CHEMISTRY, RATE OF A CHEMICAL REACTIONS

CHEMISTRY LECTURE

Rate of Chemical Reactions & Chemical Equilibrium

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1. Rate of a Chemical Reaction

The rate of a chemical reaction is the speed at which reactants are converted into products. It is measured as the change in concentration of reactants or products per unit time.

Example: If magnesium reacts faster with hydrochloric acid, the reaction rate is high.

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2. Factors Affecting Rate of Chemical Reaction

(a) Concentration

An increase in concentration increases the rate because more particles are available to collide.

Example: Higher concentration of HCl reacts faster with zinc.

(b) Temperature

Higher temperature increases kinetic energy of particles, leading to more effective collisions.

(c) Surface Area

Powdered substances react faster than lumps due to larger surface area.

(d) Catalyst

A catalyst increases reaction rate without being consumed.

(e) Pressure (for gases)

Increased pressure increases collision frequency among gas molecules.

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3. Exothermic and Endothermic Reactions

(a) Exothermic Reactions

These reactions release heat to the surroundings.

Example: Combustion of fuel: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

(b) Endothermic Reactions

These reactions absorb heat from the surroundings.

Example: Photosynthesis absorbs energy from sunlight.

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4. Activation Energy & Activated Complex

Activation energy is the minimum energy required for a reaction to occur.

The activated complex is a temporary unstable arrangement of atoms at the peak of the energy curve.

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5. Catalysed and Uncatalysed Reactions

A catalysed reaction has a lower activation energy than an uncatalysed reaction.

Example: Decomposition of hydrogen peroxide using manganese (IV) oxide.

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6. Spontaneous Reactions

A spontaneous reaction occurs naturally without continuous external energy input.

Example: Rusting of iron.

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7. Chemical Equilibrium

Chemical equilibrium occurs when the rate of forward reaction equals the rate of backward reaction.

N₂ + 3H₂ ⇌ 2NH₃

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8. Equilibrium Constant (Kc)

For a general reaction:

aA + bB ⇌ cC + dD

Kc = (Cᶜ × Dᵈ) / (Aᵃ × Bᵇ)

Simple Calculation

If: H₂ + I₂ ⇌ 2HI Kc = [HI]² / ([H₂][I₂])

Gaseous Equilibrium (Kp)

Kp is used for gases and is based on partial pressures.

Kp = (P_products) / (P_reactants)

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9. Factors Affecting Equilibrium Constant

(a) Temperature

Temperature changes the value of Kc.

(b) Catalyst

Catalyst does NOT change equilibrium position, only speeds up attainment.

(c) Concentration & Pressure

They shift equilibrium position but do not change Kc.

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10. Applications of Equilibrium Reactions

• Haber process (ammonia production)
• Contact process (sulphuric acid)
• Industrial chemical manufacturing
• Biological systems

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Summary

Chemical reactions depend on factors like temperature and concentration. Equilibrium reactions are dynamic and essential in industrial and natural processes.