Chemical Reactions and Equations – Detailed Study Notes

Page 1 – Balancing Chemical Equations (Worked Example)

Context

• Example chosen: iron reacts with steam.
• Unbalanced (skeletal) equation: $\text{Fe}+\text{H}2\text{O}\rightarrow \text{Fe}3\text{O}4+\text{H}2$
• Objective: convert skeletal equation into a balanced chemical equation that obeys the Law of Conservation of Mass.

Hit-and-Trial / Inspection Method (Step-wise)

Step I – Draw Boxes
Surround every formula with a box so that the chemical formula itself is not modified while coefficients outside the box can change.
$\boxed{\text{Fe}}\ +\ \boxed{\text{H}2\text{O}}\ \rightarrow\ \boxed{\text{Fe}3\text{O}4}\ +\ \boxed{\text{H}2}$

Step II – Atom Count Table (Initial)

Element	Reactants	Products
Fe	1	3
O	1	4
H	2	2
The equation is clearly unbalanced.
Step III – Start With the Species Having Maximum Atoms
• Compound with maximum atoms = $\text{Fe}<em>3\text{O}</em>4$ .
• Choose oxygen first (4 atoms RHS vs 1 LHS).
• Multiply water by 4 to equalise oxygen: $4\text{H}<em>2\text{O}$ . $\text{Fe}+4\text{H}</em>2\text{O}\rightarrow \text{Fe}<em>3\text{O}</em>4+\text{H}_2$
Step IV – Balance Second Element (Hydrogen)
• LHS H-atoms = $4\times2=8$ , RHS H-atoms = 2.
• Multiply hydrogen molecule by 4: $4\text{H}<em>2$ . $\text{Fe}+4\text{H}</em>2\text{O}\rightarrow \text{Fe}<em>3\text{O}</em>4+4\text{H}_2$
Step V – Balance Remaining Element (Iron)
• Fe: LHS 1 vs RHS 3 ⇒ multiply Fe by 3.
$3\text{Fe}+4\text{H}<em>2\text{O}\rightarrow \text{Fe}</em>3\text{O}<em>4+4\text{H}</em>2$
Step VI – Verification
Element	Left	Right
---------	------	-------
Fe	3	3
H	8	8
O	4	4
All atoms tally → equation balanced.
Step VII – Add Physical States & Conditions
• Steam is gaseous, iron & its oxide are solids, hydrogen is gas.
$3\text{Fe(s)}+4\text{H}<em>2\text{O(g)}\rightarrow \text{Fe}</em>3\text{O}<em>4\text{(s)}+4\text{H}</em>2\text{(g)}$

Key Take-aways

• Only the coefficients (stoichiometric numbers) may be changed; formulae inside boxes stay intact.
• The procedure systematically balances one element at a time, beginning with the most complex compound.
• When all elements are balanced and coefficients are the smallest whole numbers, the equation honours the Law of Conservation of Mass.
• Adding states (s, l, g, aq) or reaction conditions (temperature, pressure, catalysts, light, etc.) makes an equation more informative, e.g.
$\text{CO(g)}+2\text{H}2\text{(g)}\xrightarrow{340\,\text{atm}}\text{CH}3\text{OH(l)}$
$6\text{CO}2\text{(aq)}+12\text{H}2\text{O(l)}\xrightarrow[\text{Sunlight}]{\text{Chlorophyll}}C6H{12}O6\text{(aq)}+6\text{O}2\text{(aq)}+6\text{H}_2\text{O(l)}$

Page 2 – Atom-Inventory Method Revisited

Re-statement of the Example (Eq 1.4)

$\text{Fe}+\text{H}2\text{O}\rightarrow \text{Fe}3\text{O}4+\text{H}2$

Atom Inventory Before Balancing (Unbalanced Eq 1.5)

Element	LHS	RHS
Fe	1	3
O	1	4
H	2	2

Reasons for the Chosen Order of Balancing

Choose compound with maximum atom count per formula → $\text{Fe}3\text{O}4$ .
Within that compound, select element with maximum atoms → Oxygen (4).
Balance O by introducing coefficient 4 before water.
Balance H next since it’s now unbalanced (8 vs 2).
Finally balance Fe.

Partly Balanced Snapshots

• After balancing O: $\text{Fe}+4\text{H}2\text{O}\rightarrow \text{Fe}3\text{O}4+\text{H}2$
• After balancing H: $\text{Fe}+4\text{H}2\text{O}\rightarrow \text{Fe}3\text{O}4+4\text{H}2$
• After balancing Fe: $3\text{Fe}+4\text{H}2\text{O}\rightarrow \text{Fe}3\text{O}4+4\text{H}2$

Terminology

• Skeletal Equation – formulae are present but atom counts do not match.
• Hit-and-Trial Method – repeated adjustment of coefficients until balance is achieved.

Page 3 – Word Equations, Formula Equations & Balancing Rationale

From Word to Formula

• Burning magnesium:
Word form → Magnesium + Oxygen → Magnesium oxide
Formula form → $\text{Mg}+\text{O}_2\rightarrow \text{MgO}$
• The formula form offers brevity and shows actual combining entities.

Balanced vs Skeletal

• A balanced equation has equal atom counts on LHS & RHS.
• An unbalanced (skeletal) equation violates the Law of Conservation of Mass.

Conservation of Mass (Class IX Recall)

“Mass can neither be created nor destroyed in a chemical reaction.”
Mathematically for every element $i$ :
$\sum n{i,\,\text{reactants}} = \sum n{i,\,\text{products}}$

Example (Zn + Dil. H₂SO₄)

Word description (Activity 1.3):
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Formula equation:
$\text{Zn}+\text{H}2\text{SO}4\rightarrow \text{ZnSO}4+\text{H}2$
Atom check table shows it is already balanced.

Page 4 – Observing Chemical Reactions in Daily Life

Everyday Examples of Chemical Change

• Spoilage of milk in summer.
• Rusting of iron cookware/nails.
• Fermentation of grapes (→ alcohol).
• Cooking of food.
• Digestion in the human body.
• Respiration.

Diagnostic Features of a Chemical Reaction

Change in state (solid ↔ liquid ↔ gas).
Change in colour.
Evolution of a gas.
Change in temperature / enthalpy (exothermic or endothermic).

Laboratory Activities Illustrating These Clues

Activity 1.1 – Burning Mg Ribbon
Observation: dazzling white flame, formation of white powder $\text{MgO}$ .
Activity 1.2 – Lead Nitrate + Potassium Iodide
Observation: bright yellow precipitate of $\text{PbI}2$ indicates colour change & state change. Reaction: $\text{Pb(NO}3)2\text{(aq)}+2\text{KI(aq)}\rightarrow 2\text{KNO}3\text{(aq)}+\text{PbI}_2\text{(s)}$
Activity 1.3 – Zinc + Dil. H₂SO₄
Observation: effervescence of $\text{H}_2$ gas & rise in temperature.
Reaction already balanced in previous page.

Page 5 – Syntax of Chemical Equations

Components

• Reactants – substances consumed (written LHS).
• Products – substances formed (written RHS).
• Arrow (→) indicates direction; plus sign (+) separates multiple species.

Symbol Conventions for Physical States

• (s) – solid
• (l) – liquid
• (g) – gas
• (aq) – aqueous (dissolved in water)

Noting Reaction Conditions

Conditions such as temperature, pressure, catalyst, light, etc. are written above or below the reaction arrow, e.g.
$\text{N}2\text{(g)}+3\text{H}2\text{(g)}\xrightarrow[400\,\text{atm}]{\text{Fe Catalyst, }450\,\text{°C}}2\text{NH}_3\text{(g)}$

Best Practices for Writing Chemical Equations

Convert descriptive words to chemical formulae.
Check valencies to ensure correct formulae (do not tamper during balancing).
Balance using systematic hit-and-trial or algebraic methods.
Use smallest whole-number coefficients.
Append physical states and reaction conditions when relevant.

QUICK RECAP CHEAT-SHEET

• Four visual evidences of chemical change: state, colour, gas, temperature.
• Balancing order: complex compound → element with highest multiplicity → proceed until all are balanced.
• Never alter a formula; only place integer coefficients in front.
• Balanced Fe–steam reaction:
$3\text{Fe(s)}+4\text{H}2\text{O(g)}\rightarrow \text{Fe}3\text{O}4\text{(s)}+4\text{H}2\text{(g)}$
• Law of Conservation of Mass underpins all balancing.
• State symbols and reaction conditions add contextual clarity to chemical equations.