Chemistry notes · Chapter 6 of 11

Chemical Reactions, Equilibrium & Catalysis

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What Is a Chemical Reaction?

A chemical reaction changes one or more substances into entirely new substances with different properties.
The starting substances are reactants; the new substances formed are products.
Signs a reaction occurred: colour change, gas bubbles, precipitate, temperature change or new smell.
A reaction is written as a chemical equation, e.g. Zn + H2SO4 → ZnSO4 + H2.
Every equation must be balanced – equal atoms of each kind on both sides (Law of Conservation of Mass).
To balance, only add big coefficients in front of formulae; never change the small subscripts inside a formula.

Check yourself

What are the signs a chemical reaction has occurred?

Colour change, gas bubbles, a precipitate, temperature change or a new smell.

What is the rule for balancing an equation?

Only add big coefficients in front of formulae — never change the small subscripts inside a formula.

A changes one or more substances into entirely new substances with different properties.

A chemical reaction changes one or more substances into entirely new substances with different properties.

The starting substances are ; the new substances formed are products.

The starting substances are reactants; the new substances formed are products.

A reaction is written as a , e.g. Zn + H2SO4 → ZnSO4 + H2.

A reaction is written as a chemical equation, e.g. Zn + H2SO4 → ZnSO4 + H2.

Every equation must be – equal atoms of each kind on both sides (Law of Conservation of Mass).

Every equation must be balanced – equal atoms of each kind on both sides (Law of Conservation of Mass).

Exothermic vs Endothermic Reactions

An exothermic reaction gives out heat; an endothermic reaction takes in heat.
Burning fuel, respiration, and adding water to quicklime are exothermic.
Dilution of an acid (e.g. H2SO4) is exothermic – it releases heat.
Cooking food, photosynthesis, and evaporation/sublimation are endothermic (absorb heat).
Decomposition reactions are generally endothermic as they need energy to break bonds.

Type	Heat	Examples
Exothermic	Released	Burning, respiration, CaO + water
Endothermic	Absorbed	Cooking, photosynthesis, decomposition

Check yourself

Give the standard exothermic examples.

Burning fuel, respiration, adding water to quicklime (CaO), and dilution of an acid such as H₂SO₄.

Give the standard endothermic examples.

Cooking food, photosynthesis, evaporation/sublimation, and most decomposition reactions (they need energy to break bonds).

An reaction gives out heat; an endothermic reaction takes in heat.

An exothermic reaction gives out heat; an endothermic reaction takes in heat.

Dilution of an acid (e.g. H2SO4) is – it releases heat.

Dilution of an acid (e.g. H2SO4) is exothermic – it releases heat.

Type: Exothermic → Heat · Examples?

Heat: Released
Examples: Burning, respiration, CaO + water

Type: Endothermic → Heat · Examples?

Heat: Absorbed
Examples: Cooking, photosynthesis, decomposition

Types of Chemical Reactions

Combination: two or more substances join into one new substance, e.g. CaO + H2O → Ca(OH)2.
Decomposition: one compound breaks into simpler substances, e.g. CaCO3 → CaO + CO2.
Decomposition is driven by heat (thermal), light (photo) or electricity (electrolytic).
Displacement: a more reactive element pushes out a less reactive one, e.g. Fe + CuSO4 → FeSO4 + Cu.
Double displacement: two compounds swap ions, e.g. Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl.
A precipitation reaction forms an insoluble solid (precipitate) that settles out.

Type	What happens	Example
Combination	Many → one	CaO + H2O → Ca(OH)2
Decomposition	One → many	CaCO3 → CaO + CO2
Displacement	Reactive out-pushes weak	Fe + CuSO4 → FeSO4 + Cu
Double displacement	Ions swap partners	Na2SO4 + BaCl2 → BaSO4 + 2NaCl

Check yourself

What is a combination reaction?

Two or more substances join into one new substance, e.g. CaO + H₂O → Ca(OH)₂.

What is a decomposition reaction, and what drives it?

One compound breaks into simpler substances, e.g. CaCO₃ → CaO + CO₂. Driven by heat (thermal), light (photo) or electricity (electrolytic).

What is a displacement reaction?

A more reactive element pushes out a less reactive one, e.g. Fe + CuSO₄ → FeSO₄ + Cu, or Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.

What is a double displacement reaction?

Two compounds swap ions, e.g. Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. If an insoluble solid settles out it is a precipitation reaction.

Type: Combination → What happens · Example?

What happens: Many → one
Example: CaO + H2O → Ca(OH)2

Type: Decomposition → What happens · Example?

What happens: One → many
Example: CaCO3 → CaO + CO2

Type: Displacement → What happens · Example?

What happens: Reactive out-pushes weak
Example: Fe + CuSO4 → FeSO4 + Cu

Type: Double displacement → What happens · Example?

What happens: Ions swap partners
Example: Na2SO4 + BaCl2 → BaSO4 + 2NaCl

Oxidation, Reduction & Redox

Simple view: oxidation = gain of oxygen or loss of hydrogen; reduction is the opposite.
Electron view: oxidation is loss of electrons, reduction is gain of electrons (memory: OIL RIG).
A redox reaction has oxidation and reduction happening at the same time.
The oxidising agent takes electrons (gets reduced); the reducing agent gives electrons (gets oxidised).
Oxidation number is a bookkeeping value, e.g. carbon is +4 and oxygen −2 in CO2.
Disproportionation: the same element is both oxidised and reduced, e.g. decomposition of H2O2.
Redox is balanced by the oxidation-number method or the ion-electron (half-reaction) method.

Check yourself

Oxidation and reduction — the simple view and the electron view?

Simple: oxidation = gain of oxygen or loss of hydrogen. Electron view: OIL RIG — Oxidation Is Loss, Reduction Is Gain of electrons.

Which agent gets oxidised and which gets reduced?

The oxidising agent takes electrons and gets reduced; the reducing agent gives electrons and gets oxidised.

What is disproportionation?

The same element is both oxidised and reduced, e.g. decomposition of H₂O₂.

A has oxidation and reduction happening at the same time.

A redox reaction has oxidation and reduction happening at the same time.

is a bookkeeping value, e.g. carbon is +4 and oxygen −2 in CO2.

Oxidation number is a bookkeeping value, e.g. carbon is +4 and oxygen −2 in CO2.

Redox is balanced by the or the ion-electron (half-reaction) method.

Redox is balanced by the oxidation-number method or the ion-electron (half-reaction) method.

What does this diagram show?

Oxidation, Reduction & Redox

Corrosion & Rusting

Corrosion is a slow redox process where a metal is eaten away by air and moisture.
Rusting needs both oxygen and water (moisture) together.
Rust is reddish-brown hydrated iron(III) oxide (Fe2O3·xH2O).
When iron rusts its weight first increases as it combines with oxygen.
Later the flaky rust peels off, so the weight then decreases.
Prevented by painting, oiling, galvanising (zinc coat) and alloying (e.g. stainless steel).

Check yourself

What does rusting need, and what is rust?

Both oxygen and water (moisture) together. Rust is reddish-brown hydrated iron(III) oxide, Fe₂O₃·xH₂O.

What happens to the weight of iron as it rusts?

It first increases as it combines with oxygen, then decreases when the flaky rust peels off.

How is rusting prevented?

Painting, oiling, galvanising (zinc coat) and alloying — e.g. stainless steel.

is a slow redox process where a metal is eaten away by air and moisture.

Corrosion is a slow redox process where a metal is eaten away by air and moisture.

Reactivity (Activity) Series of Metals

The reactivity series ranks metals from most reactive (top) to least reactive (bottom).
A metal higher in the series displaces a lower metal from its salt solution.
Top order: K > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > Au.
Very reactive metals (K, Na, Ca) react even with cold water.
Moderately reactive metals react with dilute acids to release hydrogen gas (tested by the 'pop' sound).
Gold and platinum are so unreactive they occur free (uncombined) in nature.

Reactivity	Metals	Behaviour
Most reactive	K, Na, Ca	React with cold water
Moderate	Mg, Al, Zn, Fe	React with dilute acids → H2
Least reactive	Cu, Ag, Au, Pt	Barely react; found free

Check yourself

Give the reactivity series order.

K > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > Au. A metal higher in the series displaces a lower one from its salt solution.

Which metals react with cold water, and which with dilute acids?

K, Na, Ca react even with cold water. Mg, Al, Zn, Fe react with dilute acids, releasing hydrogen (the 'pop' test).

Which metals occur free (uncombined) in nature?

Gold and platinum — they are too unreactive to combine.

The ranks metals from most reactive (top) to least reactive (bottom).

The reactivity series ranks metals from most reactive (top) to least reactive (bottom).

Moderately reactive metals react with release hydrogen gas (tested by the 'pop' sound).

Moderately reactive metals react with dilute acids to release hydrogen gas (tested by the 'pop' sound).

Reactivity: Most reactive → Metals · Behaviour?

Metals: K, Na, Ca
Behaviour: React with cold water

Reactivity: Moderate → Metals · Behaviour?

Metals: Mg, Al, Zn, Fe
Behaviour: React with dilute acids → H2

Reactivity: Least reactive → Metals · Behaviour?

Metals: Cu, Ag, Au, Pt
Behaviour: Barely react; found free

Chemical Equilibrium

Some reactions are reversible, going forward and backward at the same time (shown by ⇌).
Equilibrium is reached when the forward rate equals the backward rate.
At equilibrium the concentrations of reactants and products stop changing.
Equilibrium is dynamic, not dead – both reactions keep happening unseen.
The equilibrium constant (K) = product concentrations divided by reactant concentrations, each raised to its coefficient.
A large K means the reaction favours products; a small K favours reactants.

Check yourself

When is chemical equilibrium reached?

When the forward rate equals the backward rate, so concentrations stop changing. It is dynamic, not dead — both reactions keep happening unseen.

What does a large equilibrium constant K mean?

K = product concentrations ÷ reactant concentrations, each raised to its coefficient. A large K favours products; a small K favours reactants.

Some reactions are , going forward and backward at the same time (shown by ⇌).

Some reactions are reversible, going forward and backward at the same time (shown by ⇌).

Le Chatelier's Principle

Le Chatelier's Principle: if a system at equilibrium is disturbed, it shifts to oppose the change.
Adding more reactant shifts equilibrium toward products (forward).
Increasing pressure shifts equilibrium toward the side with fewer gas molecules.
For an exothermic reaction, raising temperature shifts equilibrium backward (toward reactants).
A catalyst does not shift equilibrium; it only helps it be reached faster.

Change applied	Equilibrium shifts
Add reactant	Toward products
Add product	Toward reactants
Increase pressure	Toward fewer gas moles
Increase temperature	Endothermic direction

Check yourself

State Le Chatelier's Principle.

If a system at equilibrium is disturbed, it shifts to oppose the change.

How does equilibrium shift on adding reactant, adding product, raising pressure, raising temperature?

Add reactant → toward products. Add product → toward reactants. Raise pressure → toward fewer gas moles. Raise temperature → in the endothermic direction.

Does a catalyst shift the equilibrium?

No — it only helps equilibrium be reached faster.

Increasing shifts equilibrium toward the side with fewer gas molecules.

Increasing pressure shifts equilibrium toward the side with fewer gas molecules.

For an exothermic reaction, raising shifts equilibrium backward (toward reactants).

For an exothermic reaction, raising temperature shifts equilibrium backward (toward reactants).

Change applied: Add reactant → Equilibrium shifts?

Equilibrium shifts: Toward products

Change applied: Add product → Equilibrium shifts?

Equilibrium shifts: Toward reactants

Change applied: Increase pressure → Equilibrium shifts?

Equilibrium shifts: Toward fewer gas moles

Change applied: Increase temperature → Equilibrium shifts?

Equilibrium shifts: Endothermic direction

Rate of Reaction

Rate of reaction is how fast reactants are converted into products.
Higher concentration of reactants speeds up the reaction.
Higher temperature increases rate by giving particles more energy.
Greater surface area (powder vs lump) increases rate.
A catalyst speeds up the rate; an inhibitor (negative catalyst) slows it down.
Activation energy is the minimum energy needed for reactant particles to react.

Check yourself

What increases the rate of a reaction?

Higher concentration, higher temperature, greater surface area (powder vs lump), and a catalyst.

What is activation energy, and how does a catalyst work?

The minimum energy needed for reactant particles to react. A catalyst lowers the activation energy, giving an easier path, and is itself unchanged at the end.

What slows a reaction down?

An inhibitor (negative catalyst). Promoters and both homogeneous and heterogeneous catalysts all speed reactions up.

is how fast reactants are converted into products.

Rate of reaction is how fast reactants are converted into products.

Higher increases rate by giving particles more energy.

Higher temperature increases rate by giving particles more energy.

Catalysis

A catalyst changes the speed of a reaction but is itself unchanged at the end.
A catalyst works by lowering the activation energy, giving an easier path.
Positive catalysts speed up reactions; negative catalysts (inhibitors) slow them down.
Nickel (Ni) is the catalyst for hydrogenation of vegetable oil into vanaspati ghee.
Iron is used in the Haber process (ammonia); vanadium pentoxide (V2O5) in the Contact process.
Enzymes are biological catalysts that control reactions in living bodies.

Process	Catalyst
Vegetable oil → ghee	Nickel (Ni)
Haber process (NH3)	Iron (Fe)
Contact process (H2SO4)	V2O5

Check yourself

Name the three exam catalysts and their processes.

Nickel (Ni) — hydrogenation of vegetable oil into vanaspati ghee. Iron (Fe) — Haber process for ammonia. V₂O₅ — Contact process for H₂SO₄.

What are the biological catalysts?

Enzymes — they control reactions in living bodies.

A changes the speed of a reaction but is itself unchanged at the end.

A catalyst changes the speed of a reaction but is itself unchanged at the end.

reactions; negative catalysts (inhibitors) slow them down.

Positive catalysts speed up reactions; negative catalysts (inhibitors) slow them down.

Process: Vegetable oil → ghee → Catalyst?

Catalyst: Nickel (Ni)

Process: Haber process (NH3) → Catalyst?

Catalyst: Iron (Fe)

Process: Contact process (H2SO4) → Catalyst?

Catalyst: V2O5

What does this diagram show?

Catalyst lowering activation energy

Electrolytes & Conductivity

An electrolyte is a substance that conducts electricity when molten or dissolved in water.
A strong electrolyte ionises almost completely (e.g. NaCl, HCl, H2SO4).
A weak electrolyte ionises only partly (e.g. acetic acid, ammonia).
Conductivity of a strong electrolyte decreases on dilution (fewer ions per unit volume).
Non-electrolytes (e.g. sugar, urea) do not conduct as they form no ions.
Conduction occurs because ions are free to carry charge through the solution.

Check yourself

Strong vs weak electrolyte?

A strong electrolyte ionises almost completely (NaCl, HCl, H₂SO₄); a weak one ionises only partly (acetic acid, ammonia).

What happens to the conductivity of a strong electrolyte on dilution?

Specific conductivity decreases — the same ions spread into more solvent, so there are fewer ions per unit volume (molar conductivity, though, rises).

Why do sugar and urea not conduct electricity in water?

They are non-electrolytes — they form no ions, and conduction needs free ions to carry charge.

An is a substance that conducts electricity when molten or dissolved in water.

An electrolyte is a substance that conducts electricity when molten or dissolved in water.

Electrochemistry: Cells & Electrode Potential

A redox couple (e.g. Zn2+/Zn) is the oxidised and reduced forms taken together – basis of cells.
Electrode potential measures a metal's tendency to lose or gain electrons.
A more negative electrode potential means a stronger reducing power (loses electrons easily).
So for potentials 0.5 V, −3 V, −1.2 V, reducing power order is the −3 V metal > −1.2 V > 0.5 V.
In electrolysis, electric current drives a non-spontaneous redox reaction.

Quantity	Value/Meaning
Faraday constant	≈ 96,500 C per mole of electrons
More negative E°	Greater reducing power
More positive E°	Greater oxidising power

Michael Faraday
Faraday constant (F) ≈ 96,500 coulombs – the charge carried by one mole of electrons.

Electrochemistry: Cells & Electrode Potential

Check yourself

What does electrode potential measure?

A metal's tendency to lose or gain electrons. A more negative E° means stronger reducing power — so for 0.5 V, −3 V and −1.2 V, the order is −3 V > −1.2 V > 0.5 V.

What is the Faraday constant?

≈ 96,500 coulombs — the charge carried by one mole of electrons. It is a universal constant, independent of electrolyte, current or solvent.

A (e.g. Zn2+/Zn) is the oxidised and reduced forms taken together – basis of cells.

A redox couple (e.g. Zn2+/Zn) is the oxidised and reduced forms taken together – basis of cells.

In , electric current drives a non-spontaneous redox reaction.

In electrolysis, electric current drives a non-spontaneous redox reaction.

Quantity: Faraday constant → Value/Meaning?

Value/Meaning: ≈ 96,500 C per mole of electrons

Quantity: More negative E° → Value/Meaning?

Value/Meaning: Greater reducing power

Quantity: More positive E° → Value/Meaning?

Value/Meaning: Greater oxidising power

Who is this, and what should you remember about them?

Michael Faraday

Faraday constant (F) ≈ 96,500 coulombs – the charge carried by one mole of electrons.

What does this diagram show?

Electrochemistry: Cells & Electrode Potential

Some Important Reactions to Know

Limewater (Ca(OH)2) turns milky when carbon dioxide (CO2) is passed through it.
The milkiness is due to insoluble calcium carbonate (CaCO3) forming.
Zn + H2SO4 → ZnSO4 + H2 releases hydrogen, tested by the 'pop' sound.
Fe + CuSO4 → FeSO4 + Cu: blue colour fades to green as copper is set free.
CaO + H2O → Ca(OH)2: quicklime + water gives slaked lime with much heat.

Check yourself

What turns limewater milky, and why?

Carbon dioxide (CO₂). Limewater is Ca(OH)₂, and insoluble calcium carbonate (CaCO₃) forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

What do you observe when iron is put in copper sulphate solution?

The blue colour fades to green as copper is set free: Fe + CuSO₄ → FeSO₄ + Cu.

The milkiness is due to insoluble (CaCO3) forming.

The milkiness is due to insoluble calcium carbonate (CaCO3) forming.

releases hydrogen, tested by the 'pop' sound.

Zn + H2SO4 → ZnSO4 + H2 releases hydrogen, tested by the 'pop' sound.

: quicklime + water gives slaked lime with much heat.

CaO + H2O → Ca(OH)2: quicklime + water gives slaked lime with much heat.

All 11 chapters of Chemistry notes

Basic Concepts, Matter & Measurement13 sections
Structure of Atom12 sections
Periodic Table & Periodicity11 sections
Chemical Bonding & Molecular Structure12 sections
States of Matter & Gas Laws12 sections
Chemical Reactions, Equilibrium & Catalysis13 sections
Acids, Bases & Salts13 sections
Organic Chemistry & Hydrocarbons12 sections
Chemistry in Everyday Life & Important Compounds11 sections
Biomolecules (Chemistry)13 sections
Radioactivity & Nuclear Chemistry11 sections