Enthalpy Change — IAL Edexcel AS Chemistry

Enthalpy change chapter টা pure understanding-based। Definition ঠিক থাকলে, sign convention আর diagram বুঝলে exam-এ এই chapter থেকে easy marks তোলা যায়। চল casual Bangla তে পরিষ্কার করে দেখি।

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What is Enthalpy?

Enthalpy (H) হলো system-এর total energy, যার মধ্যে heat energy included থাকে। Absolute enthalpy measure করা যায় না, তাই আমরা সবসময় enthalpy change নিয়ে কথা বলি।

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What is Enthalpy Change?

Enthalpy change হলো reaction চলাকালীন heat energy change। Symbol:

$$\Delta H$$

Formula:

$$\Delta H=H$$_products − H_reactants

Bangla logic: products-এর energy reactants থেকে বেশি হলে ΔH positive।

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Exothermic Reaction

যখন reaction heat release করে:

• ΔH negative
• Surroundings গরম হয়
• Products-এর energy কম

Energy profile idea:

$$\mathrm{Energy}\left(\mathrm{Reactants}\right)>\mathrm{Energy}\left(\mathrm{Products}\right)$$

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Endothermic Reaction

যখন reaction heat absorb করে:

• ΔH positive
• Surroundings ঠান্ডা হয়
• Products-এর energy বেশি

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Standard Conditions

Standard enthalpy change measured হয়:

• Temperature = 298 K
• Pressure = 100 kPa
• Solutions = 1 mol dm⁻³

Standard symbol ব্যবহার করা হয়: ΔH°

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Types of Enthalpy Change (AS Level)

• Enthalpy change of reaction (ΔH)
• Standard enthalpy of formation (ΔH_f°)
• Standard enthalpy of combustion (ΔH_c°)

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Standard Enthalpy of Formation

Definition:

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states.

Example:

$$C+O^2\to \mathrm{CO}2$$

Note: Elements in standard state have ΔH_f° = 0

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Standard Enthalpy of Combustion

Definition:

The enthalpy change when 1 mole of a substance burns completely in oxygen under standard conditions.

Always exothermic → ΔH negative।

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Worked Example

If:

H_reactants = −200 kJ mol⁻¹
H_products = −450 kJ mol⁻¹

Calculate ΔH:

$$\Delta H=(-450)-(-200)$$

ΔH = −250 kJ mol⁻¹ → exothermic

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MCQ Practice (AS Level)

Q1. Which statement is correct for an exothermic reaction?

A) ΔH positive
B) Heat absorbed
C) Products have lower enthalpy
D) Temperature decreases

Answer: C

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Q2. Which condition is used for standard enthalpy values?

A) 273 K, 1 atm
B) 298 K, 100 kPa
C) 300 K, 1 atm
D) 298 K, 1 atm only

Answer: B

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Exam-Style Structured Question

Q. Define the term standard enthalpy change of combustion and state the sign of this enthalpy change.

Worked Answer

Standard enthalpy change of combustion is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.

The sign of ΔH is negative because the reaction is exothermic.

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Common Exam Mistakes

• Sign (+/−) ভুল করা
• Standard conditions mention না করা
• Formation আর combustion confuse করা

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Final Revision Tip

Enthalpy question মানেই: definition → sign → formula → logic। এই চারটা ঠিক থাকলে marks নিশ্চিত।