Chemical Reactions and Equations

• A chemical reaction occurs when the nature and identity of a substance changes
• Physical and chemical changes have been studied in previous classes
• The following observations help determine if a chemical reaction has taken place:

Indicators of a Chemical Reaction

• Change in state
• Change in color
• Evolution of a gas
• Change in temperature

Activities Overview

• Activity 1.1 involves burning a magnesium ribbon in air
• Activity 1.2 involves reacting lead nitrate solution with potassium iodide solution
• Activity 1.3 involves reacting zinc granules with dilute hydrochloric or sulfuric acid

Chemical Equations

• A chemical reaction occurs when a magnesium ribbon is burnt in oxygen, resulting in magnesium oxide
• Reactants are the substances that chemically change, while products are the new substances formed
• The substances involved in the chemical reaction are magnesium and oxygen (reactants) producing magnesium oxide (product)
• Word equations illustrate the change of reactants into products, using an arrow
• Reactants are on the LHS with a plus sign, products are on the RHS with a plus sign
• The arrowhead indicates the reaction's direction

Writing Chemical Equations

• Representing chemical equations using chemical formulas is more concise
• Magnesium, oxygen, and magnesium oxide formulas can be used to write: Mg + O2 → MgO
• Count and compare the number of atoms of each element on both sides of the arrow
• A balanced equation has the same number of atoms for each element on both sides; otherwise, it is unbalanced
• An unbalanced chemical equation is a skeletal chemical equation

Balancing Chemical Equations

• The law of conservation of mass states mass is neither created nor destroyed in a chemical reaction
• The total mass of elements in reactants must equal the total mass of elements in products
• The number of atoms for each element must remain the same before and after a chemical reaction
• Skeletal chemical equations need balancing to reflect mass conservation
• The word equation for Activity 1.3 is: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
• This translates to: Zn + H₂SO₄ → ZnSO4 + H₂
• In said reaction, the number of atoms of each element is the same on both sides, balanced and equal
• 1 atom of Zinc, 2 atoms of Hydrogen, 1 atoms of Sulfer, and 4 atoms of Oxygen on both sides
• Trying to balance: Fe + H2O → Fe3O4 + H2

Steps to Balancing Chemical Equations

• Step I: Draw boxes around each formula, do not change anything inside
• Therefore: Fe + H2O → Fe3O4 + H2
• Step II: List the number of atoms of each element present in unbalanced equation
• 1 iron atom, 2 hydrogen atoms, and 1 oxygen atom in the reactants (LHS)
• 3 iron atoms, 2 hydrogen atoms, and 4 oxygen atom in the products (RHS)
• Step III: Begin balancing by focusing on the compound with the most atoms
• Maximize atom count using Fe3O4 and element, oxygen
• There are 4 oxygen atoms on the RHS and only 1 on the LHS
• To balance oxygen atoms, remember that it is impossible to alter formulae of the compounds or elements involved in the reactions
• Add '4' as 4 H2O and not H2O4 or (H2O)4
• Now partly balanced equation: Fe + 4 H₂O → Fe3O4 + H₂
• Step IV: Balance elements to proceed further like Hydrogen
• To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS
• Fe + 4 H₂O → Fe3O4 + 4 H₂ (partly balanced equation)
• Step V: Examine previous equation and pick the third element, Iron
• 1 iron atom in the reactants (LHS) initially
• 3 iron atoms in the product (RHS) initially
• 3 Fe + 4 H2O → Fe3O4 + 4 H2
• Step VI: Check the correctness of the balanced equation
• Therefore: 3Fe + 4H2O → Fe3O4 + 4H2 (balanced equation)
• Numbers of atoms of elements on both sides of Eq. are equal and balanced
• Balancing chemical equations is called the hit-and-trial method
• This method makes trials to balance the equation by using the smallest whole number coefficient
• Step VII: Writing symbols of physical states, gaseous (g), liquid (l), aqueous (aq) and solid (s)
• More informative equation: 3Fe(s) + 4H2O(g) → Fe3O₄(s) + 4H2(g)
• Note: (g) is used to indicate in this reaction, water is used in the form of steam
• Reaction conditions like temperature, pressure, catalyst, may be written above and/or below the arrow: CO(g) + 2H2 (g) → CH3 OH(l)

Questions

• Why magnesium ribbon be cleaned before burning in air?
• Equations: i) Hydrogen + Chlorine → Hydrogen chloride, ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride, iii) Sodium + Water → Sodium hydroxide + Hydrogen
• Write balanced chemical equation, and include stated symbols

Types of Chemical Reactions

• During a chemical reaction atoms of one element do not change into those of another element
• Chemical reactions involve the breaking and making of bonds between atoms to produce new substances

Combination Reaction

• Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat: CaO(s) +H₂O(l) → Ca(OH)₂(aq) + Heat
• In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide
• A reaction in which a single product is formed from two or more reactants is a combination reaction.

Fun Fact

• Solution of slaked lime produced is used for whitewashing
• Calcium hydroxide reacts with the carbon dioxide in air to form a thin layer of calcium carbonate
• Chemical formula for marble is also CaCO3: Ca(OH)₂(aq) + CO₂(g) → CaCO3(s) + H₂O(l)

Further Examples of Combination Reactions

• Burning of coal: C(s) + O2(g) → CO2(g)
• Formation of water from H2(g) and O2(g): 2H2(g) + O2(g) → 2H2O(l)
• In simple language when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions
• There will be a large amount of heat evolved, in which heat is released and the reactions are exothermic
• Burning of natural gas: CH4(g) + 2O2 (g) → CO₂(g) + 2H₂O (g) is exothermic
• Respiration is an exothermic process, because food is broken down into simpler substances
• Process: C6H12O6(aq) + 6O₂(aq) → 6CO₂(aq) + 6H2O(l) + energy
• The decomposition of vegetable matter into compost is also an example of an exothermic reaction