Chemical Reactions
& Equations

Changes are chemical when new substances are formed.

5 Indicators of Chemical Change:

• Change in State
• Change in Colour
• Evolution of Gas
• Change in Temperature
• Formation of Precipitate

Teacher's Tip 💡

Burning a candle is both physical (melting wax) and chemical (burning wax)!

Law of Conservation of Mass

Mass can neither be created nor destroyed. Atoms on LHS = Atoms on RHS.

Skeletal (Unbalanced):
$Mg + O_2 \rightarrow MgO$

Balanced Equation:
$2Mg + O_2 \rightarrow 2MgO$

Use the "Hit and Trial" method. Balance the heaviest/most numerous atoms first!

Boards give extra credit for correct state symbols. Memorize these:

• (s): Solid (metals, precipitates)
• (l): Liquid (pure liquids like $H_2O$)
• (g): Gas ($CO_2$, $H_2$)
• (aq): Aqueous (dissolved in water)
• $\downarrow$: Precipitate (solid settling down)
• $\uparrow$: Gas evolving (bubbles)
• $\Delta$: Heat supplied (Thermal)

Exothermic Reaction

Heat is released along with products.

Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

Endothermic Reaction

Energy is absorbed from surroundings.

Photosynthesis: $6CO_2 + 12H_2O \xrightarrow{\text{Sunlight}} C_6H_{12}O_6 + 6O_2 + 6H_2O$

Reactants merge into one product: $A + B \rightarrow C$.

Commonly Exothermic!

Board Favorite: Whitewashing

1. Quick Lime + Water $\rightarrow$ Slaked Lime
$CaO + H_2O \rightarrow Ca(OH)_2$

2. Slaked Lime + Carbon Dioxide $\rightarrow$ Calcium Carbonate
$Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$

$CaCO_3$ provides the shiny finish after 2-3 days.

A single reactant breaks down: $C \rightarrow A + B$.

Commonly Endothermic!

Types of Decomposition:

• Thermal (Heat): $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$
• Electrolytic (Electricity): $2H_2O \rightarrow 2H_2 + O_2$
• Photolytic (Sunlight): $2AgCl \rightarrow 2Ag + Cl_2$

A stronger (more reactive) metal kicks out a weaker one based on the Reactivity Series.

Classic Example

Iron nails in blue Copper Sulphate solution fade it to light green (Ferrous Sulphate) and nails get a brownish copper coating.

Exchange of ions between reactants. Usually forms a Precipitate.

Precipitation Example

A white precipitate of Barium Sulphate ($BaSO_4$) is formed instantly.

Mnemonic: OIL RIG

Oxidation Is Loss (of Electrons)
Reduction Is Gain (of Electrons)

Oxygen-Hydrogen Rule

• Oxidation: Gain of Oxygen OR Loss of Hydrogen
• Reduction: Loss of Oxygen OR Gain of Hydrogen

Example: $MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2$

• Substance Oxidized: $HCl$ (loses Hydrogen)
• Substance Reduced: $MnO_2$ (loses Oxygen)

Finding the Agent

Oxidizing Agent: The substance that gets reduced (e.g. $MnO_2$).

Reducing Agent: The substance that gets oxidized (e.g. $HCl$).

Corrosion

• Iron: Rust ($Fe_2O_3 \cdot xH_2O$) - Reddish Brown.
• Silver: Black Coating ($Ag_2S$) due to sulfur.
• Copper: Green Coating ($CuCO_3 \cdot Cu(OH)_2$).

Rancidity

Oxidation of fats/oils leads to bad smell/taste in food.

Prevention: Flush packets with Nitrogen gas (Inert), use airtight containers, add Antioxidants.

Clean a magnesium ribbon with sandpaper to remove the protective layer of Magnesium Oxide/Carbonate.

Observation

Burns with a dazzling white flame and changes into a white powder (Magnesium Oxide).

Mix aqueous solutions of Lead Nitrate $Pb(NO_3)_2$ and Potassium Iodide $KI$.

Observation

A beautiful Yellow Precipitate of Lead Iodide ($PbI_2$) is formed instantly.

This is a classic Double Displacement & Precipitation reaction.

Add dilute Sulphuric Acid or Hydrochloric Acid to Zinc granules in a conical flask.

Observation

Gas bubbles form. Bring a candle, burns with a 'pop' sound (Hydrogen gas). The flask gets hot (Exothermic).

Take a small amount of calcium oxide (quick lime) in a beaker and slowly add water.

Observation

Vigorous reaction with a hissing sound. Beaker gets very hot (Highly Exothermic). Slaked lime is formed.

Heat 2g of Ferrous Sulphate crystals ($FeSO_4 \cdot 7H_2O$) in a dry boiling tube.

Observation

Green crystals turn white (lose water), then brown ($Fe_2O_3$). Characteristic smell of burning sulphur ($SO_2, SO_3$).

Heat 2g of Lead Nitrate powder in a boiling tube over a flame.

Observation

Emission of thick brown fumes of Nitrogen Dioxide ($NO_2$). A yellow residue of Lead Oxide ($PbO$) is left behind.

Pass electric current through acidified water.

Observation

Hydrogen gas collects at Cathode (-), Oxygen at Anode (+).

Volume ratio is exactly 2:1 ($H_2:O_2$).

Place 2g of Silver Chloride ($AgCl$) in a china dish out in sunlight for some time.

Observation

White $AgCl$ turns grey due to the formation of silver metal. Chlorine gas escapes.

Used in black and white photography!

Dip iron nails into a blue solution of Copper Sulphate ($CuSO_4$) for 20 minutes.

Observation

The blue color fades to light green ($FeSO_4$). Iron nails get a brownish coating of Copper.

Mix 3mL of Sodium Sulphate solution with 3mL of Barium Chloride solution.

Observation

A completely insoluble white precipitate of Barium Sulphate ($BaSO_4$) is formed instantly.

Heat about 1g of brown copper powder in a china dish.

Observation

The surface of copper powder becomes coated with black copper(II) oxide ($CuO$). Oxygen is added (Oxidation).

If $H_2$ gas is passed over this, the black coating turns brown again!