on both sides without changing the formulas themselves. For example, the reaction of zinc with dilute sulphuric acid is Zn + H₂SO₄ → ZnSO₄ + H₂, which is already balanced. Another example is the reaction of iron with steam producing iron oxide and hydrogen, initially unbalanced as Fe + H₂O → Fe₃O₄ + H₂. By balancing oxygen, hydrogen, and iron atoms stepwise, the balanced equation becomes 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Physical states of reactants and products can also be indicated using symbols (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions. Reaction conditions such as temperature, pressure, or catalysts can be noted above or below the reaction arrow to provide more information. Thus, chemical equations are essential tools for representing chemical reactions clearly and accurately. **Table on page 3 (5×3)** | Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) | | --- | --- | --- | | Zn | 1 | 1 | | H | 2 | 2 | | S | 1 | 1 | | O | 4 | 4 | **Table on page 4 (4×3)** | Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) | | --- | --- | --- | | Fe | 1 | 3 | | H | 2 | 2 | | O | 1 | 4 | **Table on page 4 (3×3)** | Atoms of oxygen | In reactants | In products | | --- | --- | --- | | (i) Initial | 1 (in H₂O) | 4 (in Fe₃O₄) | | (ii) To balance | 1×4 | 4 | **Table on page 4 (3×3)** | Atoms of hydrogen | In reactants | In products | | --- | --- | --- | | (i) Initial | 8 (in 4 H₂O) | 2 (in H₂) | | (ii) To balance | 8 | 2 × 4 | **Table on page 5 (3×3)** | Atoms of iron | In reactants | In products | | --- | --- | --- | | (i) Initial | 1 (in Fe) | 3 (in Fe₃O₄) | | (ii) To balance | 1×3 | 3 | **Table on page 13 (6×1)** | QUESTIONS | | --- | | 1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? | | 2. Give an example of a double displacement reaction other than the one given in Activity 1.10. | | 3. Identify the substances that are oxidised and the substances that are reduced in the following reactions. | | (i) $4\mathrm{Na}(\mathrm{s}) + \mathrm{O}_2(\mathrm{g}) \rightarrow 2\mathrm{Na}_2\mathrm{O}(\mathrm{s})$ | | (ii) $\mathrm{CuO}(\mathrm{s}) + \mathrm{H}_2(\mathrm{g}) \rightarrow \mathrm{Cu}(\mathrm{s}) + \mathrm{H}_2\mathrm{O}(\mathrm{l})$ |

• Chemical equations symbolically represent chemical reactions using formulas and symbols.
• Reactants are written on the left, products on the right, separated by an arrow.
• Equations must be balanced to satisfy the law of conservation of mass.
• Balancing involves adjusting coefficients without changing chemical formulas.
• Physical states of substances can be indicated using (s), (l), (g), and (aq).
• Reaction conditions like temperature, pressure, and catalysts can be shown above/below the arrow.
• 📌 Chemical equation: Symbolic representation of a chemical reaction.
• 📌 Balanced equation: Chemical equation with equal numbers of atoms on both sides.
• 📌 Coefficient: Number placed before a formula to balance atoms.