1.4 Hess’s Law — IAL Edexcel AS Chemistry

Hess’s Law টা খুব straightforward কিন্তু exam-এ অনেকেই ভুল করে। Simple কথায় বললে, reaction যেভাবেই ঘটুক না কেন, total enthalpy change একই থাকবে, যদি initial আর final state একই হয়।

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Statement of Hess’s Law

Hess’s Law: The total enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same.

মানে reaction direct হোক বা কয়েকটা step দিয়ে হোক, মোট ΔH একই হবে।

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Why Hess’s Law Works

• Enthalpy হলো state function
• Only initial and final state matter করে
• Path বা route matter করে না

এই logic-টাই exam-এ explain করতে বললে লিখবে।

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Enthalpy Change Symbol

Reaction enthalpy change লেখা হয়:

$$\Delta H$$

Negative ΔH → exothermic
Positive ΔH → endothermic

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Hess Cycle Method (Most Important)

1. Reaction লিখো
2. একটা Hess cycle আঁকো (usually formation বা combustion)
3. Arrows follow করো
4. Cycle complete হলে equation লেখো

Exam-এ diagram না আঁকলেও equation clearly দেখাতে হবে।

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Worked Example (Formation Enthalpy)

Calculate ΔH for:

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

Given:

ΔH_f(CO₂) = −394 kJ mol⁻¹
ΔH_f(C) = 0
ΔH_f(O₂) = 0

Using Hess’s Law:

$$\Delta H=\Delta H$$_products − ΔH_reactants

So:

ΔH = −394 − (0 + 0) = −394 kJ mol⁻¹

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Using Combustion Enthalpy

Formula:

$$\Delta H$$_reaction = ΣΔH_{combustion (reactants)} − ΣΔH_{combustion (products)}

Bangla tip: combustion data থাকলে, reactants − products করলেই বেশিরভাগ সময় ঠিক আসে।

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MCQ Practice (AS Level)

Q1. Which statement correctly describes Hess’s Law?

A) Enthalpy depends on reaction pathway
B) Enthalpy depends on temperature only
C) Enthalpy change is independent of route
D) Enthalpy changes only apply to combustion

Answer: C

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Q2. If ΔH for a reaction is −120 kJ mol⁻¹, the reaction is:

A) Endothermic
B) Exothermic
C) Neutral
D) Reversible

Answer: B

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Exam-Style Structured Question

Q. The enthalpy of combustion of carbon and carbon monoxide are given below:

C + O₂ → CO₂  ΔH = −394 kJ mol⁻¹
CO + ½O₂ → CO₂ ΔH = −283 kJ mol⁻¹

Calculate the enthalpy change for:

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

Worked Answer

Step 1: Write Hess equation

ΔH = −394 − (−283)

Step 2: Calculate

ΔH = −394 + 283 = −111 kJ mol⁻¹

So, the formation of CO is exothermic.

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Common Exam Mistakes

• Sign error (− and + ভুল করা)
• Reactants − products উল্টো করা
• Equation balance না করা

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Final Revision Tip

Hess’s Law question মানেই: equation → cycle → subtraction → sign check। এই চারটা ঠিক থাকলে full marks almost guaranteed।